The significance of heterochrony to the evolution of Hispaniolan palm-tanagers, genus Phaenicophilus

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The significance of heterochrony to the evolution of Hispaniolan palm-tanagers, genus Phaenicophilus behavioral, morphological and genetic correlates
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McDonald, Mara Alessandra
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Tanagers   ( lcsh )
Passeriformes -- Hispaniola   ( lcsh )
Heterochrony (Biology) -- Hispaniola   ( lcsh )
Zoology thesis Ph.D
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Thesis:
Thesis (Ph. D.)--University of Florida, 1988.
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Includes bibliographical references.
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by Mara Alessandra McDonald.
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Typescript.
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Vita.

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THE SIGNIFICANCE OF HETEROCHRONY TO THE EVOLUTION OF HISPANIOLAN
PALM-TANAGERS, GENUS PHAENICOPHIL US: BEHAVIORAL, MORPHOLOGICAL
AND GENETIC CORRELATES











By

MARA ALESSANDRA MCDONALD


A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL
OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF
DOCTOR OF PHILOSOPHY



UNIVERSITY OF FLORIDA

1988





















Copyright 1988

by

Mara Alessandra McDonald




















To Margaret K. Langworthy, who has maintained her personal and professional integrity

through all these trying times, and to all women and men who remain ethical and devoted to

excellence in their scientific and personal endeavors.
















ACKNOWLE DGMENTS

Members of my committee who have kindly supported me during my graduate tenure

include Lincoln B. Brower, John Wm. Hardy, Larry D. Harris, Carmine A. Lanciani, and

Michael H. Smith. Bill Hardy edited the several versions of the dissertation and graciously

helped get all the paperwork together in my absence. I am particularly grateful to Mike

Smith for his input into the final analyses, for his moral and financial support, and for his

many hours of advice, help, and guidance.

Robert M. Zink, Curator of Frozen Tissues, Louisiana State University Museum of

Zoology, patiently provided me with material I requested for the systematic analysis; helped

me with some cold concepts in phylogenetic analysis; and warmly encouraged my work. Bob

Crawford, formerly of Tall Timbers Research Station in Tallahassee, Florida, provided many

of the warbler specimens. James Bond at the Philadelphia Academy of Sciences, Storrs Olson


at the U.S.


National Museum in Washington, D.C.,


Kenneth Parkes at the Carnegie Museum


of Natural History in Pittsburgh, and Charles Sibley now at San Francisco State University,

made significant contributions through their writings, correspondence, and discussions.


Mercedes Foster of the U.S.


National Museum and Marcy and Robert Lawton of the


University of Alabama in Huntsville, pioneers in the embryonic field of avian heterochrony,

have provided me with the theoretical and empirical foundation for pursuing my research.

I can never repay James M. Novak at the Savannah River Ecology Laboratory (SREL)

for all the time he has taken to discuss minor points or major issues, both conceptual and

statistical. His patience in teaching me computer and other essential skills helped me










Philip M. Dixon at SREL for their moral and intellectual support.


They accepted and


encouraged me under all circumstances and provided a challenging, but supportive

atmosphere in which to work. I owe much to my friends and teachers at the University of

Florida, including Mike Binford, Mark Brenner, Lincoln Brower, Kris Brugger, Peter

Feinsinger, Rob Ferl, Larry Harris, Susan Jacobson, Rich Kiltie, Carmine Lanciani,

Margaret Langworthy, Brian McNab, Frank Nordlie, Theresa Pope, Jon Reiskind, John

Robinson, and to Bill Kilpatrick at the University of Vermont.

I am indebted to the Organization of American States for financial support while in

Haiti and to Ragnar Arneson, Roland Roi, and the staff in Port-au-Prince who helped make

my stay in Haiti a success. I am also indebted to the Chapman Memorial Fund of the

American Museum of Natural History, Sigma Xi Grants-in-Aid, the Oak Ridge Associated

Universities Program, the Department of Zoology at the University of Florida, and to the

University of Georgia Savannah River Ecology Laboratory for financial assistance. I thank

Morris and Cecilia Maizels for their friendship and financial support.

I thank the many Government of Haiti officials including Edmond Magny, Florence

Sergile, Joseph Felix, Jean-Edner Francois and Paul Paryski, who spent much time and effort

in making my field work possible. I owe much to Jean-Phillipe Audinet, Robert Cassagnol,

Stephan Dix, Santa and Scott Faiia, Joyce Flores, Tom Greathouse, Jim Keith, Susana

Molnar, Pierre-Yves Roumain, Jim Talbot, and John Thorbjarnarson, for all their help,


support, guidance and encouragement while working in Haiti.


Without the cordial help of the


various office personnel, including Janet Ziegler, Carole Binello, Grace Kiltie, Rhoda Bryant

at the University of Florida and JoAn Lowery, Miriam Stapleton and Jan Hinton at SREL,

many of the tedious little tasks could not have been finished. I thank Jean B. Coleman at

SREL for advice on the illustrations.










Vertebrate Zoology, was instrumental in providing me with the conceptual tools to undertake

my dissertation research. His lecture on heterochrony in 1978 set the stage for my

subsequent interest in the subject and his excellent teaching provided me with the conceptual

framework necessary to pursue questions in evolutionary biology. I am also deeply grateful to

Michael H. Smith, Director of the University of Georgia Savannah River Ecology Laboratory .

Although his very broad interests did not include birds, heterochrony, Hispaniola, feminism,

or systematics, he committed himself to a critical evaluation of my work and provided

significant input into the generation of hypotheses and the formulation of ideas. His


influence significantly improved the anal


yses and presentation of my data; his tenacity pulled


me through some bleak moments. I have the highest esteem for the dedication, critical

thinking, broad perspectives, and integrity of these two gentlemen.

This research was supported under contract DE-AC09-76SROO-819 between the U.S.

Department of Energy and the University of Georgia's Savannah River Ecology Laboratory.











TABLE OF CONTENTS


ACKNOW LEDGMENTS . . . .. . ........................

LIST O F TA BLES . . .. . . . . . . .. .. .. .

LIST O F FIG U RES ...............................................................

A B ST R A C T .....................................................................


CHAPTERS


INTRODUCTION


Purpose and Definitions . . . . . . .

Hispaniolan Palm-Tanagers . . . . . . . . . . .


The Significance of Heterochrony and Paedomorphosis


HISPANIOLAN PALM-TANAGERS: BEHAVIORAL AND
MORPHOLOGICAL CONSEQUENCES OF HETEROCHRONY


Introduction


Methods and Materials


R results . . . . . . . . . . . .

D discussion ................................................................


HISPANIOLAN PALM-TANAGERS: THE GENETIC CONSEQUENCES OF
H ETEROCH RON Y . . . . .....................................


MI ethod s ........ . . . . ......... . . .............. ..

R es lts . . . . . . . ................................ ...

D iscussion . . . . . . . . ................... ..
x-i& -o x i .*** .*^ .**************.


BIOCHEMICAL SYSTEMATICS OF HISPANIOLAN PALM-TANAGERS


Introduction . . . . .......................

Mpfhnrlc










CONCLUSIONS AND FUTURE DIRECTIONS


APPENDICES


DESCRIPTION OF MORPHOLOGICAL MEASUREMENTS


ILLUSTRATION OF PLUMAGE CHARACTERS MEASURED ON


HISPANIOLAN PALM-TANAGERS


LIST OF SUBSTRATES USED BY HISPANIOLAN PALM-TANAGERS


WHILE FORAGING


HORIZONTAL SUBSTRATE USE OF HISPANIOLAN PALM-


TANAGERS


LITERATU RE CITED ..........................................................

BIOGRAPHICAL SKETCH .....................................................












LIST OF TABLES


TABLES


PAGE


Principal Components for seventeen morphological variables measured in
Hispaniolan palm-tanagers . . . . . . . . . . .


Mean, sample size, and significance of differences of 8 foraging
behavior variables . . . . . . . . . .


List of gray-crowned Black-crowned Palm-Tanagers collected in late winter or


the earlypart of the breeding season (late March to early April


with associated


comments


Estimates of genetic variability for Hispaniolan palm-tanagers and their hybrids
for 39 enzym e loci .......................................................


Mean % heterozygosity for adult and juvenile Black-crowned palm-tanagers
after Jackknife simulations (Lanyon, 1987) for 13 variable loci ...........


List of species, number of specimens, and genetic variability for 25 species of
Emberizidae across 20 assayed enzyme loci .............................


Allelic designations and frequencies for 20 loci used in the systematic analysis


25 species of Emberizidae


. 35


. 80


Nei's (


1978) unbiased genetic distance (above diagonal) and modified Rogers'


distance (Wright, 1978) (below diagonal) for 25 taxa used in the systematic
analysis . . . . . . . . . . . . . . .


Results of Jackknife procedure on the stability of phylogenetic affinities of tanagers
and warblers based on 20 electrophoretic loci ................................


Manhattan distance matrix of morphological features used in comparison of 15
tanagers and warblers . . . . . . . . . . . .


. 83


. . . . .. 33










LIST OF FIGURES


FIGURES


PAGE


Map of the current distribution of Hispaniolan palm-tanagers


Three-dimensional graph of morphological variables that separate Hispaniolan
palm -tanagers . . . . . . . . . . . . . .


Unweighted Pair-group Method of Averaging (UPGMA) foraging behavior
phenogram for six species of birds found in Haiti .............................


Frequency of foraging behavior on each substrate types


pine, braodleaf tree,


and shrub below 3 m in height) observed in mixed pine habitat for adult
(BPA) and juvenile (BPI) Black-crowned Palm-Tanagers and Gray-crowned
Palm-Tanagers (GPT) .. . ................ . .


Expected heterozygosity (He) of colonists based on founding population size
(No) and observed heterozygosity (Ho) in the antecedent population .......

Unweighted Pair-group Method of Averaging (UPGMA) phenogram (A) and
Distance Wagner tree (B) based on modified Rogers' genetic distance
(Wright, 1978) across 20 loci for 25 Emberizidae species ..............


Unweighted Pair-group Method of Averaging (UPGMA)phenogram based on
21 plumage and skeletal features measured on 15 species of Emberizidae .















Abstract of Dissertation Presented to the Graduate School
of the University of Florida in Partial Fulfillment of the
Requirements for the Degree of Doctor of Philosophy




THE SIGNIFICANCE OF HETEROCHRONY TO THE EVOLUTION OF HISPANIOLAN
PALM-TANAGERS, GENUS PHAENICOPHIL US: BEHAVIORAL, MORPHOLOGICAL
AND GENETIC CORRELATES



By

MARA ALESSANDRA MCDONALD


AUGUST 1988


Chairperson: John William Hardy
Cochairperson: Michael Howard Smith
Major Department: Zoology



Heterochrony, or changes in developmental timing, may explain the rapid speciation

and diversification of avian taxa. The purpose of this study is to investigate the significance of

heterochrony to the evolution of Hispaniolan palm-tanagers, genus Phaenicophilus, using

behavioral, morphological, and genetic characteristics at the population and systematic

levels. Phaenicophilus palmarum is characterized by a significant age-dimorphism in

foraging behavior, morphology, and genetic variability, as detected by starch gel

electrophoresis. Significantly higher levels of genetic variability were observed for juveniles


(H= 0.121) than for adults (H


= 0.074) across 13 of 17 variable loci in the sample of 39 assayed


1 f .










is its sibling species. Levels of genetic variability observed in P. poliocephalus


(H= 0.104) are not significantly different from either age-class of P. palmarum. A colonization

model, using observed heterozygosity of juvenile P. palmarum and varying founding

population sizes and compositions, with subsequent rapid reproduction after founding

supports the hypothesis that a small number of founders, composed mostly of juvenile P.

palmarum, could explain the high levels of genetic heterozygosity observed in current


populations of P. poliocephalu


s. The smaller body size, different social structure, retention of


juvenile behavior and morphology, and absence of a long-term age-dimorphism in relation to

P. palmarum suggest that P. poliocephalus is paedomorphic to P. palmarum.

Biochemical systematic analyses were used to determine the polarity of evolution of

the two species of Phaenicophilus. Phaenicophilus was aligned most frequently with Piranga

on Distance Wagner trees. Furthermore, average genetic distance was lower and the number

of shared alleles was higher between Phaenicophilus and Piranga than for other comparisons.

This result is consistent with the hypothesis that Phaenicophilus poliocephalus is most likely

derived from P. palmarum and paedomorphic to it.

















CHAPTER 1
INTRODUCTION



Purpose and Definitions


The purpose of this research was to investigate the importance of heterochrony to the

evolution of two species of Hispaniolan Palm-Tanagers, genus Phaenicophilus, using


behavioral, morphological and genetic characteristics.


Population and systematic analyses


were employed to determine how the palm-tanagers were related to one another and to other

species. Heterochrony is the selective modulation of patterns of development" (Larson,

1980:1) and can be divided into two processes: recapitulation, the retention of ancestral adult

features in the early ontogeny of the descendant species, and paedomorphosis, defined on a


systematic level


as the retention of ancestral juvenile features in the descendant species


(Gould, 1977; Alberch etal., 1979).


Use of the term "juvenile" in this discussion refers to


retained morphological features and not the state of sexual maturation. Paedomorphosis has

been invoked to explain the adaptive radiation of plethodontid salamanders (Wake, 1966;

Larson, 1980) and to characterize salamander populations that delay metamorphosis (Gould,

1977). Its significance as a microevolutionary process leading to macroevolutionary change


has only recently been appreciated for other taxa, such


Lawton, 1986; Foster


birds (Gould, 1977; Lawton and


1987).


Hispaniolan Palm-Tanagers


't n "i n H fin n C./1 I 4- nrjr r^ a n< ,^ ,. ^ ,. ,f .n 4 LI; C. nfl 1 n 'T t n I rt aK n. iL al i n r n a a. up A










palmarum. Phaenicophilus poliocephalus resembles the juvenile of P. palmarum in crown


color.


This resemblance suggested that heterochrony might be a process important to the


evolution of these species.


To determine the significance of heterochrony to the evolution of


Phaenicophilus, it is first necessary to reexamine the species designations of the two forms

and establish that they are not geographic variants of a single species. Both morphological

and genetic criteria are used in the analysis. Second, alternative hypotheses to explain the

similarity in morphology between the two species need to be compared to the hypothesis that


invokes heterochrony. These alternative hypotheses include those that consider character

divergence and convergence. Third, if heterochrony is involved, it is essential to know which


Phaenicophilus species was derived and which was antecedent.


The answer to this question


depends upon a knowledge of phylogenetic relationships among tanagers. Finally, changes in

behavior and body size, which correspond to the retention of juvenile morphology, can be used

as further support for the importance of heterochrony. If predictions from a model of

heterochrony by paedomorphosis are consistent with the observed patterns of foraging


behavior, social structure, body size and other morphological characters, and genetics,


this would be strong support for heterochrony


as the most parsimonious model.


Distribution.


The interpretation of the speciation of the two forms of Phaenicophilus


is dependent on


the distributions in Hispaniola. Early ornithologists described the eastern limit of P.


poliocephalus as extending no farther than the western edge of the Trouin Valley


and Swales


(Wetmore


,1931; Bond, 1980; Fig. 1-1). However, sightings of P. poliocephalus in the


Dominican Republic (Dod, 1981) suggest that earlier reports were either incorrect or that

changes in the distributions have occurred. Bond (pers. comm.) originally determined the

distribution of P. poliocephalus by spending a few hours in the xeric northern part of the








3

in this habitat (pers. obs.). Wetmore and Swales (1931) based the description of the

distribution ofPhaenicophilus at least partially on Bond's observations made in 1928. Fifty

years later, I observed P. poliocephalus in mesic habitat around the village of Trouin but not

in the xeric habitat of the Trouin Valley.

