ArticlePDF Available

S18-2 Phylogenetic studies of plumage evolution and speciation in New World orioles (Icterus)


Abstract and Figures

Detailed molecular phylogenies of closely related species provide an unprecedented opportunity to study the relationship between plumage evolution and speciation. Through reconstruction of ancestral character states, phylogenies enable us to separate convergence from similarity due to shared ancestry, and gains of plumage ornaments from losses. Molecular phylogenies also provide information for inferring the details of speciation: which species split, when the splits occurred, and even whether one species is nested genetically within another. We have used these approaches in a series of studies on plumage evolution and speciation in New World orioles (Icterus). A genus-wide study of 44 individual plumage ornaments revealed evidence of repeated convergence and reversal. Two overall plumage types, moreover, have evolved independently in the three clades of orioles. We then conducted a detailed study focused on the "northern oriole" group. Multiple samples from throughout the ranges of the Baltimore oriole (Icterus galbula; eastern North America) and the black- backed oriole (Icterus abeillei; central Mexico) confirm that these two species are each other's closest relatives, and that they probably split very recently. They differ from each other in many plumage traits, providing a dramatic example of rapid divergence in signal characters. In orioles, it seems likely that much of this plumage divergence occurred in allopatry. Nevertheless, lineages that have evolved plumage differences in allopatry may be less likely to remerge upon secondary contact. Such a process could account for published correlations between signal divergence and species richness.
Content may be subject to copyright.
Acta Zoologica Sinica
© 2006 Acta Zoologica Sinica
S18-2 Phylogenetic studies of plumage evolution and speciation in New
World orioles (Icterus)
, Beatrice KONDO
Dept. of Biological Sciences, University of Maryland, Baltimore County, Baltimore, MD 21250, USA;
Abstract Detailed molecular phylogenies of closely related species provide an unprecedented opportunity to study the
relationship between plumage evolution and speciation. Through reconstruction of ancestral character states, phylogenies
enable us to separate convergence from similarity due to shared ancestry, and gains of plumage ornaments from losses.
Molecular phylogenies also provide information for inferring the details of speciation: which species split, when the splits
occurred, and even whether one species is nested genetically within another. We have used these approaches in a series of
studies on plumage evolution and speciation in New World orioles (Icterus). A genus-wide study of 44 individual plumage
ornaments revealed evidence of repeated convergence and reversal. Two overall plumage types, moreover, have evolved
independently in the three clades of orioles. We then conducted a detailed study focused on the “northern oriole” group.
Multiple samples from throughout the ranges of the Baltimore oriole (Icterus galbula; eastern North America) and the black-
backed oriole (Icterus abeillei; central Mexico) confirm that these two species are each other’s closest relatives, and that they
probably split very recently. They differ from each other in many plumage traits, providing a dramatic example of rapid
divergence in signal characters. In orioles, it seems likely that much of this plumage divergence occurred in allopatry. Nevertheless,
lineages that have evolved plumage differences in allopatry may be less likely to remerge upon secondary contact. Such a
process could account for published correlations between signal divergence and species richness.
Key words Ancestral state reconstruction, Icterus, Phylogeny, Plumage evolution, Speciation
1 Introduction
In recent years, molecular phylogenies based on mi-
tochondrial DNA sequences have provided an unprec-
edented source of information for studying plumage evolu-
tion and speciation. Prior to this technology, phylogenies
for closely related bird species were often not even
attempted. For example, before our studies of the New World
orioles, only one study examined the entire genus of 25+
species, and then without attempting a comprehensive phy-
logeny (Beecher, 1950). Although closely related species of
birds often have well-marked morphological differences,
these differences may involve only one or two plumage
characters. Thus there are generally too few informative
characters to allow construction of well-resolved phylog-
enies for close relatives (Omland and Lanyon, 2000).
Similarly, earlier molecular methods such as DNA-DNA
hybridization, allozyme electrophoresis, and nuclear cod-
ing sequences are generally not sensitive enough to posi-
tion closely related species. Early phylogenies based on
mitochondrial DNA restriction sites and sequences (e.g.,
Kessler and Avise, 1984; Zink and Avise, 1990) opened up
new possibilities for studying species limits, speciation, and
rapidly evolving characters such as plumage and song.