There are no published ornithological records describing the area south of the Massif de


La Selle or Cap Rouge.


The major collecting efforts occurred before the end of American


occupation of Haiti in 1930; since then there have been few ornithological studies in the

country. In the last half-century, significant environmental changes have occurred


throughout Haiti including extensive clearing of lands.


The resulting habitat disturbance


probably affected the distribution of the palm-tanagers. Habitat disruption and subsequent

contact between the species could have resulted in hybridization and called into question


their species status (Bond, 1986).


Finally, the sightings of purported P. poliocephalus within


the range of P. palmarum in the Dominican Republic suggested that P. poliocephalus might

simply be a geographic variant of P. palmarum. Consequently, the relationship between the

two forms required further investigation because of the new information on the distribution of

the black- and gray-crowned phenotypes.

Phylogeny of Tanagers.

The relationship between two forms can often be better understood when viewed from a

broader phylogenetic perspective. The affinities of Phaenicophilus to other tanagers are not

clear. Sibley (pers. comm.), using DNA-DNA hybridization techniques, regarded

Phaenicophilus as a sister group to the genus Piranga, which includes the North American

scarlet tanager, P. olivacea, summer tanager, P. rubra, and the western tanager, P.

ludoviciana. Sibley did not include species that might be considered closely allied to the


palm-tanagers based on external morphology.


The common bush-tanager, Chlorospingus








4

Microligea palustris and Xenoligea montan are so similar in appearance to Phaenicophilus


that their local name,


"Petite Kat-je" translates "Little Four-eyes," the diminutive form of the


Creole name given to the palm-tanagers. Hispaniolan warblers are about the


same size


bush-tanagers, and both groups have darkish gray crowns. In addition, the warblers have


white eye-spots similar to those of the palm-tanagers.


If a clear sister relationship could be


shown between Chlorospingus-Microlige and Phaenicophilu, then the polarity of the

Phaenicophilus derivation might be more easily determined using morphological features.

The establishment of this relationship would allow a test of the hypothesis that the

Chlorospingus-Microligea-Phaenicophilu group forms a paedomorphic assemblage.



The Significance of Heterochrony and Paedomorphosis


Heterochrony depends on the evolution of regulatory genes. Small changes in

regulation can alter the growth rate of the organism, simultaneously changing suites of


characters not directly selected upon


Larson


1980).


Neo-Darwinian theory assumes that


macroevolution occurs by the accumulation of large numbers of changes across the genome


over long time periods. Heterochrony


is an alternative hypothesis to gradualism; rapid


evolutionary change can result with fewer genetic changes


Larson, 1980). There are several


processes


that can result in heterochronic change in lineages (Gould, 1977


They include


terminal additions, deletions, or substitutions, and nonterminal additions, deletions or


substitutions of morphological characters in the adult


vs. juvenile life history stage.


Terminal additions result in the evolution of novel features and are equivalent to


peramorphosis.


Terminal deletions are equivalent to paedomorphosis and result in the


retention of juvenile features of the ancestor in the adult of the descendant species (Alberch et


al., 1979; Kluge and Strauss,


1985). Juvenile features can be retained either by delay of








5

Before predictions from a model of heterochrony can be made, the polarity of

evolutionary relations (i.e., which species is derived from which) must be known. The problem

is to detect whether a terminal deletion or terminal addition has occurred; it may be resolved

by the use of outgroup analysis (Kluge and Strauss, 1985). Outgroup analysis involves the

choice of a taxon (i.e., sister group) that is closely related to the taxa being compared to

determine whether a character is common or unique to one or more of the taxa examined. If a

character is shared by one but not the other taxon with the sister group, the first taxon is most

likely antecedent to the second.

Heterochrony provides an alternative mechanism for macroevolution to occur without

major genetic changes. Paedomorphosis may result in rapid evolutionary diversification,

parallelisms, and convergences simply by changing ontogenetic sequences for one or more


characters.


The degree of concordance between genetic and morphological characters in


species assemblages is expected to be low because convergence may obscure the relationships.

Paedomorphic changes may occur in only one or a few characters producing mosaic evolution.

Congruent changes over several character states of the organism may be observed in closely

related species that have not diverged significantly from each other. Hispaniolan palm-

tanagers are closely related and may be ideal for the study of the importance of heterochrony

to their evolution.

Paedomorphosis was not expected to be a significant process in the evolution of birds

(Gould, 1977). Only recently was neoteny equated with delayed maturation in birds (Lawton

and Lawton, 1986; Foster, 1987), although delayed maturation had already been documented

for numerous bird species (Selander, 1965; Rohwer et al., 1980; Flood, 1984; Hamerstrom,


1986)


Common to delayed maturation hypotheses are several assumptions that provide the


critical link between delayed maturation and Gould's (1977) ecological constraints model to










intense intraspecific competition;


(4) some proportion of juveniles are able to breed, under


certain conditions, with the consequence that sexual maturation is decoupled from


morphological maturation; (5


the costs of sexual and somatic maturation are greater than


the increased fitness accrued to inexperienced or subordinate individuals that may

successfully breed; and (6) juveniles may benefit from cryptic or deceptive morphology


reducing predation and/or intraspecific competition (Selander, 1965; Rohwer, 1978;


1984; Lawton and Lawton, 1986; Foster,


Flood


1987). Consequently, delayed maturation, or


neoteny, results from a balance between selection to breed early and


longer to ensure successful breeding.


selection to survive


Under conditions of limiting resources, the relative


increase in fitness accrued by individuals breeding at an earlier age would be offset by the

decrease in fitness due to the inexperience of juveniles competing for scarce resources. For


example, younger-aged flocks


of Brown Jays, Cyanocorax morio, are less successful breeders


than their older-aged counterparts (Lawton and Lawton, 1985). In years of resource

abundance, the fitness differential would favor juvenile breeding.


In contrast, when species colonize new habitat, resource leve


may be relatively more


abundant. Under these circumstances, early sexual maturation is no longer constrained.

Brown Jay flocks moving into recently cleared habitat have a lower average age than flocks


observed in the older habitat (Lawton and Lawton, 1985).


With attainment of sexual


maturity, somatic development slows down or stops (Gould, 1977), resulting in individuals

with smaller body size, juvenile morphology, juvenile behaviors associated with the


morphology, and a concomitant increase in group behaviors (Geist, 1971


Gould, 1977;


Lawton and Lawton, 1986). If the colonizing population becomes isolated from the main

stock, divergence can occur. It is unnecessary to invoke selection against adult phenotypes, if

rates of somatic development are tied to rates of sexual maturation, though the endpoints of








7

selection, can change in concordance with a selected character. For example, derived species

of mountain sheep, that have retained the juvenile morphologies of their antecedents, are


relatively more social than their antecedents (Geist, 1971).


This sociality is not a result of


selection for juvenile behaviors, but a consequence of retaining juvenile morphologies,

resulting in reduced intraspecific aggression (Lawton and Lawton, 1986).

Additional predictions can be made once the particular type of paedomorphosis can be

inferred, i.e., neoteny or progenesis. Both neoteny and progenesis achieve the same endpoint,


terminal deletion, but by different routes.


The mechanism for paedomorphic changes may be


controlled by changes in regulatory gene activity governing maturation rates. Sexual

maturation is accelerated in progenetic species, purportedly in response to abundant

resources. Progenetic species should breed earlier and be smaller than their antecedent

species; in addition, they should exist under density-independent conditions. Gould (1977)

invoked limiting resources as the necessary ecological constraint that leads to the evolution of

neoteny. Neoteny may be a function of temporal variability in resources; it should be favored

when resources are generally limiting but periods of limitations are sometimes relaxed.

Under crowded conditions, individuals delay sexual maturation with a concomitant delay in


somatic development.


The delay in maturation results in evolutionary change if sexual and


somatic maturation are decoupled (Lawton and Lawton, 1986).


The conditions for this


decoupling are associated with a high variance around first age to reproduction, resulting in

juveniles that breed in response to relaxation of resource limitations.

The ability to respond to fluctuating resources has genetic consequences as well. Gould

(1977) did not link changes in life history traits with genetic variation. Predictions for the

study of palm-tanagers concerning changes in genetic variability of paedomorphic systems

could not be made a priori, because patterns of genetic variability are not apparent for








8

heterozygosity was greater in habitats consisting of later successional stages and in higher

densities. Redfield (1973) concludes that colonizers (i.e., individuals in the earlier


successional habitat) are more homozygous than their counterparts in older habitat.


assumes that dispersal occurs into earlier successional habitat, and not throughout all

successional stages. It is unclear from Redfield's study whether heterozygous yearlings are

more successful than their homozygous counterparts in dispersing into older aged habitat,

due to behavioral dominance (Baker and Fox, 1978), or whether they result from selection for

heterozygotes in the age-class under stress. Heterozygotes may be favored under conditions of


limiting resources (Samollow and Soule, 1983: Smith, Teska and Smith,


1984). either due to


increased metabolic efficiency, superior competitive behavior, or a combination of both.


Genetic variability may be favored as a response to unpredictable resources.


Genetic


variability may be higher in neotenic species, if neotenv is evolved in response to


limiting


resources.


To understand how heterochrony influences the evolution of groups,


general approaches. First, species assemblages, characterized by


due to developmental changes, can be studied.


one can take two


similarities in morphology


This approach was successfully applied to


plethodontid salamanders (Wake,


1966; Larson, 1980; Alberch, 1981).


Unfortunately,


concordance between morphological and genetic evolution is likely to be obscured by

convergences, parallelisms, and mosaic evolution in these assemblages because of significant


amounts of differentiation.


The particular ecological constraints that may eventually lead to


evolutionary change are lost in broad comparisons such as these.

The second approach focuses on population level differences of a few closely related

species. Recent ancestry will avoid obscuring the relationships of several character sets

between these species. Ideally, such species should exist in similar habitats so that selective








9

geographic areas imposed by island boundaries may provide less opportunity for confounding

geographic variation. Increased isolation from the the mainland results in fewer competitive


interactions that might confound patterns based on heterochrony.


Thus, Hispaniolan palm-


tanagers would seem to be suitable for the study of heterochrony because they are closely

related island endemics, and exhibit morphological patterns of resemblance that suggest

heterochrony is involved.
































Figure 1-1. Map of the current distribution of Hispaniolan palm-tanagers. Diagonal lines
represent the known distribution of Gray-crowned Palm-Tanagers; Black-
crowned Palm-Tanagers occur throughout the rest of Hispaniola. Area southwest
of Port-au-Prince, bounded by Marbial and Decoze and Furcy is undescribed.
Stippled areas define the Trouin Valley and the Cul-de-Sac Plain. Enlargement
features the area of contact and hybridization bounded by Fond Jean Noel (Fd Jn
Noel) in the east, Marigot on the Caribbean Sea to the south and includes a region
southwest of Seguin. The western extent of the hybrid zone is still undefined but
may extend into the valley between the Massif de la Selle and Cap Rouge.




























HISPANIOLA
















CHAPTER 2
HISPANIOLAN PALM-TANAGERS:
BEHAVIORAL AND MORPHOLOGICAL CORRELATES OF HETEROCHRONY



Introduction


Few attempts have been made to integrate alterations in developmental sequence with

evolutionary processes in birds. An understanding of heterochrony (i.e., shifts in

developmental sequences) should prove useful to such an integration. Heterochrony by

paedomorphosis, resulting in the retention of juvenile features in reproductively mature


individuals, can be achieved in two ways: progenesis and neoteny. Progenesis


characterized by early sexual maturation which often results in smaller body size, while


neoteny, or delayed somatic maturation results in larger body


size(


Gould, 1977; Lawton and


Lawton, 1986), and increased variation to age of first reproduction. Selection for change in

one character often results in simultaneous change in nonselected but associated characters


(Larson, 1980). Juvenile behaviors


associated with juvenile morphologies can be retained in


breeding individuals due to paedomorphosis (Geist, 1971; Coppinger and Coppinger, 1982;

Lawton and Lawton, 1986; Coppinger et al., in press). Suites of characters, such as behavior


and morphology, can evolve without selection acting on every character.


Thus, speciation


between closely related forms may be facilitated by paedomorphosis.

Phaenicophilus palmarum occurs throughout the island of Hispaniola except on Isle La

Gonave and on the southern peninsula of Haiti, where P. poliocephalus occurs (Fig. 2-1).

Prior to this study. the distributions of the two nalm-tananers were thought to meet but not







13


poliocephalus had been reported in the Dominican Republic (Dod, 1981) and eastern Haiti.


These sightings were later found to be mistaken identifications of juvenile P. palmarum


pers. comm.). In July, 1983, I observed P. poliocephalus approximately 30 km east of the

Trouin Valley. In May, 1985, I documented, with specimen collections, a narrow hybrid zone

in the area north of Marigot, extending to about 8 km south of Seguin and bordered on the


east by Fond Jean Noel (Fig. 2-1).


The western extent of this hybrid zone is not known,


although hybrids were not found along the Riviere Gosseline approximately 10-15 km east of


Marbial.


There were no P. poliocephalus or hybrids found in the Massif de La Selle. Both


species occur in all major habitat types of Haiti, including cloud forest, mixed pine, mesic

broadleaf woodland, xeric thornscrub, desert, mangrove swamp, and disturbed rural and

urban areas, while hybrids are found in mesic woodland, thornscrub, and disturbed rural

areas.

Phaenicophilus palmarum has a yellow-green back, a gray nape, three white eyespots

on a black face mask, a black crown and a diffused white chin and throat. It is also

characterized by an age-dimorphism in that juveniles have gray crowns that range from the

same shade of gray on the nape to darker gray. Phaenicophilus poliocephalus resembles

juvenile P. palmarum by having gray crowns, but it is distinguished from this species by a

distinct white chin against a gray throat. Juvenile P. palmarum have frequently been

confused with P. poliocephalus in the field. The resemblance of adult P. poliocephalus to

juvenile P. palmarum, the existence of an undescribed hybrid zone, and their estimated recent

divergence (Chapter 3) suggested that these taxa would be suitable for the study of


heterochronic processes on the evolution and diversification of avian species.


The effects of


heterochrony on the relationship of the two species is not likely to be confounded by

significant differentiation due to long periods of isolation.










(3) describe the relationship between behavior and morphology in these species; and

(4) determine whether P. poliocephalus is paedomorphic to P. palmarum.




Methods


Morphometric anal


Seventeen morphological variables were measured on fresh


specimens of Phaenicophilus palmarum (NI1


), P. poliocephalus (N2 = 20), and P.


palmarum


X P. poliocephalus hybrids


(N3 =


14), before skin or skeletal preparations were


made (Baldwin et al., 1931; Table 2-1: App. 1 and 2). Hybrid specimens and representatives of


both parental species are deposited in the American Museum of Natural History in


York.


The remaining individuals are deposited in the Florida State Museum in Gainesville.


Additional museum specimens (NI = 104 and N2 = 46) were included for the morphological

analysis to increase the sample size to conduct multivariate morphometric analysis. Hybrids

were identified in the field by the intermediate extent of black on the crown and white on the


chin. Analyses of


age, sex,


and species differences were subjected to Mann-Whitney U-tests


for the morphological variables.


The value for the crown character in juvenile P. palmarum


zero, because no black occurs on the crown before the final molt.


The average value for a


species class was substituted for missing values and the de


grees


of freedom were adjusted


accordingly. Homogeneity of variance for morphological traits were tested using the Fmax

test (Sokal and Rohlf, 1969).


Juvenile P. palmarum are distinguishable in the field from adul


but that


not true


for P. poliocephalus. Juveniles were initially identified as fledglings when adults were seen


feeding them.


Once I established that juveniles had gray crowns but did not have a distinct


chin pattern, I could differentiate juvenile P. palmarum and P. poliocephalus without
lr r r *1 1 i i *, n







15


museum collections favors adults because juveniles are often in duller plumage and are not


collected as frequently (R. Zink, pers. comm.).


The proportion of juveniles, as detected by a


yellow wash in head plumage, in the collections I examined varied dramatically between the

two Phaenicophilus species. Juvenile P. palmarum represented 55% of collected P.

palmarum; juvenile P. poliocephalus represented only 9% of the collected specimens for this

species. Because juveniles are easily identified in P. palmarum but not in P. poliocephalus,

any collecting bias that favors adults should be greater in the former species, if juveniles take

equal time to mature in both species.

Foraging behavior. A modified version of the Cody-stopwatch method (Cody, 1968) was

used to collect foraging data for five species in Haiti, including the following: the two palm-

tanagers; the Stripe-headed tanager, Spindalis zena; the Green-tailed Ground warbler,

Microligea palustris, which is similar in plumage to Phaenicophilus; and the Black-and-white

warbler, Mniotilta varia, which was used as an outgroup in the systematic analysis of the


warbler-tanager groups (Chapter 4).


In addition to the foraging variables detailed by Cody


(1968), I included activity level (perch changes per unit time), food-catching attempts (FCA)

per perch change, FCA per minute, food-catching successes (FCS) per minute, FCS/FCA

(Table 2-2), as well as average distance per flight (Kepler, 1977; Rabenold, 1980), and

substrate zone used (see below). Multiple observations on some individuals were no doubt

made because the birds were not individually marked. Manhattan distances between the

Operational Taxonomic Units (OTU's) were calculated from the data for eight of the foraging


variables (Table 2-2) and these distances were used to cluster the OTU's


with the Unweighted


Pair-group Method of Averaging (Sokal and Rohlf, 1969; Cherry et al., 1982; Norusis, 1985).

Substrates were identified by their common Creole names (Pierre-Noel, 1971) or

classified more broadly (e.g., shrubs, broadleaf; App. 3) for subsequent diversity calculations.







16


of time spent in each zone for each species and for age-classes with P. palmarum were

computed. Significant differences in diversity of substrates used by the species and age-


classes were computed using


the Shannon Information Index (Peet, 1974; Pielou, 1977), and


were evaluated with a t-test (Zar, 1984).

Frequency data for flight distances were grouped into 3 m intervals, combining the


observations that occurred in the intervals


greater than 27 m.


The frequency of foragin


observations at different heights was calculated for each 1.8 m interval, up to


27 m, and


combined for the intervals beyond that. Standardization for differences in maximum

vegetation height between habitats were made by dividing the foraging height by the height

of the maximum substrate height.


Data analysis.


Statistical analyses were conducted using parametric and


nonparametric tests from SAS or SPSS (SA


1985; Norusis, 1985). Data were standardized


to a mean of 0 and a standard deviation of 1 to perform Discriminant Function Analysis.


Data


were not standardized prior to Principal Component Analysis (PCA) because standardization

would weight variables equally, thus eliminating the utility of the method. Covariance


instead of correlation matrices were used in the PCA.


Principal components were accepted if


they accounted for 5% or more of the variation. Null hypotheses were rejected at a probability


level of P


- 0.05; highly significant differences occurred at P


< 0.01.


Type 1 error


reduced when multiple comparisons of data were made using the same hypothesis with the


formula:


1 (0.95)"", where n is the number of comparisons (Harris, 1975).


This procedure


adjusted the experimentwide error rate to P1


< 0.05. Statistical differences between


frequency distributions were evaluated using the Kolmogorov-Smirnov test (Sokal and Rohlf,

1969). Comparisons of the tendency for the two species to form groups in the nonbreeding

season were evaluated by a modification of the Kolmogorov-Smirnov test, using a chi-square


was










Results


Morphologyv.


The first two Principal Components (PC) extracted from the data for 17


morphological variables explained 92.4% of the total variance (Table 2-1). PC 1 had high

positive loadings for the width of white on the chin and PC 2 had high loadings for the extent

of black on the crown. Neither PC 1 nor PC 2 accounted for significant variation of body size

traits. Differences between the two species were highly significant. Discriminant Function

Analysis (DFA) reclassified two of the P. palmarum individuals collected on the edge of the

hybrid-contact zone near Fond Jean Noel as hybrids based on phenotype (P= 0.984 and

P= 0.998). One hybrid clustered with the parental species on the crown character and may

either represent a backcross or an outlier for one or more of the morphological characters,


since DFA weights characters equally (Fig.


as P. poliocephalus or vice versa.


No juvenile P. palmarum was misclassified


Two adult P. palmarum were misclassified as juveniles, but


the probability of correct reclassifications


was marginal (P


< 0.57), so they were still


considered adults in subsequent analyses.

Highly significant differences between age-classes within P. palmarum were detected


for the extent of black on the crown, wingchord, and proximal depth of bill (Pi


< 0.01:


Table 2-1). There were only 6 juvenile P. poliocephalus in the samples, thus making it


statistically impossible to test for age differences in morphology.


The three variables listed


above along with width of white on the chin, length of malar stripe, bill length, bill mid-depth

and proximal width, length of dorsal anterior, posterior and ventral eyespots, and tail length

were significantly different between P. palmarum and P. poliocephalus. Significant

differences between the sexes were detected within P. palmarum for proximal depth, proximal


width, and mid-width of the bill, length of the dorsal posterior eyespot, and tail length.


Sexes


within P. poliocephalus differed only in tail length.







18


distinct from adults for the extent of black on the crown and from P. poliocephalus for the

width of white on the chin. Adult P. palmarum and P. poliocephalus were distinct from one

another for both characters. Hybrids graded from one parental species to the other, though

they resembled P. poliocephalus more closely. Juveniles were morphologically distinct from

the hybrids and were not intergrades between the two parental species.

Homogeneity of morphological character variance between hybrids and each parental

species was evaluated under the hypothesis that hybrids were no more variable than the

parentals and that they represented first generation crosses and not backcrosses (Lerner,


1954).


Variances for all characters between P. palmarum and the hybrids were not


significantly different. Hybrid variances were highly significantly different from those of P.


poliocephalus for extent of black on crown (F


.87) and width of white on chin


=18.8).


The question of convergence or divergence was evaluated for bill mid-width and 12

morphological characters that differed significantly between the two taxa (Table 2-1).

Sympatric populations were designated as those that occurred in and around the contact zone

extending from Foret des Pins to LaVallee (Fig. 2-1). Differences between sympatric and

allopatric populations within P. poliocephalus and within age-classes of P. palmarum were

evaluated for each of the species-distinct characters. Significant regional differences in P.

poliocephalus were found for bill taper (mid-width to proximal width of bill) and tail length


(Mann-Whitney U-test).


The length of the ventral eyespot was significantly different


between sympatric and allopatric populations of adult P. palmarum. No differences were

found between sympatric and allopatric populations of juvenile P. palmarum.

Once regional differences were established between sympatric and allopatric

populations within a species, then the issue of convergence vs. divergence could be resolved.







19


(Grant, 1972; Cody, 1973). Differences in character means between sympatric populations of

P. palmarum and P. poliocephalus were evaluated against those in allopatry. Differences

between adults of the two species were more than two times greater for tail length and 18

times greater for bill taper in sympatry than in allopatry. For the length of the ventral

eyespot, differences were 1.5 times greater in allopatry than in sympatry.

Foraging behavior. Juvenile P. palmarum clustered more closely with adult P.

poliocephalus using data for foraging behavior than with adults of their own species (Fig. 2-2).

No significant differences between juvenile P. palmarum and P. poliocephalus were found for


the eight variables analyzed (Table


Adult and juvenile P. palmarum differed


significantly for five variables, whereas adults of both species differed significantly for four

variables (Table 2-2).


Homogeneity of variances for each of the eight foraging variables

poliocephalus and P. palmarum was evaluated under the hypothesis tha


between P.


t P. poliocephalus


observations should be more variable than juvenile P. palmarum observations if juvenile P.

poliocephalus were included in the collection of the data but not correctly identified. Only one

variable, average speed of foraging, showed significant heteroscedasticity between the two

groups. Significant differences in the use of substrate type between adult and juvenile P.

palmarum (Dmax = 0.310) and adults and P. poliocephalus (Dmax= 0.267) were found

(Fig. 2-3). Phaenicophilus poliocephalus did not differ from juveniles in substrate use

(Dmax = 0.064). Adult P. palmarum (BPA) foraged primarily in pine (58%) whereas juveniles

(BPI) and P. poliocephalus (GPT) chose broadleaf trees and shrubs (67-69%). Because foliage-


gleaning was the predominant foraging mode for all groups (BPA


= 67%,


BPI=68%, and


73%), foraging differences occurred as a consequence of substrate use and not the mode


of foraging.


The diversities of the substrates used by P. poliocephalus and juvenile P.







20


Adult P. palmarum were highly significantly different from juveniles (Dmax = 0.158)

and from P. poliocephalus (Dmax= 0.190) and juveniles were highly significantly different

from P. poliocephalus (Dmax = 0.201) in foraging heights. Juvenile P. palmarum occurred at

equal or higher heights when foraging than did adults in all habitats except the mixed pine.

The lower foraging heights of adult P. palmarum may result from differences in the

vegetation structure of their habitat and/or resource distribution.

Highly significant differences in the frequency distribution of horizontal substrate use

were found between adult and juvenile P. palmarum (Dmax= 0.053), adults and P.


poliocephalus


(Dmax = 0.092), and juveniles and P. poliocephalus (Dmax = 0.046) when data


were pooled across habitats


App. 4). Because sample sizes differed substantially for upper


and mid-elevation sites, horizontal substrate use was examined only for lowland habitat;

there were still highly significant differences in horizontal substrate use. In open savannah

habitat, at mid-elevation,where trees stand alone or in small clusters, foraging was more


frequent on the middle and inner zones for both species (BPA


=88%,


GPT = 62%;


no data for


BPI). In closed canopy forests, foraging was concentrated on the outer zone of the substrate


(BPA = 49%, BPI = 42%, GPT= 50%).


Thus, horizontal substrate use varied with habitat


structure and/or elevational gradients.

The frequency distribution of flight distances for adult P. palmarum differed

significantly from the distributions for juveniles (Dmax = 0.209) and for P. poliocephalus

(Dmax = 0.193). Juveniles did not differ significantly from P. poliocephalus (Dmax = 0.147) in


the distribution of flight distances.


This frequency pattern is true for all habitats sampled,


supporting the hypothesis that juvenile P. palmarum and P. poliocephalus perceive the


habitats differently from adult P. palmarum.


These differences may result from differential


dispersion of resources within the habitats. For example, in mixed pine, shrubs are not as







21


nonbreeding season (August-April) was evaluated against the same tendency during the

breeding season. A significantly greater tendency to form groups in the breeding rather than


the nonbreeding season occurred for P. palmarum (x2(1


between seasons for P. poliocephalus.


= 3.9). Differences were not observed


Therefore, the distributions of the number of


individuals observed together on transects in the nonbreeding season were compared between

P. palmarum and P. poliocephalus. Phaenicophilus poliocephalus had a significantly greater

tendency to be observed in groups of more than two individuals than did P. palmarum in the


nonbreeding season (x2


=7.3).


Discussion


Several hypotheses can explain the close similarity between two taxa.


They include


lack of divergence due to insufficient time, character convergence, selection due to similar

environments, and/or heterochrony. Because the two taxa were originally chosen for study on

the basis of their close relationship, many of the similarities are due to lack of divergence.

Ninety-two per cent of the variance explained by Principal Component Analysis was

attributed to the chin and crown characters which are the primary distinguishing features of


these two taxa. However, the crown character is polymorphic within P. palmarum.


variation within this character and the frequency of its occurrence might be explained by

geographic variation within one species.

One of the predictions from the one species model of geographic variation is that

different morphotypes should correspond to particular habitat types. Observations from the

same habitats (e.g., cloud forest, mangrove swamp) within different parts of the range of each

taxa failed to document such a correspondence. For example, in mangrove swamp and cloud

forest in the southern peninsula, individuals are all gray-crowned (i.e., P. poliocephalus)










morphology, occur in areas where the two taxa are sympatric. Thus, hybrids do not occur in a

unique habitat distinct from that of the two species of Phaenicophilus. The most


parsimonious hypothesis is that this zone ofintergradation resulted from a secondary contact


between the forms which were geographically isolated in the past.


The patterns of similarity


in crown features between juvenile P. palmarum and adult P. poliocephalus, the lack of

concordance of crown color and habitat type, and the distribution of hybrids are not totally

explained by the lack of divergence model or the geographic variation model.

Additional consideration of the hybrids indicates that selection is probably involved in

reinforcing the distinctness of the two species. If selection against hybrids occurs, differences

in morphology between the parental species in sympatry should be greater than in allopatry.

Differences between the adults of the species were greater for bill taper and tail length in

sympatry than in allopatry, although comparable differences were not found for crown color.

The similarity of adult P. poliocephalus to juvenile P. palmarum in the crown character is not

easily explained by a model of character divergence between the adults of the two species,


unless the species overlapped more extensively in the past.


There is no evidence for such an


overlap and no reason to believe it occurred.

Variances of the crown and chin characters were significantly higher for the hybrids

than for P. poliocephalus, but hybrids were no more variable than P. palmarum for any


morphological character.


The general lack of significant differences in the degree of


morphological variation between hybrids and either of the parental species suggests that


hybrids are the result of parental crosses and are not backcrosses (Lerner, 1954).


apparent absence of introgression and the narrowness of the hybrid zone suggests that

selection against the hybrids is occurring. Similarly, convergence due to similar selective

regimes does not explain the age-dimorphism in one species and its reduction in the second for







23


1966; Geist, 1971) and delayed maturation within a species (Lawton and Lawton, 1986;

Foster,1987). Gould (1977) did not relate delayed maturation in birds to neoteny because of

his emphasis on phylogenetic endpoints rather than on population processes that produce


them.


There is an absence of evidence that bird species are derived through heterochronic


changes, although variation in degrees of delayed maturation occur among closely related

species of manakins (Chiroxiphia:Foster, 1987), suggesting derivation is through

heterochronic processes. Age-dimorphisms in characters usually accompany delayed

maturation. Phaenicophilus palmarum shows a striking age-dimorphism and is likely an


example of a species characterized by delayed maturation.


The systematic consequences of


heterochrony could be a species retaining the juvenile characters of an antecedent species, as

may be the case for Phaenicophilus poliocephalus. For this to occur, there must be significant

character differences between the adults and juveniles of the antecedent species. Such a

dimorphism is often associated with differences in resource exploitation by the adult and

juvenile age-classes when resources are periodically limiting (e.g., Northern Harrier;

Hamerstrom, 1986).

Juvenile P. palmarum were significantly different from adults in the extent of black on

the crown, wingchord, and proximal depth of the bill. Gray crowns in juveniles darken with

age. Although gray-crowned P. palmarum were not observed breeding, dark gray-crowned

individuals have been collected at the beginning of the breeding season in late March or early

April (Table 2-3); some individuals probably delay maturation through their first breeding


season.