Mitochondrial DNA (mtDNA) sequences continue to
provide the best estimates of phylogenies for closely re-
lated species because nuclear DNA sequences have sev-
eral serious limitations. First, nuclear autosomal DNA has a
larger effective population size than mtDNA, and is there-
fore more likely to share ancestral polymorphisms between
species (Palumbi et al., 2001). Secondly, nuclear DNA does
not accumulate mutations as rapidly as mtDNA, and is
harder to work with than mtDNA, both in the laboratory
and in the analysis stage (Avise, 2000).
Having well-resolved species-level phylogenies from
mtDNA studies provides two main advantages for our un-
derstanding of plumage evolution and speciation. First, we
can use phylogenies to reconstruct ancestral character
states. By scoring the characteristics of present day species,
we can infer the likely evolutionary changes that have taken
place in the past through the principle of parsimony or maxi-
mum likelihood (e.g., Cunningham et al., 1998). For example,
Fig. 1 shows a hypothetical phylogeny of two sister clades
of four species each. The presence or absence of an elabo-
rate plumage ornament is reconstructed on the phylogeny
using simple parsimony (see Omland, 1997).
This reconstruction enables us to infer two key as-
pects of signal evolution. First of all, it provides evidence
of convergent evolution of the ornament: it evolved once in
species B in the left clade, and it also evolved early in the
history of the right clade. Secondly, the phylogeny enables
us to distinguish gains from losses. For example, if A and B
52(Supplement): 320–326, 2006
Kevin E. OMLAND et al.: Speciation and phylogeny of plumages
are sister species, either species A lost the ornament or
species B gained it. Knowing that species C and D are
unornamented sister lineages enables us to infer that there
was a recent gain in species B. Similarly, parsimony recon-
structs a recent loss of the feather ornament in species Y,
which is nested within a group of three species that all have
This example highlights another advantage of phylo-
genetic information: phylogenies can be used to pinpoint
the best species for behavioral studies. For example, spe-
cies B is much better than species W, X or Z in the right
clade for investigating why species gain ornamentation.
The right clade may well have evolved elaborate ornamen-
tation in the common ancestor of that clade but a long time
ago. A behavioral ecologist with no knowledge of the phy-
logeny would have a 75% chance of noticing and studying
the ornament in species W, X or Z, holding him/her back
from inferring the selective forces that led to its origin. Much,
moreover, can be learned by studying species Y, which has
recently lost the ornament. Admittedly, many explicit and
implicit assumptions need to be acknowledged in such an-
cestral state reconstructions (Omland, 1997; Cunningham
et al., 1998; Omland, 1999); yet when applied across many
characters (Omland and Lanyon, 2000) or across multiple
groups, ancestral reconstructions can provide a sound ba-
sis for evolutionary inference.
The other advantage of molecular phylogenies in
studies of speciation and signal evolution is their provision
of sound data on speciation itself. A phylogeny can tell us
which taxa have speciated most recently. For example, with-
out a phylogeny, we might assume that two parapatric taxa
are each other’s closest relatives, and make assumptions
about how the two taxa split. This problem surfaced in the
“northern oriole” group (Omland et al., 1999), as detailed
below. Genetic distances among taxa can also be used to
infer when two species split. Using molecular clocks to date
evolutionary events may be controversial, and indeed many
assumptions go into such calculations which gives rise to
skepticism (Hillis et al., 1996; Fleischer et al., 1998). Even so,
much can be learned about speciation by comparing rela-
tive levels of divergence. A well-known example of how
molecular clocks have been applied to studies of bird spe-
ciation is the work of Klicka and Zink (1997), who showed
that levels of divergence between putative sister species in
the eastern and western US were much deeper than would
be expected if speciation was caused by the most recent
cycles of late-Pleistocene glaciation (cf. Avise and Walker,
1998; Arbogast and Slowinski, 1998).
Molecular data can also provide evidence of the ge-
netic nesting of one species within another (e.g.,
paraphyletic species). Such findings provide a unique op-
portunity to study speciation and character evolution, es-
pecially because it enables reconstruction of character
changes and the timing of speciation with more precision
than allowed by other means. In birds, there are several
cases of likely paraphyly resulting from recent speciation,
involving, among others, mallards (Anas platyrhynchos)
and the American black ducks (Anas rubripes) (Avise et al.,
1990; Omland, 1997), though this example could also reflect
hybridization (Broadsky et al., 1988).