Wetmore collected two of these gray-crowned individuals which he designated as


adult females, presumably basing his judgment on ovarian maturation. Pairs consisting of

adults and juveniles were also observed in the post-breeding season, after the time of fledging,

thus increasing the likelihood of pair-bond formation. In addition, the tanager, genus







24


likely a neotenic species, and one reflection of this is the observed age-dimorphism. Neoteny


may be a common condition in closely related tanagers.


The principal difficulty in


establishing whether P. poliocephalus is paedomorphic to P. palmarum rests with

determining the polarity of their evolution, i.e., which species is derived from which. Derived


characters are presumed to be those unique to derived taxa (Wiley,


1981). Phaenicophilus


poliocephalus has a distinct white chin and gray throat not shared with other tanagers.

Furthermore, neoteny is not a derived condition in palm-tanagers. Because P. poliocephalus

lacks a distinct age-dimorphism, in addition to the above facts, P. palmarum are presumed to

be antecedent to them, making P. poliocephalus the paedomorph.


Several predictions follow from a model of paedomorphosis. Species


derived from other


species in a manner that involves paedomorphosis should be more social relative to their

antecedents (Lawton and Lawton, 1985). Data on mountain sheep support this prediction

(Geist, 1971). Derived species, living in environments where resources are plentiful reach


reproductive maturity earlier.


These derived species retain the juvenile morphologies and


associated behaviors of their antecedents. Moreover, they are relatively more socia


Geist,


1971).


There is evidence in red-winged blackbirds, Agelaius phoeniceus, that subadult


plumage reduces intraspecific aggression (Rohwer,1978).


Therefore, delayed plumage


maturation, or the retention of juvenile morphology, may reduce intraspecific aggression,

resulting in increased sociality. Phaenicophilus poliocephalus was observed in groups of 4-6

individuals during the nonbreeding season, whereas P. palmarum was observed singly or in


pairs.


This tendency to form groups in P. poliocephalus is consistent with the hypothesis that


gray crowns reduce intraspecific aggression. Additional predictions involving body size

follow from this general paedomorphic model.







25


sexual maturation) results in smaller body size, due to slower somatic growth after sexual


maturation (Gould, 1977).


Development arrested at an early stage often results in smaller


individuals (Larson, 1980; Alberch,1981). Thri

size between the two species were evaluated as


ee hypotheses concerning the relation of body

follows: (1) Adult P. palmarum are larger


than post-fledging juveniles; (2) Adult P. palmarum are larger than adult P. poliocephalus;


(3) Juvenile P. palmarum are no larger than adult P. poliocephalus.

needs to be tested because post-fledging juveniles can often be as lai


birds


The first hypothesis


rge or larger than adults in


The second hypothesis tests the prediction that arrested development occurs early


enough in the ontogeny of the antecedent species that the adult of the derived species will be


smaller


The third hypothesis tests the prediction that adults of the derived species


juveniles of the antecedent species should be comparable in size.

Juvenile P. palmarum are significantly smaller (one-tailed test) than adults in


wingchord, braincase length, and proximal depth of the bill (P1


< 0.025).


Phaenicophilus


poliocephalus is significantly smaller than juvenile P. palmarum in bill length, medial depth

of bill, and length of second toe (one-tailed), and is smaller than adults in the same three


characters


as well


as wingchord, braincase length, and proximal bill width and depth (one-


tailed)


Data from juveniles of differing ages were pooled because the criterion for


designating juvenile P. palmarum was the presence of a gray crown; elevated variances for

various characters might be expected to confound the tests for size differences. However,

variances were homogeneous between the juvenile and adult classes. Juveniles were


significantly smaller than adults in three of the eleven size-related characters.


That P


poliocephalus was significantly smaller than adult P. palmarum in seven of the eleven

characters as predicted, supports the hypothesis of evolution by progenesis for this species.

The average first age of reproduction and/or its variance should also be lower in P.







26


poliocephalus is not a likely result of selection, but the consequence of its association with

juvenile morphology (Coppinger et al., in press). Juvenile P. palmarum differ significantly in


their foraging behavior from and are less efficient than adults.


This is a common observation


in birds and has been attributed to a lack of experience of juveniles (Orians, 1969; Recher and

Recher,1969; Sutherland etal., 1983; Stevens,1986). Juvenile P. palmarum are significantly

slower, spend more time stopped, and try more food-catching attempts per perch change than

adults, but the success-to-attempts ratio is lower (Table 2-2). Significant differences between

juveniles and adults were not observed in the type of foraging behavior; they are both

primarily insectivorous. Significant differences were observed in choice of substrates. In

mixed pine habitat, juveniles used broadleaf rather than pine substrates. In mesic woodland,

juveniles had a significantly lower substrate diversity than did adults. Foraging height and

flight distances were also significantly different between juveniles and adults; differences in

foraging behavior are probably not a simple function of learning and effectively result in


partitioning the foraging space between birds of different age-classes.


There were also


differences in horizontal substrate use. Juveniles and adults use the same foragin


behavior


but in different parts of the habitat.

Arrested morphology in the paedomorph should correspond to retention of juvenile

behaviors (Geist, 1971; Lawton and Lawton, 1986; Coppinger et al., in press). Adult

Phaenicophilus poliocephalus resemble juvenile P. palmarum in foraging behavior.

Homogeneity of variances were equivalent for both groups except for average speed of

foraging thus suggesting that errors in aging P. poliocephalus in the field did not contribute

significantly to the results. A lack of experience in foraging is not a likely explanation for

behavior in adult P. poliocephalus. Phaenicophilus poliocephalus uses broadleaf substrates in

mixed pine habitat as do juvenile P. palmarum and has a significantly lower substrate










selective regimes because their habitats vary.


The behavioral similarities are a result of


retaining juvenile behaviors which are associated with arrested morphological development

in P. poliocephalus.

In conclusion, the polymorphism in crown color is not due to geographic variation

within a species, nor is character displacement a viable hypothesis. Low morphological

variances in hybrids and a narrow hybrid zone point to a restriction in hybridization most


likely due to selection against the intermediates.


These data, in conjunction with a


substantial lack of gene flow (Chapter 3) support the species status of the two forms of


Phaenicophilus.


The close resemblance between the species is likely due to their recent


divergence, although the pattern of resemblance requires further explanation.


resemblance of adult P. poliocephalus to juvenile P. palmarum is best explained by a model


involving heterochrony by paedomorphosis.


This study demonstrated that selection among


individuals from different age-classes has significant phylogenetic consequences under a

model of heterochrony by paedomorphosis.

Phaenicophilus poliocephalus is most likely derived from and progenetic to P.


The smaller size, retention of juvenile foraging characteristics, and the


propensity to form groups in P. poliocephalus supports this conclusion.


juvenile patterns cannot be simply explained by


The retention of these


selection for every character, especially


because juvenile P. palmarum are less efficient foragers than adult P. palmarum.


retention of juvenile foraging behavior and smaller size of P. poliocephalus is probably a

consequence of arrested development in morphology. Selection for early maturation in P.

poliocephalus was probably concomitant with the retention of gray crowns, which serves to

reduce intraspecific aggression. Reduction of intraspecific aggression resulted in changes in

the degree of sociality between species of Phaenicophilus, which can have significant


palmarum.












Table 2-1.


Principal Components for 17 morphological variables measured in Hispaniolan
palm-tanagers.


PRINCIPAL COMPONENTS


VARIABLE


SPECIES


MEAN


(85.1


(7.3%)


Extent ofb


26.48


0.1646


0.9612


black on


crown


GPT

HYB


16.08

6.08

12.82


Lateral
extent of
face mask


GPT

HYB


22.66


0.0011


0.0072


22.54

22.35

23.50


Length of
nape from
exposed
culmen


38.24


-0.0130


0.0150


38.99


GPT

HYB


38.68

39.01










Table


1. (Cont'd)


PRINCIPAL COMPONENTS


VARIABLE


Width oft
white on


SPECIES


MEAN


BPA


(85.1%)


19.34


0.9785


(7.3%)


0.1668


GPT

HYB


35.18

10.33

18.74


Lateralb
extent of


10.57


-0.1083


-0.0122


23.06


malar
stripe


GPT

HYB


28.15

29.02


Wingchord"&b


85.29


0.0210


0.2100


81.00


83.49


HYB


85.73


Braincase


BPA


22.04


0.0072


0.0311


length


BPI

GPT

HYB


21.36

21.30

22.89










Table 2-1. (Cont'd)


PRINCIPAL COMPONENTS


VARIABLE


SPECIES


MEAN


(85.1%)


(7.3%)


20.33


0.0177


-0.0006


length


BPI

GPT

HYB


20.34

18.89

19.25


Proximalb,"


0.0021


0.0054


width


GPT

HYB


Mediale


-0.0001


-0.0006


width


GPT

HYB


Proximalab,


0.0044


0.0083


depth


GPT

HYB










Table 2-1. (Cont'd)


PRINCIPAL COMPONENTS


VARIABLE


SPECIES


MEAN


(85.1%)


(7.3%)


Medialb


0.0032


-0.0012


depth


GPT

HYB


20.60


Tarsus
length


0.0000


0.0188


BPI

GPT

HYB


19.70

20.44

20.88


Length ofb
Dorsal
Anterior


0.0092


-0.0041


Eyespot


GPT

HYB


Length ofP'
Dorsal
Posterior


0.0465


-0.0119


10.00


Eyespot


GPT

HYB










Table 2-1. (Cont'd)


PRINCIPAL COMPONENTS


VARIABLE


SPECIES


MEAN


(85.1%)


(7.3%)


Length ofl
Ventral


0.0208


-0.0196


Eyespot


GPT

HYB


Length of)lcd


68.00


0.0040


0.0406


BPI

GPT

HYB


67.96

66.79

71.90


Notes:


Sample size (N), mean in mm, standard error


S.E.), and loadings on the first two


Principal Components for 17 morphological variables of adult (BPA) and juvenile (BPI)
Black-crowned Palm Tanager (BPT), Gray-crowned Palm Tanager (GPT), and their
hybrids (HYB).

Percent variation accounted for by each Component given in parentheses.


Significant differences in morphological character between BPA and BPI (P1
Significant differences between BPT and GPT.
Significant differences between sexes within BPT.
Significant differences between sexes within GPT.


< 0.01).









Table


Mean, sample size, and significance of differences of eight foraging behavior
variables.


VARIABLE BPA BPI GPT


Average
Activity


(100)


(170)


-$$



I--------------------------------------I


Average
Speed


(269)


(100)


(169)


&I
$*

I-----------


Duration


of stop


(260)


(163)


- - - - -------------------------- -

I--------------------------I


Percent


time stopped


(258)


(163)


FCA pera
minute


(230)


I__________________










Table


(Cont'd)


VARIABLE BPA BPI GPT



FCSperb 2.05 1.80 1.53
minute (126) (30) (54)

Successes/ 0.59 0.46 0.39
attempts


I ------------------ ----------------- --- I

FCA pera 0.26 0.37 0.30
perch change (233) (84) (136)

I-------------*--------------


Note:


Black-crowned Palm-Tanagers are divided into adults (BPA) and juveniles (BPI).


Gray-crowned Palm-Tanagers (GPT) are not.


Significance differences between


pairwise comparisons


of classes,


evaluated


Mann-Whitney


U-tests, are


designated by a line with asterisks,
P < 0.001 (***).


where P


< 0.025


P s 0.01


FCA
FCS


= food-catching attempts.
= food-catching successes.


(**),









Table


List of gray-crowned Black-crowned Palm-Tanagers collected in the early part of


the breeding


SPECIMEN
NUMBER


season


(late March to early April) with associated comments.


DATE COLLECTED


COMMENTS


CARN 70717


6 Feb 1916


Yellow wash on crown, nape, chin and
throat, and Dorsal Anterior Eyespot


PNAS


82404


1 Feb 1928


Dark gray crown;


yellow


wash


Dorsal Anterior Eyespot


PNAS


11950


9 Feb 1932


Some gray on crown


USNM 327928


26 Mar 1931


Gray in crown.
female


Wetmore lists


as adult


USNM 252708


16 Apr 1919


Crown more dark gray than black;
difficult to distinguish crown from
nape


USNM 264808


11 Apr 1927


Back of crown dark gray;
listed this as adult female


Wetmore


USNM 250465


9 Mar 1917


Dark gray crown;


yellow


wash


Dorsal
Eyespots


Anterior


Posterior


USNM 280406


26 Mar 1929


Crown mostly dark gray


Note:


Abbreviations for museums are:


CARN


= Carnegie Museum of Natural History in


Pittsburgh; PNAS


= National Academy of Sciences in Philadelphia; USNM = U.S.


National Museum at the Smithsonian Institute in Washington, D.C.













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CHAPTER 3
HISPANIOLAN PALM-TANAGERS:
THE GENETIC CONSEQUENCES OF HETEROCHRONY



Introduction


Heterochrony, or variation in developmental patterns, provides a framework from

which to integrate populational processes into a general evolutionary model (Gould, 1977)

and to explain how these processes lead to different levels of genetic variability within and

between species. Paedomorphosis, a type of heterochrony, results in the retention of juvenile

characters in reproductively capable individuals (Gould, 1977; Lawton and Lawton, 1986) and


can be achieved by small changes in regulatory genes early in development


Larson, 1980).


Alterations in body size and shape, fecundity, age-structure, and significant changes in social

structure within populations can result by increasing growth rates and decreasing age of

sexual maturity (Geist, 1971; Gould, 1977; Lawton and Lawton, 1986). Life history

characteristics, such as fecundity and growth rates, are correlated to levels of genetic


variability (Smith et al.,


1975; Cothran etal., 1983; Mitton and Grant, 1984), although there


is no unifying theory to predict the genetic consequences of paedomorphosis or its effects on

the rate of speciation.

Avian groups that are paedomorphic may represent ideal systems from which to

develop a coordinated theory relating ecological constraints, age-structure, life history traits,

and genetic variability to rapid speciation. Many avian groups, with the possible exception of

ratites, have only recently been recognized as paedomorphic (Lawton and Lawton, 1986;










recognized as a paedomorphic process. The alternative process to delayed somatic maturation

is early onset of sexual maturation, or progenesis. This latter process has not been


documented for avian groups. Both neoteny and progenesis result in individuals breeding in

juvenile morphology (i.e., paedomorphosis). Hispaniolan palm-tanagers represent a species

pair in which one is paedomorphic to the other. Gray-crowned Palm-Tanagers,

Phaenicophilus poliocephalus, resemble the juveniles of Black-crowned Palm-Tanagers, P.

palmarum, in foraging behavior and morphology (Chapter 2).

My purpose is to describe the patterns of genetic variation within and between the two


palm-tanager species. My specific objectives are to: (1


quantify the degree of genetic


differentiation between these species and use it to calculate estimated divergence time; (2)

compare genetic variability, as measured by multilocus heterozygosity, between the two

species; (3) describe age-specific genetic variation within these species and relate it to


differences in behavior and morphology; and, (4) discuss speciation in these tanagers


relates to isolation, founder effect, and genetic variability.



Methods and Materials


Collections were made of two species of Phaenicophilus and their hybrids in Haiti from

May through September, 1985. Field identifications were based on crown and chin characters


(Chapter 2)


Juveniles of both species could be identified in hand by the presence of a yellow


wash in the plumage. Because no dry ice or liquid nitrogen is available in Haiti, tissues were

stored in 1.5% buffered 2-phenoxyethanol solution 0.5 to 4 hours after collection (Nakanishi


., 1969; Barrowclough, pers. comm.), and vials were stored in a conventional freezer at -


4oC until September 1985.


Tissues were stored at -600C thereafter.


Liver and muscle extracts were ground in the phenoxyethanol solution. Samples


were







44


Thirty-nine presumptive loci were assayed. Locus designations, abbreviations, and

buffer conditions are given in Table 3-1 except where listed below. Loci were numbered

according to the mobility of the products with the most anodal as 1 when two or more isozymes


appeared on the same gel. Monomorphic loci included AAT-2 (Acid Citrate 6.2

Malate 7.4 buffers), CK-1, FH-2, aGPD-1, lactate dehydrogenase (LDH)- 1 and


and Tris


malate


dehydrogenase (MDH)- 1 and 2, malic enzyme (ME), and SOD-2 (all on Acid Citrate 6.2


buffer), and general proteins (GP)-1 and


(Lithium Hydroxide


8.2 buffer). Alleles were


designated alphabetically, with A corresponding to the allele with the fastest migrating

product. No locus had more than three alleles. A third locus for AAT appeared occasionally in

the most anodal position, although attempts to visualize this third and highly variable locus

on a regular basis were futile. Because only two AAT loci have been reported for birds, the

appearance of this third AAT locus may be a staining artifact.

General statistical tests were conducted with the Statistical Analysis System (SAS,


1985) or Statistical Package for the Social Sciences (SPSS, Norusis, 1985). Significance


was


set at P<0.05; highly significant rejection of null hypotheses occurred when P <0.01.

Acceptance levels for multiple comparisons involving the same data set were adjusted to an

experiment-wide error of P < 0.05 (Harris, 1975). Degrees of freedom are given as subscripts


to the reported statistics.


Tests are reported as two-tailed except where noted. Substrate


diversities were calculated for the two species ofPhaenicophilus and age-classes within P.

palmarum, using the Shannon Information Index (Peet, 1974; Pielou, 1977). Differences

between diversities were evaluated using t-tests (Zar, 1984).

Allele frequencies and genetic variability, as measured by the proportion of

heterozygous loci determined by direct count per individual averaged across 39 loci (H), the

average number of alleles per locus (A), and the per cent loci polymorphic (P) with the










1981).


Statistical analyses of individual heterozygosities that were arcsine square-root


transformed (Archie, 1985) were performed using the t-test. Because samples were small in

some categories, data for rare or unique alleles were excluded on subsequent analyses if the


expected frequency of observation


was less than one individual


as calculated from the gene


frequencies in the alternate sample.


Results


Genetic distance (Nei, 1978) between P. palmarum and P. poliocephalus


was D=


0.010.


F-statistics (Fst) (Wright, 1965, 1978) were employed to evaluate the amount of genetic


differentiation between P. palmarum and P. poliocephalus (Fst =


case


0.122). Fst were used in this


to make comparisons to within-species values possible. No significant differences in


genetic heterozygosity existed between the two parental species (t(40)= -


0.81),


between P


palmarum and hybrids (t34


(Table 3-1


=-0.17), or between P. poliocephalus and hybrids (t(32)= 0.54)


No fixed allelic differences were found between the species although there were


shifts in allele frequencies and the distribution of rare alleles.


Unique alleles, defined as


those observed in only 1 group, were observed at


21 of the


polymorphic loci. Phaenicophilus poliocephalus had 15 and P. palmarum had 16 unique


alleles


out of a total number of 63 and 64 alleles, respectively.


Sample


size for each species


was sufficient to detect 56


.2% and 74.1%


, respectively, of the unique alleles in at least one


individual at the observed frequencies in the other species.


Nine rare alleles across seven loci


were shared between the two species. Rare alleles are those found in both samples with a


frequency of 0


.25 or less.


There were 9 unique alleles in juvenile P. palmarum and 6 unique


alleles in adults across 11 loci; they shared 10 rare alleles. Differences in heterozygosity were


highly significant between juvenile (H


= 0.121) and adult P. palmarum (H = 0.074)(t(19)=







46


Juvenile or adult P. palmarum did not differ significantly from either P. poliocephalus or the

hybrids in genetic heterozygosity (P1 0.05).

To assess the size of the founding populations required for colonization and

maintenance of current heterozygosity levels in P. poliocephalus, expected heterozygosity

(He) was computed for the colonists as follows (Crow and Kimura, 1970; Baker and Moeed,

1987):


He= (1-1/2No)Ho,


where No is the size of the founding population and Ho is heterozygosity of the founders.

Founding populations derived from ancestral P. palmarum were assumed for the calculation

to consist of one of three groups: all adults, all juveniles, or a mixture of adults and juveniles


collected at random.


Three curves were generated and compared with the current levels of


heterozygosity in P. poliocephalus (H =


0.104) (Fig. 3-1


The curve generated assuming a


founding population of all juveniles asymptoled at He =


0.106 at No = 4, where the curve


generated assuming a mixed group reached He = 0.090 at No = 40.


This latter value is more


than one standard error (S.E.) below the current level of heterozygosity of P. poliocephalus.

The curve generated assuming a founding population of all adults reached an aymptote at


He= 0.061,


more than two S.E. below the current heterozygosity level of P. poliocephalus.


Of the 17 variable loci in P. palmarum (aGPD-2


was omitted due to low sample sizes),


only four did not have higher levels of heterozygosity in juveniles than adults.


This trend


across loci was significantly different from random (x2(1)= 4.8). Data from the 4 loci (glucose

dehydrogenase, phosphoglucosemutase-1, peptidase-1, and xanthine dehydrogenase) that did


not follow this trend were dropped.


The jackknife procedure (Lanyon,


1987) was performed on


the remaining data to evaluate single-locus effects on heterozygosity differences between age-







47


loci. Heterozygosity difference between age-classes was tested under the hypothesis that the


differences were not due to significant single-locus effects. Levels


significantly different between age-classes (t(24)=-24.33).


of heterozygosity were still


Therefore, differences in


heterozygosity between age-classes were not due to the effects of one or two loci. Moreover,


the direction of the difference between age-classes remained unchanged; juvenile


palmarum were always more heterozygous than were adults, regardless of which data were

removed (Table 3-2).


Substrate and behavioral diversities were used


as estimates of niche breadth. Adult


and juvenile P. palmarum differed significantly from P. poliocephalus in foraging behavior


diversity (t(964)= 9.95 and t(819)= 6.49).


Age-classes within P. palmarum were not


statistically different from one another (t(724)= 2.18).


Adult P. palmarum were highly


significantly different from juveniles of the same species and from P. poliocephalus


5.00 and t(235)=


, respectively) in the diversity of substrates used.


Discussion


F-statistics were used as an indirect measure of gene flow in palm-tanagers.


interpret the statistic, both species were assumed to represent populations of a single form


(Wright, 1965, 1978). The differentiation observed between the two forms ofPhaenicophilus

(Fst= 0.122) is twice as high as that observed among populations of most other avian species


(Barrowclough, 1980b, 1983), suggesting that gene flow is restricted within Phaenicophilus.

Comparable Fst's are observed for species with low dispersal rates that are caused by


geographic isolation between populations (Corbin et al.,


1974; Yang and Patton, 1981


Baker


and Moeed, 1987).


Because species of Phaenicophilus are not now isolated by any geographic


barrier, the high Fst is indicative of reduced gene flow on the same order as most avian


(t(48)=







48


and the apparent selection against hybrids in the contact zone (Chapter 2) supports the

hypothesis of reduced gene flow.


The genetic distance found between the two Phaenicophilus taxa was low


D=0.010),


even for avian species that have an overall average D= 0.044 (Barrowclough, 1980a; 19


However, Johnson and Zink (1983) observed D= 0.004 in two closely related species of

sapsuckers (Sphyrapicus) that were undergoing character displacement due to assortative

mating in sympatry. Low genetic distance should not be used a priori as a criterion for species


recognition in birds.


Genetic distance is not just a function of current gene flow, but is a


measure of the time since gene flow has been restricted (Yang and Patton, 1981). Moreover, D

is a function of the relative magnitude and divergence of allelic frequencies and consequent


shifts in heterozygosities and associated variances (Novak, pers. comm.).


The low D for


Phaenicophilus is likely an indication of two recently diverged but distinct species.


The estimated time of divergence


Nei, 1975; Johnson and Zink, 1985) is between 5.0


104 and 2.6


x 105 years ago in the middle of the Pleistocene.


This range in the estimate


corresponds remarkably well to the time of the most recent interglacial period when sea levels

rose 8-10 m some 6.5 x 104 years ago and multiple times throughout the Pliocene and


Pleistocene (Pregill and Olson, 1981).


The rise in sea level would have inundated the Cul-de-


Sac Plain, which runs from west to east across Hispaniola, thus cleaving it into north and


south islands.


This plain is presently below sea level; during interglacial periods when


glaciers melted and sea levels rose it would have formed an open water barrier to gene flow

(Pregill and Olson, 1981).

Phaenicophilus is likely to have fragmented in allopatry on the two islands. Current

distributions of the two species suggest that P. poliocephalus arose on the south island. If the

divergence is a result of a vicariance event, then P. poliocephalus would be expected to be







49


in P. poliocephalus is consistent with the hypothesis that it is derived from P. palmarum as a

result of colonization to the south island. Low competition and relatively abundant resources

on the colonized south island could have favored the evolution of progenesis, which is

characterized by earlier sexual maturation often at smaller body size than normally expected


(Gould, 1977).


If the colonizers were few in number, then genetic variability would have


declined due to founder effect (Crow and Kimura,1970; Kilpatrick, 1981).


Both species have


similarly high levels of heterozygosity (H = 9-10%). Relatively high H is not unusual for

island birds (Yang and Patton, 1981) or for other vertebrates (Nevo et al., 1984), though an

"island" effect has been observed in small mammals (Kilpatrick, 1981; Aquadro and

Kilpatrick, 1981). A decline in heterozygosity could be avoided by rapid population growth

after colonization, a large number of founders, or multiple invasions. If heterozygosity were

reduced by founder effect, it could have been restored over long periods through mutation.

The simplest explanation for the current situation is that heterozygosity was not


reduced on the south island in ancestral P. poliocephalus.


There seems to be insufficient time


for the re-establishment of high heterozygosity since the estimated divergence. Large

numbers of colonizers could explain the high heterozygosity, but is inconsistent with the idea

of geographic isolation and rapid subsequent speciation (Templeton, 1980). A small number

of highly heterozygous colonizers and rapid population growth would also explain

maintenance of high heterozygosity in the founder population on the south island (Nei, 1975;


Senner, 1980).


The major difficulty with this explanation is how to get a group of highly


heterozygous colonizers. A founding group of adults would not have had sufficient levels of

genetic variability to explain levels of heterozygosity currently observed in P. poliocephalus

(Fig. 3-1). A mixed group of adult and juveniles colonizers would have had to consist of four

times the number of founders than a group composed only of juveniles to maintain levels of







50


other avian species (Greenwood and Harvey, 1982). Flocks of neotenic Brown jays,

Cyanocorax morio, colonizing recently cleared habitat, have a lower mean age than flocks in

the main population (Lawton and Lawton, 1985). Geographic isolation, small numbers of


founders, and few founding events set the stage for rapid speciation (Templeton,


1980) and


progenesis provides a basis for explaining the resemblance of P. poliocephalus to juvenile P.

palmarum in behavior and morphology. An alternative explanation, using a vicariance

model, could be used but it is not as parsimonious as the colonization model in explaining both

levels of genetic variability and the resemblance of adult P. poliocephalus to juvenile P.

palmarum.


Furthermore, leve


of heterozygosity are similar between adult P. poliocephalus and


juvenile P. palmarum. Although no significant difference between age-classes exists within

P. poliocephalus (P= 0.10), adults are relatively more heterozygous than are juveniles

(H = 0.115 and 0.069, respectively), and have comparable levels of variability with juvenile P.

palmarum (H = 0.121). Juvenile P. palmarum are significantly more heterozygous than

adults (H = 0.074) of their own species. Age-related changes in multi-locus heterozygosity

have not been documented previously for birds, but are known for other vertebrates (Cothran


et al., 1983; Samollow and Soule, 1983).


The similarity of genetic variability between adult P.


poliocephalus and juvenile P. palmarum may be serendipitous, but the pattern across several

character sets (i.e., behavior, morphology; Chapter 2) suggests there may be a single


underlying biological proce


ss. Rapid divergence of P. poliocephalus on the south island may


have been facilitated by selection in a new habitat with abundant resources.


Colonists on the


south island may have experienced abundant resources relative to their north island

counterparts, resulting in increased fecundity, faster growth rates, and earlier sexual


maturation.


These characteristics are the essential features of progenesis.


Thus, P.







51


founded by a few individuals and isolated from the ancestral form satisfy the requirements of


Templeton's


1980) model for rapid speciation.


The size of founder populations should be just


small enough to cause a rapid accumulation of inbreeding without a severe reduction in


genetic variability.


These conditions enhance the probability of a reorganization of the


genome, with especially important consequences for regulatory genes (Templeton, 1980).

Even small changes in regulatory loci can effect significant phenotypic changes (Larson,

1980) and provide the basis for the establishment of isolating mechanisms. Few differences in

structural loci nor are radical shifts in ecological niches expected in Templeton's model or in


Gould's


(1977) model of paedomorphosis.


Phaenicophilus poliocephalus appears to be


exploiting the foraging niche of juvenile P. palmarum (Chapter 2) and there are few


differences between the two species in structural gene loci (Table 3-1


The conditions that


favor rapid speciation after a founding event are the same ones as prescribed by Gould's


1977)


model for the evolution of progenesis in colonizing populations.


Rapid speciation in paedomorphic assemblages would be promoted by higher genetic

variability in juvenile age-classes. Higher levels of genetic variability in the founding

populations would result in greater evolutionary changes than populations with lower levels


of genetic variability (Futuyma, 1979:p. 442).


Faster evolution may increase the probability


of speciation and not require as long a period of geographic isolation.


Thus, paedomorphic


species in which juveniles are more heterozygous and are also the primary dispersers should

be more prone to speciation.

Age-related differences in heterozygosity observed in P. palmarum might be spurious


and not justify the predictions for rapid speciation in Phaenicophilus.


Significantly higher


levels of heterozygosity are observed for juvenile P. palmarum in 13 out of 17 variable loci.


Removal of the data for any single locus does not alter this pattern (Table 3-2).


Age-related







52


Age-related differences in heterozygosity observed for Phaenicophilus might also be

expected to occur in amphibians that undergo paedomorphosis. No changes in the levels of

genetic variability have been observed in populations of salamanders that experience varying

degrees of paedomorphosis (Pierce and Mitton,1980; Shaffer, 1984). However, age-class

differences in single- and multi-locus heterozygosity have been observed in other vertebrates

(Redfield, 1973; Tinkle and Selander, 1973; Ramsey et al., 1979; Chesser et al., 1982; Cothran

et al., 1983; Samollow and Soule, 1983), although higher genetic variability is not always

found in juveniles. If the heterozygosity levels in adults are relatively constant over time and

juveniles are more variable than adults, one mechanism is required to explain higher

heterozygosity in juveniles and another for its reduction in adults.


The differences in genetic variability across life history stages


can b


e explained in


several ways that are not mutually exclusive.


These explanations include developmental


changes in isozyme patterns, negative assortative mating, or dispersal of individuals with

different genotypes across spatially heterogeneous areas for allele frequencies. None of these


processes account for the loss of heterozygosity in adults observed for 13 loci.


the number of rare alleles in juveniles compared to adults


operating (Samollow and Soule,


The decrease in


indicates selection is probably


1983). Increased genetic heterozygosity in juveniles may be


due to one process while selection eliminates certain adults, thus decreasing heterozygosity

for that age-class. Selection may also act on both life history stages, first increasing, then

decreasing genetic variability. Juvenile Blue grouse (Dendrogapus obscurus), another

neotenic species (Lewis and Jamieson, 1987), are more heterozygous for the Ng locus than are


adults in the same late successional habitat.