Phylogenies are, in addition, particularly useful in re-
search that employs the comparative method (sensu Harvey
and Pagel, 1991). This paper will not present results based
on the comparative method, but the discussion will address
several studies that have used it to evaluate correlations
between rates of speciation and plumage coloration. Rather,
we simply review results of our research into speciation
and plumage evolution in Icterus.
2 Oriole plumage reconstruction and
2.1 Phylogenetic reconstruction of plumage patterns
Sexually selected characters such as plumage colora-
tion have long been assumed to evolve rapidly and be sub-
ject to high levels of convergence (Omland and Lanyon,
2000). However, no empirical studies of all plumage traits
had been conducted using a well-resolved independent
phylogeny. mtDNA sequences were obtained for 45 taxa of
New World orioles: all 25 recognized species and 20 addi-
tional subspecies from the genus Icterus (Omland et al.,
1999). We obtained over 2000 base pairs of sequence from
the cytochrome b and ND2 genes. All methods of analysis
and data combinations identified three main clades, desig-
nated A, B and C) (Fig. 2). Over half of the nodes on the tree
received 95% bootstrap support or more. This well-resolved
tree provided the phylogenetic framework for reconstruct-
ing plumage evolution (sensu Lanyon, 1993). We studied
two aspects of male plumage coloration: 1) individual feather
areas, and 2) overall plumage patterns.
Using museum skins, we scored all the individual
feather areas that varied among oriole species (Omland and
Lanyon, 2000). We found 44 plumage areas that varied, and
scored whether these areas were white, black or pigmented
Fig. 1 Model phylogeny of nine species showing the most
parsimonious reconstruction of changes in a hypothetical
plumage ornament (e.g., colored wing patch or head crest)
The ancestral state reconstruction suggests two convergent gains
of the elaborate ornament, and one subsequent loss.
Acta Zoologica Sinica
with carotenoid (yellow, orange, chestnut, etc). The 44 plum-
age patches were then mapped on to the molecular phylog-
eny to reconstruct ancestral plumage changes. Forty two
of the 44 plumage characters showed at least some conver-
gence or reversal (homoplasy) (Omland and Lanyon, 2000);
the two characters that did not show any homoplasy in-
volved character states that simply united different sub-
species of the same species. Most plumage characters ap-
peared independently many times on the phylogenetic re-
construction (i.e., high levels of homoplasy). For example,
Fig. 2 incorporates reconstruction of crown coloration, sug-
gesting independent gains of colored crown feathers (e.g.,
orange or yellow) at least six times, and at lease one subse-
quent reversal to black. Other less parsimonious reconstruc-
tions are possible, but clearly individual feather areas in
orioles are evolving rapidly, and with high levels of conver-
gence and reversal.
Reconstruction of overall patterns also revealed much
evidence of convergence and reversal (Omland and Lanyon,
2000). We identified two main plumage types that had
evolved multiple times within the genus Icterus. Species
with the “Baltimore” plumage type have completely black
heads, and consistent white edging in the secondary co-
verts and flight feathers. Species with the “Altamira” plum-
age type have colored heads and crowns, but black fore-
heads and necks, and a distinct white spot on the outer
primaries. These two plumage types represent extremes in a
continuum of plumage convergence values (Omland and
Lanyon, 2000: Fig. 8), involving species that show greater
than 8% sequence divergence and range from only 3 to as
many as 37 differences in plumage. Three species that show
the “Baltimore” type are found in different parts of two
clades: Baltimore oriole (I. galbula) and Scott’s oriole (I.
parisorum) in clade C, and orchard oriole (I. spurius) in
clade A. Species with the “Altamira” type are found in all
three clades: clade A, hooded oriole (I. cucullatus), clade B,
spot-breasted oriole (I. pectoralis), and clade C, Altamira
oriole (I. gularis) (Fig. 2).
The occurrence of both plumage types throughout
the oriole tree strongly suggests convergence, but this pat-
tern could also occur if the mtDNA phylogeny is misleading.
Sequences from a nuclear intron (ODC; Friesen et al., 1999)
from 10 oriole species confirm the basic structure of the
mitochondrial tree, and reveal the same three main clades
(E. S. Allen and K. E. Omland, unpublished data). They also
verify that species within each of the two plumage types
are not each others’ closest relatives, thus providing strong
support for convergence and reversal in producing the two
Fig. 2 Ancestral state reconstruction of crown and nape pigmentation on to the oriole mtDNA phylogeny (from Omland et al.,
1999: Fig. 6)
“Colored” refers to orange or yellow coloration likely to come from carotenoid pigments. Species that exemplify the two overall plumage
types are indicated above the taxon names.
main plumage types.