These data suggest that genetic variability may


also be positively correlated with resource availability (Redfield, 1973).

Genetic variability is often correlated with various characteristics closely associated







53


individuals might be at a selective disadvantage among adult P. palmarum. If exploratory

behavior is positively correlated with heterozygosity (Garten, 1977), then heterozygous

juveniles might be more active and exposed to higher levels of accidental mortality.

Alternatively, since adult birds tend to return to the same breeding territory (Greenwood and

Harvey, 1982), selection may result in microgeographic adaptation to particular habitats.

The type of genetic data collected do not allow an evaluation of the relative importance of any

of these mechanisms or the basis of selection for or against relatively heterozygous


individuals.


Low heterozygosity in adults might be a response to decreased niche breadth


(Levins, 1968). Contrary to this prediction, substrate diversity for adults is higher than that

of juveniles. Because niche breadth is an N-dimensional concept (Hutchinson, 1958), other

variables need to be examined to test this hypothesis more fully.

In conclusion, neoteny in P. palmarum seems to be expressed both behaviorally and


morphologically (Chapter 2).


The age-dimorphism in behavior and morphology is congruent


with observed heterozygosity differences between age-classes. Selection is probably operating

to produce the observed life history shifts in genetic variability between juveniles and adults.

Higher juvenile genetic variability, combined with a greater amount of dispersal by juveniles

and the high genetic variability observed in P. poliocephalus, are consistent with the

derivation of P. poliocephalus from small founding populations of juvenile P. palmarum on the

south island of Hispaniola during the Pleistocene. Speciation will be more rapid when life


history traits of the ancestral populations are variable


Derivation of new species by


progenesis is more likely if the ancestral species is neotenic. Speciation will occur for neotenic

species expanding into new environments, if (1) the colonizers possess high genetic

variability, (2) the colonizers have high reproductive and low mortality rates at low densities,

(3) founder populations are small, and (4) reproductive isolation follows the reorganization of
















Table 3-1.


Estimates of genetic variability for Hispaniolan palm-tanagers and their
hybrids for 39 enxyme loci.


ENZYME INFORMATION


POPULATION


NAME/NUMBERa


BUFFER/pHb


LOCUS


HYB


Aspartate
aminotrans-
ferase
(2.6.1.1)


AAT-1: (N)
A
B
C

ACON-1: (N)


Aconitate
hydratase
4.2.1.3


ACON-2: (N)
A
B


14
0.143
0.857
0.000

13
0.000
1.000


12
0.000
1.000


8
0.188
0.813
0.000

8
0.000
1.000


6
0.083
0.917


20
0.000
0.850
0.150

17
0.029
0.971


16
0.063
0.938


14
0.036
0.929
0.036

13
0.000
1.000


12
0.042
0.958


Catalase
1.11.1.6


Creatine
kinase
2.7.3.2


CAT: (N)


CK-2: (N)


14
0.000
0.857
0.143

14
0.000
1.000


8
0.000
0.875
0.125

8
0.000
1.000


20
0.100
0.750
0.150

16
0.063
0.938


14
0.214
0.750
0.036

13
0.038
0.962


Diaphorase
1.6.2.2


1.6.4.3


DIA-2: (N)


DIA-3: (N)
A
B
C


12
0.167
0.792
0.042

14
0.036
0.929
0.036


7
0.143
0.643
0.214

8
0.000
0.875
0.125


12
0.000
1.000
0.000

19
0.000
0.921
0.079


12
0.000
1.000
0.000

13
0.000
0.923
0.077









Table 3-1. (Cont'd)


ENZYME INFORMATION


POPULATION


NAME/NUMBERa


BUFFER/pHb


LOCUS


HYB


FH-1: (N)


Fumarate
hydratase
4.2.1.2


GDH: (N)


Glucose
dehydrogenase
1.1.1.47


13
0.885
0.115

12
0.042
0.958
0.000


8
0.750
0.250

6
0.000
1.000
0.000


16
1.000
0.000

17
0.000
0.912
0.088


13
0.962
0.038

12
0.042
0.917
0.042


Alpha
glycero-
phosphate
dehydrogenase
1.1.1.8


Glucose
phosphate
isomerase
5.3.1.9


aGPD2: (N)


GPI: (N)


bGUS: (N)


Beta-
Glucuronidase
3.2.1.31


Hexokinase
2.7.1.1


HK: (N)


1
0.000
1.000
0.000


14
0.214
0.000
0.786


14
0.000
1.000
0.000

9
0.000
1.000
0.000


3
0.167
0.333
0.500


8
0.188
0.000
0.813


8
0.188
0.750
0.063

6
0.000
1.000
0.000


20
0.000
1.000
0.000


20
0.200
0.000
0.800


20
0.050
0.925
0.025

17
0.000
1.000
0.000


9
0.000
0.833
0.167


14
0.107
0.036
0.857


13
0.192
0.808
0.000

12
0.125
0.833
0.042


Isocitrate
dehydrogenase
1.1.1.42


ICD-1: (N)
A
B
C


14
0.000
1.000
0.000


8
0.000
1.000
0.000


17
0.118
0.853
0.029


14
0.000
1.000
0.000


Mannose
phosphate


TC 8.0
EDTA 8.6


MPI: (N)
A


9
0.000


5
0.100


14
0.000


10
0.000









Table 3-1. (Cont'd)


ENZYME INFORMATION


POPULATION


NAME/NUMBERa


BUFFER/pHb


LOCUS


HYB


Purine
nucleoside
phosphorylase
2.4.2.1


EDTA 8.6


Phospho-
glucomutase
2.7.5.1


NP: (N)
A
B
C

PGM-1: (N)
A
B
C


14
0.143
0.107
0.750

12
0.042
0.917
0.042


7
0.214
0.214
0.571

7
0.000
1.000
0.000


19
0.316
0.158
0.526

17
0.000
1.000
0.000


12
0.083
0.833
0.083

14
0.036
0.821
0.143


PGM-2: (N)
A
B

PGM-3: (N)
A
B
C


Peptidasec
3.4.11


PEP-Al: (N)
A
B
C


PEP-G: (N)
A
B

PEP-A2: (N)
A
B
C

PEP-L: (N)
A
B
C


5
1.000
0.000

4
0.000
1.000
0.000


9
0.000
1.000
0.000


13
0.038
0.962

14
0.000
1.000
0.000

14
0.071
0.929
0.000


6
1.000
0.000

3
0.000
1.000
0.000


6
0.000
1.000
0.000


8
0.063
0.938

7
0.071
0.929
0.000

7
0.000
1.000
0.000


14
0.929
0.071

13
0.038
0.962
0.000


15
0.000
0.967
0.033


16
0.000
1.000

18
0.000
0.889
0.111

16
0.000
0.906
0.094


9
1.000
0.000

13
0.000
0.962
0.0383


14
0.036
0.893
0.071


14
0.143
0.857

14
0.036
0.964
0.000

14
0.000
1.000
0.000









Table 3-1. (Cont'd)


ENZYME INFORMATION


POPULATION


NAME/NUMBERa


BUFFER/pHb


LOCUS


BPA


GPT


HYB


Phospho-
gluconate
dehydrogenase
1.1.1.44


TM 7.46


PGD-1


0.000
1.000


0.000
1.000


0.219
0.781


0.000
1.000


SOD-1:


Superoxide
dismutase


1.15.1.1


0.000
1.000
0.000


0.000
0.917
0.083


0.167
0.833
0.000


0.429
0.571
0.000


Xanthine
oxidase


1.2.3.2


TC 8.0


XDH:


0.100
0.900
0.000


0.000
1.000
0.000


0.079
0.895
0.026


0.000
0.964
0.036


0.074

0.011

1.4


0.121

0.009

1.5


0.104

0.012

1.6


0.095

0.011

1.7


Note: Allele frequencies, direct count heterozygosity (H


, percent polymorphic loci


(common allele


0.99)


, mean number of alleles (A), and


sample sizes (N) for


Hispaniolan palm-tanagers and their hybrids across 39 enzyme loci.


Information on


enzyme names and international nomenclatural numbers, according to Harris and


Hopkinson (1976), and pH/buffer systems is included for each isozyme


assayed.


= Adult Black-crowned palm-tanager; BPI=juvenile


Black-crowned


palm-


tanager; GPT = Gray-crowned palm-tanager; HYB = hybrids.

Enzyme names and numbers recommended by the Commission on Biological
Nomenclature ("Enzyme Nomenclature", Elsevier, Amsterdam, 1973).


Abbreviations for buffers are: AC


=Acid Citrate (Clayton and Tretiak, 1972);


=Tris Citrate 7.0 (Ayala et al., 1972); TM =


Tris Malate; TC


=Tris Citrate 8.0


EDTA = Ethylenediamine Tetraacetic Acid; LIOH = Lithium Hydroxide (Selander et


al., 1971; Harris and Hopkinson, 1976).


When more than one buffer condition is


JI 1 f r '1 I, *4 ..A- '









Table 3-2.


Mean % heterozygosity for adult and juvenile Black-crowned palm-tanagers


after Jackknife simulation


Lanyon, 1987) for 13 variable


LOCUS%


% HETEROZYGOSITYa,b
AFTER REMOVAL


HETEROZYGOSITY
BEFORE REMOVAL


SAMPLE
SIZE


LOCUS


ADULTS


IMMATURES


REMOVED


(AD)


ACON-2


CAT


DIA-2


DIA-3


FH-1


AAT-2


bGUS


PEP-C


PEP-B


SOD-1


Note:


This procedure removes


data for one


ocus at a time and computes


resulting


heterozygosity after removal. A t-test was performed to test the null hypothesis that
the average difference across all 13 loci between age-classes is not significant.
Differences were highly significant (t(24)= 24.3); the null hypothesis was rejected at
P< 0.001. Abbreviations for loci are explained in Table 3-1.

Standard errors ranged from 0.010-0.013 for adults and 0.007-0.013 for immatures.
Probabilities, after removal, ranged from 0.004-0.037 for single locus t-tests.












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CHAPTER 4


BIOCHEMICAL SYSTEMATICS


OF HISPANIOLAN PALM-TANAGERS


Introduction


In recent years, increasing confidence has been placed in the genetic resolution of

phylogenetic relationships and the molecular tools used to assay the genetic differences


underlying these relationships.


What is often not appreciated is that the various molecular


techniques provide indirect or incomplete assessments of genetic differentiation. Moreover,


correlations between character sets, such


as morphology and electrophoretically detectable


proteins, are often weak (Zink et al.,


1985).


Incongruencies in character sets often raise more


questions than they answer, thus generating more phylogenetic hypotheses. Character

reversals and convergences also create problems in resolving phylogenetic relationships as

has been amply demonstrated in birds (Sibley and Ahlquist, 1986; Neff, 1987).

The resolution of the derivation of Hispaniolan palm-tanagers from one another cannot

be based on morphological criteria alone. Crown color is an important character in

distinguishing between these species and may represent an example of character reversal in

relation to other tanagers. Additional characters must be used to resolve this relationship.

Resulting phylogenies must withstand rigorous scrutiny and be understood from a broader

taxonomic perspective. Although avian taxa are well studied, many phylogenetic

relationships remain to be resolved, including those of neotropical tanagers.

The subfamily Thraupinae tanagerss) in the Emberizidae represents a diverse South


American radiation (


> 200 species) of small to medium-sized birds that are largely







62


(Storer, 1969) and how they are related to warblers, vireos, and finches remain unclear. Bond

(1978) proposed that Greater Antillean tanagers were originally derived from Central

American species that are now extirpated on the mainland. Alternatively, some Greater

Antillean tanagers may have been derived from migrants either island hopping across the

Caribbean or flying over during annual migrations from and to North America. In general

plumage characters, Hispaniolan palm-tanagers resemble the Central American bush-


tanagers, genus Chlorospingus.


Therefore, the derivation of Phaenicophilus from a


Chlorospingus-like ancestor is plausible. Sibley (pers. comm.) placed species belonging to the

genus Piranga in close association to Phaenicophilus, though Chlorospingus was not included

in the DNA hybridization analysis. Because Hispaniola lies on one of the major migratory

corridors between North and South America, and Piranga olivacea is a migrant along this

corridor, derivation of Phaenicophilus from a Piranga-like ancestor is also possible. Species of

the two genera resemble one another in overall body-size, but otherwise are not similar in

plumage patterns.


A third phylogenetic hypothesis depends on ontogenetic similarities in plumage


being potentially important for the analysis.


Whereas bill morphology


changes a great deal


across species, plumage patterns tend to be conservative (Storer, 1969).


Therefore,


phylogenies built on plumage characters may be robust. Although adults of Spinda


lis zena


are morphologically and behaviorally distinct from adult Phaenicophilus, female Spindalis


from Jamaica are quite similar in appearance to Phaenicophilus fledglings.


may be phylogenetically significant.


This resemblance


Spindalis zena is common throughout the West Indies.


Colonization during the Pleistocene with subsequent isolation on Hispaniola could have


resulted in differentiation of populations and subsequent divergence of Phaenicophilus


from a


Spindalis-like ancestor.










plumage patterns may be important to determine phylogenetic relationships among tana


Consequently, there are five taxa that might be the closest relative to Phaenicophilus.


gers.


These


taxa include the following: the Central American common bush-tanager, Chlorospingus

ophthalmicus, based on plumage patterns and geography; the scarlet tanager, Piranga

olivacea, based on DNA hybridization and biogeographical patterns; the stripe-headed

tanager, Spindalis zena, based on ontogenetic plumage patterns; the palm tanager endemic to

the Lesser Antilles, Thraupis palmarum, based on geography, behavior and ecology; and the

South American hooded tanager, Nemosia pileata based on plumage patterns.

Which of these species is the closest relative to Phaenicophilus is critical in the


understanding of the evolution of the two species in this genus.


The determination of the


polarity of the speciation event (i.e., which species is derived from which) depends on

phylogenetic reconstruction to demonstrate that one species is derived from another either by

terminal deletion (i.e., paedomorphosis) or by terminal addition (i.e., peramorphosis) of


characters (Kluge and Strauss, 1985).


The assumption made throughout earlier discussions


of Phaenicophilus (Chapter 2) was that Piranga is the closest sister group to Phaenicophilus.

Because neoteny occurs in Piranga, it is a shared character with Phaenicophilus palmarum.

The chin and throat pattern observed in P. poliocephalus is not found in other tanagers


included in the analysis.


The existence of a shared character state between Piranga and


Phaenicophilus palmarum and the presence of a unique character in P. poliocephalus support

the hypothesis that the latter species is derived from P. palmarum. If true, the absence of a

black crown in adult P. poliocephalus constitutes a terminal deletion and P. poliocephalus

would be paedomorphic to P. palmarum. In contrast, if Chlorospingus is the closest relative of

Phaenicophilus, then the polarity of the derivation of Phaenicophilus species is not clear,

because there is not much information on the breeding biology of Chlorospingus. An







64


warblers (Parulinae) represent a group of small, colorful insectivorous birds with North

American affinities (Bond, 1978). Beecher (1953) considered warblers and tanagers to be

sister groups derived from the same vireo-like ancestor. Subsequent work with DNA

hybridization removed vireos to the corvine assemblage (Sibley and Ahlquist, 1982), thus

suggesting similarity in plumage and size among these three groups constitutes a case of

convergence rather than a close relationship. Additional analyses (Sibley, pers. comm.) point

to a more distant relationship between tanagers and warblers than previously thought.


Plumage similarities between the Hispaniolan palm-tanagers, Phaenicophilus,


the endemic


green-tailed ground warbler, Microligea palustris, and the mainland common bush-tanager,

Chlorospingus ophthalmicus raise questions about the true affinities of the palm-tanagers to

other tanagers and to warblers.


If the taxonomic designations of the Hispaniolan forms are


correct, then plumage


similarities could be due to the retention of primitive characters in both warblers and

tanagers. Alternatively, the similarity in plumage may be the consequence of convergences

or is due to closer taxonomic affinities of these species than DNA hybridization reveals. If the

latter is true, then the Hispaniolan complex might provide clues to the origin of warblers and

tanagers, possibly putting Hispaniola as the site of origin of these two groups.

If Microligea and Xenoligea are more closely allied to Chlorospingus and

Phaenicophilus than current taxonomic designations suggest, then the relationship might be

crucial to the final determination of the polarity of speciation in Phaenicophilus.

Consequently, several morphological features could be used in the determination of the

polarity if Hispaniolan warblers link Chlorospingus to Phaenicophilus to form a species


assemblage


. For example, both Microligea and Chlorospingus have gray crowns. If


Phaenicophilus were derived from a Chlorospingus-like ancestor, then gray crowns are a







65


addition. Consequently, the relationship of Microligea and Chlorospingus to Phaenicophilus

is crucial to the determination of the polarity of evolution in Phaenicophilus and whether or

not paedomorphosis has occurred.

My purpose was to describe the relationships of Hispaniolan palm-tanagers to other


tanagers and to Hispaniolan warblers.


The specific objectives were as follows: (1


determine from which group Hispaniolan palm-tanagers are most likely derived;


2) to use


close relatives as outgroups in the determination of the polarity of speciation in palm-

tanagers; and (3) to determine the relationship of Microligea palustris to Hispaniolan palm-

tanagers.


Methods


A total of 163 specimens from


25 species of tanagers


and warblers was used for the


electrophoretic analysis of 20 presumptive loci.


(Operational Taxonomic Units


An additional 422 specimens from 15 OTU's


representing each of the genera used in the genetic analysis,


were chosen for morphological analysis (Table 4-1


and Chlorospingus, Microligea palustris was included to test the relationship of warblers to


tanagers. Xenoligea montana


was


too rare to collect in Haiti.


With the inclusion of


Microligea, seven species of wood warblers were added as outgroups for the tanager-warbler

comparisons. These species included Mniotilta varia, Geothlypis trichas, Dendroica


pensylvanica, D. palmarum, D. pinus, D. coronata, and D. dominica. The addition of the

warblers also allowed comparisons between this study and earlier ones. The White-eyed


vireo, Vireo griseus, was chosen as an outgroup for the tanager-warbler comparisons


(Beecher, 1953).


Species were selected from several additional genera including Euphonia,


Eucometis, and Coereba based on the phylogeny of Sibley (pers. comm.).


Hemispingus


In addition to Phaenicophilus, Piranga,


was










Floridian material are deposited in the Louisiana State University Museum of


oology


(LSUMZ) frozen tissue collection.

Whole tissue homogenates of liver and pectoral muscle tissue were prepared for

horizontal starch gel electrophoresis (Chapter 3). Stain recipes were from Selander et al.


(1971


and Harris and Hopkinson (1976); isozyme nomenclature followed from Harris and


Hopkinson (1976).


Collection of the Haitian material is described in Chapter


Material


collected in Florida and South Carolina was treated similarly, except that tissues were stored


within 8 hours at


-600C in an ultracold freezer. Specimens from Tall Timbers Research


Station near Tallahassee, Florida, may have been dead for up to 8 hours before being frozen.

Neotropical tanagers and some North American wood warbler tissues were obtained from the


LSUMZ tissue collection.


These tissues were collected and stored in liquid nitrogen (Johnson


et al.


1984).


Subsamples of the material were stored in 1


.5% 2-phenoxyethanol solution for


subsequent analysis.


Three to five gels per stain were scored for different sequences of species


to determine the relative mobility of allelic products.


Two or more buffer systems were used


on all enzymes except fructose diphosphate aldolase (FDA-1), fumarate hydratase (FH),


malate dehydrogenase (MDH) and malic enzyme


ME) (Table 4-2).


The computer program BIOSYS-1 (Swofford and Selander, 1981) w


allelic frequencies, expected heterozygosity (He


used to calculate


observed heterozygosity by direct count


averaged over loci (Ho


Nei's


1978) and modified Rogers' distance


D:Wright, 1978)


(Table 4-3),


UPGMA (Unweighted Pair Group Method of Averaging) phenograms and


Distance-Wagner trees (Fig. 4-1). Rogers' D was used in the generation of UPGMA and

Distance Wagner trees and in comparisons with morphological distances, since it satisfies the


triangle inequality (Swofford and Selander, 1981


Distance Wagner trees were evaluated


before and after optimization and after using the multiple additions criterion for 20 loci







67


The Jackknife procedure (Lanyon, 1987), deleting data from loci or Operational

Taxonomic Units (OTU's) one at a time, with replacement, was used to determine the

stability of phylogenetic clusters. Data for fifteen variable loci were subject to jackknifing;

the data for the remaining five loci were not sufficiently variable to provide useful


information for this analysis. Data for OTU's of single species or


several species from a single


genus were systematically jackknifed. Stability of phylogenetic affinities was assessed by per


cent of association in clusters for each phylogram when data for loci or OTU's


were removed


after corrections were made for changes in sample


sizes


(Table 4-4).


Clusters were defined by


the presence of "core" species,


such as Piranga olivacea, Chlorospingus ophthalmicus, or


Eucometis penicillata. For example, Piranga olivacea, P. rubra, and P. ludoviciana were

associated 100% of the time on both UPGMA and Distance Wagner algorithms, whether data


from loci or OTU's


were removed.


Morphological data were collected for 21 skin and skeletal variables.


Where possible,


at least ten males and ten females of each species were measured to the nearest 0.01 mm


using Helios dial calipers (Table 4-1


Manhattan distances, considered to be the most robust


metric for cross taxonomic comparisons (Cherry et al.,


1982), were calculated for all species


used in the genetic analysis, except Piranga ludoviciana, Hemispingus atropileus and H.

superciliaris, Geothlypis trichas, Dendroica pensylvanica, D. palmarum, D. pinus, D. coronata,


and Vireo


griseus.


Manhattan distances are calculated as the absolute difference between two


variables.


When more than one variable is used in comparing groups, the sums of distances


between pairs of variables are used in calculating the final distance. Morphological distance

matrices were constructed for all 21 morphological features, for body-size related features

only (Table 4-5) and for plumage features only. Body-size related features included

wingchord, length of braincase, body length from the gonys to insertion of rectrices, length of







68


from exposed culmen, extent of color on the chin, lateral extent of chin color from the gonys,

and length of the dorsal anterior, dorsal posterior, and ventral eyespots. Bill size and shape,

measured by proximal and medial width and depth of the bill, were included in the total body


analysis.


The congruency between genetic and morphological matrices was tested using


Mantel analysis (Mantel,1967). Significance levels for Mantel analyses for rejecting the null


hypothesis are P


s 0.05 and P


2 0.95.


Results

Levels of genetic variability within species are given in Table 4-1; allelic frequencies

are presented in Table 4-2. No locus was monomorphic across all species. Average observed

heterozygosity was significantly higher in tanagers, excluding Microligea palustris and


Coereba flaveola, (Ho


= 0.108) than in warblers


= 0.055) (t(19)


= 2.60; P


< 0.02) and was


higher than the average heterozygosity reported for other birds (Ho


= 0.063: N


= 85; Evans,


1987).


Observed heterozygosity for each species was within one standard error of expected


heterozygosity except for Dendroica pinus and D. coronata.


Values of observed heterozygosity


for each warbler species fell within one standard error of values reported elsewhere

(Barrowclough and Corbin, 1978; Avise et al., 1980), with the exception of Mniotilta varia.

The mean value found for this species was about four times higher than previously reported

(Barrowclough and Corbin, 1978; Avise etal., 1980).

Heterozygosity (S.E.) for Microligea palustris, an wood warbler endemic to Hispaniola,


was 0.087 (0.042).


This value


was higher than that reported earlier (Ho


= 0.017; McDonald,


1987) based on 14 loci. Because estimates of heterozygosity are dependent on sample size and

the number and kinds of enzymes assayed, variation in levels of genetic variability across

studies is not unusual (Johnson, 1974; Archie, 1985; Simon and Archie, 1985). For example,










39 loci Ho


= 0.091 whereas for P. poliocephalus the values were Ho


= 0.099 and Ho


= 0.104,


respectively.


The number of alleles


, A (S.E


, averaged over loci for all species


was 4.6 (0.41).


ranged within species from 1.0 for Chlorospingus canigularis to 1.8 for Phaenicophilus

poliocephalus while per cent polymorphic loci (frequency of common allele s 0.99) ranged from


5% to 60%, respectively.


There were 15 unique alleles (found only in a single species):


one for


creatine kinase-2, amino aspartate transaminase (AAT


glucose phosphate


isomerase (GPI)


-2, lactate dehydrogenase (LDH)-2,


malic enzyme (ME), purine nucleoside


phosphorylase (NP), 6-phosphoglucose dehydrogenase (6PGD)-2, and phosphoglucose


mutase (PGM)-3; two for LDH-1 and peptidase (PEP-L); and three for 6PGD-1.


Four alleles in


fumarate hydratase (FH)-1, isocitrate dehydrogenase (ICD)-1, NP, and PGM-1 were shared


between Phaenicophilus and Chlorospingus but not with Piranga. Eight alleles in GPI-


beta-glucorunidase (bGUS), ICD-1, PEP-L and PGD-1 were shared between Phaenicophilus

and Piranga but not with Chlorospingus (Table 4-2).


Matrices of Rogers' and Nei's


genetic distances (D) are provided in Table 4-3.


Nei's (1978) interspecific genetic distance in tanagers was D


= 0.128 and in warbler


Average

S was


= 0.234 (D


= 0.265 calculated using Nei, 1972).


The warbler value was an order of


magnitude higher than those reported by Avise et al. (1980)


= 0.043 (Nei, 1972).


Eleven


loci were common to the two studies. Small sample


sizes,


different kinds of enzymes assayed


and the distribution of variation across the enzymes will affect estimates of genetic distance


(J. Novak, pers. comm.).


Nei's (1978) D for intergeneric comparisons of tanagers (excluding


Microligea palustris and Coereba flaveola) was D


= 0.600 (N


= 110), for all warblers was


.272 (N


= 11), and between tanagers and warblers was D


= 0.650 (N


= 119).


Genetic


distance was highest between species of Chlorospingus and between Piranga ludoviciana and










Average Nei's


(1978) genetic distance between species of Phaenicophilus and


Chlorospingus was D


= 0.542, while it


was D


= 0.404 between Phaenicophilus and Piranga.


Average Manhattan distances, based on 21 morphological characters, were 23.6 and 15.4,

respectively.

Both UPGMA and Distance-Wagner trees are presented since the affinities of the


palm-tanagers differed with the two methods (Fig. 4-1).


UPGMA phenograms are calculated


assuming equal importance for all sources of genetic variation, while Distance Wagner trees

weight certain types of variation. In addition, Distance Wagner trees make no assumptions

about homogeneous rates of divergence for different characters and thus, the branch lengths


for the OTU's may vary.


The cophenetic correlation for the UPGMA phenogram was 0.932,


with a % standard deviation (Fitch and Margoliasch,

Wagner trees, the goodness-of-fit statistics were 0.95


1967) equal to 7.12. For Distance


and 5.72, respectively, before


optimization. Neither the multiple additions criterion nor optimization added to the

interpretation of the patterns observed in the Distance Wagner trees.

The resolution of the relationships of the tanagers and the warblers based on the

dendrograms was complicated by the variation in the intra- and intergeneric branch lengths

(Fig. 4-1). Several species retained their same relative affinities when the data were


jackknifed: four main groups could be distinguished.


The groups included Piranga olivacea,


Chlorospingus ophthalmicus, Thraupis palmarum, and Eucometis penicillata as core species.

Euphonia music and Vireo griseus were more distantly related to the tanagers and warblers


than tanagers and warblers were to one another.


Wood warblers, with the exception of


Microligea palustris, branch off directly from the Eucometis penicillata-Coereba flaveola line.


Estimated divergence time for the warbler-tanager split was 1.7 to 10.5


X 106 years ago.


Microligea palustris, a purported wood warbler, was never aligned with the Eucometis-










were removed under these conditions; Spindalis zena was aligned with the Eucometis-C


oereba


flaveola cluster with Microligea palustris more distantly related to this group. Microligea and

Spindalis were most frequently aligned with the Piranga complex in the UPGMA phenogram


(100%) and in the Distance Wagner tree (67-95


%), except when the data for GPI-2 and PGM-3


were removed. The Piranga assemblage formed a sister group to the Chlorospingus


assemblage.


The three species of Piranga were associated 100%


of the time on both methods.


Eucometis was directly associated with Coereba 93-100% of the time (Table 4-4; Fig.


The Chlorospingus-Hemispingus association was less stable under all conditions with a


per cent association ranging from 60-90%.


Phaenicophilus was chiefly aligned with Piranga


in the Distance Wagner tree (87-95%) but


was asso


ciated with Chlorospingus in the UPGMA


phenogram (80-81%). Thraupis palmarum formed a consistent association with Nemosia

pileata and often was associated with the species in the Chlorospingus-Piranga branch


(27-70%).


Thraupis palmarum was not closely associated with Spindalis or Phaenicophilus.


Wood warblers formed two main branches with one containing Mniotilta varia and the other


Geothlypis trichas.


Dendroica coronata was closely associated with Mniotilta varia, and D.


palmarum and D. pinus were associated with Geothlypis.

The morphological phenogram (Fig. 4-3) showed patterns different from either the


genetic phenogram or tree.


Microligea palustris was paired with Chlorospingus


ophthalmicus, and this species pair formed a sister group to the Euphonia-Nemosia-Dendroica

dominica association. Phaenicophilus formed a sister group to the Piranga-Eucometis-


Thraupis association.


Mantel analysis of the relationship of genetic and combined


morphological (21 characters) distances gave a significant r


= 0.332 (P


= 0.992).


Thus, only a


small amount of the pattern of variation in morphology can be accounted for by the


correlation with the pattern of variation in genetics.


The pattern of genetic differences among










Discussion


Two clustering techniques, UPGMA and Distance Wagner trees, were used to

summarize phylogenetic relationships among tanagers with a focus on the relationship of

Phaenicophilus to other tanagers. Relationships hypothesized from UPGMA phenograms do

not necessarily reflect historical relationships among species because the phenograms are

based on statistical similarity between groups (Neff, 1987). Convergences can obscure the

true phylogenetic relationships, particularly if character differences are few and the direction


of character change is critical to defining phylogenetic branching sequences.


Wagner trees


are sensitive to changes in character polarity and represent phylogenetic relationships more

accurately (Neff, 1987).

Three of the phylogenetic hypotheses proposing a close relationship between Spindalis,

Thraupis or Microligea to Phaenicophilus can be rejected based on the branching patterns


observed for the UPGMA phenogram and the Distance Wagner tree (Fig. 4-1).


The affinity of


Phaenicophilus to either Piranga or Chlorospingus remains ambiguous when the results from


the two methods are considered.


When data from loci or OTU's used to construct UPGMA


phenograms and Distance Wagner trees were jackknifed, the results were also equivocal.


degree of association (80-95%) of Phaenicophilus to Chlorospingus or to Piranga varied with


the clustering technique used.


The instability in these associations may result from to


problems of convergences in both morphology and genetics, and these convergences may be a

common feature of heterochronic systems. If the Distance Wagner tree more accurately

reflects true phylogenetic relationships, then Piranga is more closely related to

Phaenicophilus. Piranga olivacea is neotenic (Lawton and Lawton, 1986); therefore, neoteny

would not a derived condition in Phaenicophilus palmarum. Because this relationship is

critical in determining the polarity of evolution within PhaenicoDhilus. criteria other than







73


Data based on the number of shared rare alleles and genetic and morphological

distances can be used to test the two phylogenetic hypotheses regarding the closest relative of


Phaenicophilus.


Average Nei's


(1978) distance between species of Phaenicophilus and


Chlorospingus ophthalmicus (D


= 0.542) is greater than that between species of


Phaenicophilus and Piranga olivacea (D


= 0.404).


When all morphological features are


combined to make the same comparisons, the average Manhattan distances were Dm


= 23.56


and 15.38, respectively. In addition, Phaenicophilus shares twice


as many


unique


" alleles


with Piranga than it does with Chlorospingus (8


vs 4).


Therefore, Phaenicophilus seems to be


more closely related to Piranga than with Chlorospingus, and thus, Phaenicophilus

poliocephalus is derived from P. palmarum.

The alignment of Phaenicophilus to Chloropsingus on the UPGMA phenogram seems

counterintuitive to the above conclusions. Chlorospingus is less similar to Phaenicophilus

based on genetic distance and shared rare alleles. Average genetic distance between two

points in the UPGMA phenogram is influenced by the average distance between clusters and

would probably differ from the absolute genetic distance between two taxa. Average genetic


distance between the species clusters of Phaenicophilus and Chlorospingus


the effect of averaging in the technique.


is lower because of


The polarity of the relationship of the two species of


Phaenicophilus should depend more on its relationship to its closest relative (i.e.,


olivacea) rather than its average relationship to tanagers.

The relationship of Microligea to tanagers and not warblers is clear from the UPGMA


phenogram and the Distance Wagner tree (Fig. 4-1


The relationship remains stable even


when data are jackknifed. If the genetic relationship of Microligea palustris to tanagers is

accepted, then similarities in body size and bill morphology must be due to morphological

convergence and not to their close phylogenetic affinities.


Piranga







74


general foraging ecology, although Microligea palustris is considered more closely related to


the warbler genus Dendroica (A.O.U., 1983).


The morphological and ecological similarities


may result in competitive replacement of one species for the other that essentially result in

allopatric distributions in Haiti. Microligea palustris occurs in the Massif de La Selle and the

northwest peninsula of Haiti (McDonald, 1987) where Geothlypis is rare or absent. In


contrast, Geothlypis is common in the Massif de La Hotte where Microligea is absent.


similarity in general foraging ecology may be indicative of selection for convergence in bill

and body size and shape. However, a detailed analysis of foraging behavior reveals that

Microligea is more similar to tanagers than to warblers (Chapter 2). Interspecific similarities


in morphology


between species may result from evolutionary convergences more often than


do similarities in behavior or genetics. For example, New World vultures resemble Old World

vultures in morphology and foraging ecology. Analyses based on DNA hybridization suggest

that New World vultures are related to storks (Sibley and Ahlquist, 1986). Furthermore, both

New World vultures and storks share a common behavior of defecating on their legs for

thermoregulation. In this example, consideration of the superficial morphological data leads

to the wrong conclusion, while the conclusion based on the concordance of behavior and

genetics seems correct.

The genetic and behavioral relationship of Microligea to tanagers and its similarity in

plumage to Phaenicophilus and Chlorospingus initially suggested the possibility that it

formed a link between Central American and Hispaniolan species. Microligea's alignment

with Piranga may seem perplexing. However, if heterochrony is an important process to the

adaptive radiation of tanagers, interspecific similarities due to convergences would not be

surprising. Microligea did cluster with Chlorospingus based on morphological similarities,

although these species were not closely aligned with Phaenicophilus because of body size







75


species of Phaenicophilus may be due either to retention of a primitive pattern or

heterochrony that leads to convergence. My analysis of plumage patterns cannot distinguish

between these two alternative hypotheses.


Convergence in morphology, relying on repetition of ontogenetic themes,


is more likely


if heterochrony commonly occurs in the adaptive radiation of species (Larson, 1980).


similarities in basic plumage patterns of the four Hispaniolan species, two Phaenicophilus,

Microligea, and Xenoligea, can be explained by assuming convergence due to heterochrony.

Using this explanation, the Hispaniolan species and Chlorospingus probably represent a

paedomorphic assemblage. Characters may be common within an assemblage because of

repetition of ontogenetic themes. Heterochrony may be more prevalent in tanager evolution

than considered previously. For example, the gray crown in Phaenicophilus poliocephalus

may be a terminal deletion with respect to the crown color in its closest antecedent, P.

palmarum, but when compared to the character state in Chlorospingus may represent a

character reversal. If true, then the lack of concordance between the UPGMA phenogram and

the Distance Wagner tree with respect to the affinities of Phaenicophilus could be attributed

to character reversal in P. poliocephalus.

The close affinities of warblers to Eucometis-Coereba require further evaluation by


other molecular techniques.


There may be a closer phylogenetic relationship than previously


thought based on the electrophoretic results.


Warblers do not form a distant sister group to


tanagers as suggested from the DNA hybridization data (Sibley, pers. comm.) but branch off

directly from Eucometis. Jackknifing data from loci or OTU's failed to disturb the stability of


this relationship.


The relationship of warblers to tanagers is closer than that of Euphonia to


other tanagers. Coereba flaueola forms the link between Eucometis and wood warblers and is

not clearly a tanager or a warbler based on the genetic data.







76


Phaenicophilus rather than Spindalis seems justified. Genetic similarities between Thraupis


palmarum and Nemosia pileata were unexpected because of differences in body


size.


Although Thraupis and Nemosia resemble Phaenicophilus in basic plumage patterns, they

are not the closest relatives based on the genetic data (Fig. 4-1).

Intergeneric genetic distances are higher in tanagers than in warblers, though this


may be an artifact of the diversity of tanager genera sampled (9


between congeneric species of warblers (D


congeneric species of tanagers


vs 3). Genetic distances


= 0.238) are greater than those between


= 0.128) and exceed values reported for warblers elsewhere


(Barrowclough and Corbin, 1978; Avise et al., 1980).


The reason for these differences is not


obvious, but could be due to differences in heterozygosity reported in various studies, with

high heterozygosities possibly resulting in higher genetic distances. Average heterozygosity

for Dendroica species is essentially the same as values reported in other studies. Higher

average D values may result from fewer shared alleles between warbler species, although

there are fewer fixed allelic differences between warblers than in tanagers (Table 4-2).

Distances for tanagers fall within the range of those in other avian groups (Johnson and Zink,


1983; Christidis, 1987).

Phaenicophilus. This ih


The smallest genetic distance occurs between species of


s probably due to their recent divergence during the Pleistocene that


may have been facilitated by heterochronic mechanisms that rely on small changes in


regulatory genes.


The largest genetic distance in tanagers is between Piranga ludoviciana


and its North American congeners. Although Piranga ludoviciana superficially resembles

subadult P. olivacea, it more closely resembles P. erythrocephala, a resident species of Mexico

and Central America (R. Greenberg, pers. comm.) and subadult P. rubriceps, a South


American species.


Therefore, P. ludoviciana may not be directly derived from its North


American congeners, but rather from a series of heterochronic speciation events within










evolution.


Tanagers have higher heterozygosities than do other birds and have significantly


higher levels than do warblers.


The higher tanager heterozygosity is not an artifact of the loci


examined, because values for both groups depend upon the same enzymes, and warbler values

are generally the same as those reported elsewhere. Higher genetic variability may be


correlated with differences in life history.


Although life history studies have not been


conducted for all of the tanagers, there may be a number of species with predefinitive

plumages (Isler and Isler, 1987) indicating a more widespread occurrence than is currently

recognized. For example, both Phaenicophilus and Piranga are neotenic tanagers that also


have higher than average levels of heterozygosity.


Thus it is tempting to speculate that high


levels of heterozygosity and the occurrence of heterochrony are correlated. Plasticity in life

history characteristics associated with high genetic variability may allow selection for

heterochrony to occur.

Convergence in a species assemblage might be expected to be more frequent if


heterochrony is a common phenomenon.


The low congruence observed between


morphological and genetic characters is not unique (Zink et al.,


1985) nor is it unexpected for


groups that diverged long ago. Rates of evolution for a variety of characters differ (Schnell

and Selander, 1981). Few studies have demonstrated a strong correlation between the degree

of morphological and genetic divergence in homeotherms as measured by structural gene loci.

Part of the reason for this may be that there is not a strong correlation because quantitative


assessments of structural gene changes may not provide the appropriate measure.


Some


measure of assessing regulatory gene change is necessary. Standard methods of quantifying

structural gene changes may not be appropriate to assess changes in regulatory genes. A

small change in regulatory genes could effect a large change in morphology whereas a change

of similar magnitude in structural gene loci might have little or no affect on external










significant (Mantel test).


The relationship abstracted from the morphological phenograms


are most likely the result of similarities in body size (Fig. 4-2).


The six species in the upper


part of the phenogram have the largest body size; the remaining nine are generally small,


with the exception of Spindalis zena that is intermediate in body size.


When only body-size


related features are analyzed, the relationships among the largest species change little.

When only plumage features are analyzed, striking changes occur in the phenogram. Coereba


flaveola and Dendroica cluster separately; Piranga, Chlorospingus, Phaenicophilus,


Microligea-Spindalis cluster together, because they share similar plumage characters.


similarity in basic plumage pattern may be the result of small changes in regulatory genes


and this


is consistent with the frequent convergences in morphology leading to a lack of


correlation between genetic and morphological features as observed.

Phylogenies should be evaluated by independent sets of characters. Some of these may

be more useful than others in calculating correct phylogenies. In the case of tanagers,

plumage similarities are more likely the result of convergences. Microligea is more closely

allied with tanagers based on the genetic and behavioral evidence (Chapter 2) but based on

body size and bill shape is convergent to warblers. If a choice of competing phylogenetic

hypotheses must be made, patterns based on two sets of characters, such as genetics and

special behaviors, may be more correct than those based on a limited set of morphological

features.


The assessment of the utility of character


sets in defining phylogenies must be based on


an understanding of the limitations of the methodologies used in measuring characters.

Although the biochemical evidence supported the close relationship between Phaenicophilus

and Piranga originally determined by DNA hybridization data (Sibley, pers. comm.), the

conclusions from this evidence conflicted with the higher taxonomic relationships of warblers







79


study, there were sufficient numbers of shared alleles among species to compare the

phylogenies generated from electrophoresis and DNA hybridization. Based on the UPGMA

phenogram and Distance Wagner tree generated from the electrophoretic data, warblers are


more similar to Eucometis penicillata than Eucometis is to Euphonia music.


There are


problems in assessing higher ordered relationships using DNA hybridization data

(Houde,1987; but see Ahlquist etal., 1987). Consequently, higher order relationships remain

ambiguous. Apparent similarities in structural genes used to support higher levels of

taxonomic relationships may be influenced by convergences for electrophoretic mobility more

than true genetic similarities. Future analyses must continue to test the robustness of the

various methodologies and to compare results from molecular techniques to those obtained

from other character sets.









Table 4-1.


List of species, number of specimens, and genetic variability for


25 species of


Emberizidae across


20 assayed enzyme loci.


Number/a,b


Species


Hobs
(S.E.)


Hexp


Piranga olivacea
(Scarlet tanager)


10 F
10 M


0.151
(.046)


0.171
(.046)


summer tanager)


10 F
10 M


0.083
(.049)


0.062


P. ludoviciana "
(Western tanager)

Spindalis zena d


(Stripe-headed tanager)


13 F
22 M


0.170


0.183
(.058)


0.168


0.207
(.060)


Microligea palustris d
(Green-tailed Ground


0.087
(.042)


0.134
(.049)


Warbler)


Chlorospingus
ophthalmicus


19 (.024)


0.066
(.029)


0.082


(Common bush-tanager


10 F
10 M


C. canigularis e
(Ashy-throated
bush-tanager)

Nemosia pileata t
(Hooded Tanager)


0.050
(.050)


0.150
(.082)


0.033


0.150
(.082)


13 M


Euphonia musict
(Antillean Euphonia)


10 F
10 M


0.100
(.069)


0.100
(.069)


P. rubra









Table 4-1.


(Cont'd)


Number/


Hobs


Hexp


Species


Eucometis penicillata c
(Gray-headed Tanager)


10 F
10 M


0.175
(.075)


0.158
(.065)


Coereba flaveola c


(Bananaquit)


10 F
10 M


0.050
(.034)


0.117
(.054)


Thraupis palmarum t


(Palm Tanager


10 F


0.075
(.041)


0.142
(.057)


Hemispingus atropileus C


0.069
(.034)


(Black-capped
Hemispingus)

H. superciliaris0
(Superciliaried
Hemispingus)

Phaenicophilus d


palmarum


(Black-crowned
Palm-Tanager)

P. poliocephalus d
(Gray-crowned
Palm-Tanager)


41 F
58 M


23 F
32 M


0.089
(.038)


0.061


0.025
(.018)


0.120
(.033)


0.099
(.030)


0.118
(.031)


0.114
(.035)


P. palmarum


poliocephalus hybrids


0.073
(.024)


0.068
(.021)


Geothlypis trichas e
(Common Yellowthroat)

Dendrlim a npnsvhnunir* e


0.050
(.039)


0.114
(.047)

n nR7


n 022










Table 4-1. (Cont'd)


Number/ a,b


Species


D. palmarum "
(Palm Warbler)


D. pinus
(Pine Warbler)

D. coronata '
(Yellow-rumped Warbler)

D. dominica c


(Yellow-throated


Warbler)


Mniotilta varia t
(Black-and-white


10 F
10 M


10 F


Hobs
(S.E.)


0.073
(.039)


0.013
(.013)

0.020
(.014)

0.078
(.034)


0.117
(.050)


Hexp
(S.E.)


0.115
(.044)

0.098
(.047)

0.078
(.038)

0.119
(.049)


0.119
(.053)


Warbler)


Vireo griseus '
(White-eyed Vireo)


0.067
(.031)


0.080
(.039)


Note: Observed heterozygosity (Hobs), expected (Hexp), per cent polymorphic loci (P), and
average number of alleles per locus (A) are computed by BIOSYS-1 program (Swofford
and Selander, 1981). First sample size given is for the genetic analysis; sample sizes
listed for male (M) and female (F) are for morphological analysis.


Number and sex of specimens used in analysis. F


= female; M = male; U = unknown.


Specimens used in the morphological analysis were from Harvard University Museum of
Comparative Zoology, Smithsonian Institute U.S. National Museum in Washington, D.C.,
Carnegie Museum of Natural History in Pittsburgh, Philadelphia Academy of Sciences,
and collections made in Haiti in 1985.
Tissue samples provided by Louisiana State Museum of Zoology (LSUMZ) Frozen Tissue
Collection, Robert M. Zink, Curator.


Tissue samples and skins collected in Haiti from May-September, 1985.


Tissues are


deposited in LSUMZ; specimens are deposited in the American Museum of Natural
History in New York and the Florida State Museum in Gainesville.







































































































































































































































































































































































































































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