Our phylogenetic studies of both individual areas and
overall patterns of plumage provide a much clearer picture
of plumage evolution. Plumage characters are indeed chang-
ing very rapidly, probably due to sexual selection. However,
the repeated convergence, reversal, and high levels of ho-
moplasy that we found are generally not predicted by mod-
els of sexual selection (Andersson, 1994; cf. Ryan et al.,
1990). Similarly, convergence would not be predicted if plum-
age divergence was strongly correlated with speciation. If
plumage plays a major role in species recognition, then there
is no reason why unrelated species of Icterus should be
expected to evolve similar plumage areas or overall patterns.
Rather, it seems much more likely that genetic or develop-
mental processes have constrained the numbers of colors
and patterns in New World orioles (Omland and Lanyon,
2000). Individual plumage patches may be changing rapidly
but only according to a restricted set of character states.
Convergence in overall pattern, moreover, may result from
a few genes turning modular plumage elements on and off.
Price and Pavelka (1996) studied plumage patterns in
Old World warblers, and also suggested the importance of
developmental constraints. Such constraints may operate
within many other genera that seem to have an overall plum-
age template with variations on that theme (e.g., Old World
orioles, cardueline finches, Australasian sericornithine
warblers). In contrast, other groups of birds seem much
more free to vary (e.g., Anas ducks, birds of paradise), with
nearly every species evolving novel patterns and
autapomorphic ornaments. Eventually we will need
genomics and other approaches to understand the genetic
and developmental control of plumage coloration in birds
(e.g., Theron et al., 2001). As a first step, we are using spec-
trophotometry and other methods to reconstruct changes
in pigment types and better understand the mechanistic
basis of plumage color and pattern in orioles. Some oriole
species, for example, have colored patches that may not be
carotenoid (C. Hofmann and K. E. Omland, unpublished
2.2 Speciation in the northern oriole group
The “northern oriole” group has served as the focus
for more detailed studies of speciation and plumage
evolution. Three taxa with distinct plumage patterns had
previously been combined in one species, “northern oriole”,
because of hybridization (reviewed in Rising and Flood,
1998). The eastern Baltimore oriole (I. galbula) has an ex-
tensive hybrid zone with the western Bullock’s oriole (I.
bullockii) in the midwestern US. The black-backed oriole
(I. abeillei) from Mexico was also lumped into this group
because it also hybridized with Bullock’s in northern Mexico.
No previous studies had suggested, however, that other
Mexican species such as the streak-backed oriole (I.
pustulatus) might be included in this species group as well.
Our mtDNA phylogeny of the whole genus revealed
some surprising relationships among these species (Fig. 2).
Bullock’s oriole is not at all close to the Baltimore oriole —
the two are over 4% divergent in mtDNA coding sequence.
The only monophyletic group that unites these two spe-
cies also includes six other species from Mexico, South
America and the Caribbean (Omland et al., 1999). The most
surprising outcome was the sister relationship between
black-backed oriole and Baltimore orioles. The two indi-
viduals sequenced were extremely closely related — ap-
proximately 0.5% for the combined cytochrome b and ND2
Because Baltimore and black-backed orioles are so
closely related, they provide an unusual opportunity to in-
vestigate when, where and how speciation may have
occurred. We obtained samples of both taxa from through-
out their respective breeding ranges in North America and
Mexico, and sequenced cytochrome b and the control
region. This extensive sampling revealed extremely small
levels of divergence between the two taxa: there is only a
single base pair substitution in cytochrome b (from over
900 bp sequenced) that separates the most closely related
individuals of the two species (B. Kondo and K. E. Omland,
unpublished). These two species provide the most dramatic
example of rapid plumage divergence in Icterus. Baltimore
and black-backed males differ in 17 individual plumage
areas, and have quite different overall patterns (Omland
and Lanyon, 2000), yet are about as closely related as any
two oriole species can be.
2.3 Reconstructing dichromatism and delayed plumage
maturation in Icterus
We are also using the phylogeny to reconstruct the
history of sexual dichromatism and delayed plumage matu-
ration in the genus. Rigorous scoring of female and imma-
ture plumages requires more subtle methods, including
spectrophotometry. Some general trends are already
emerging. Most tropical oriole species are sexually
monochromatic, both males and females having contrast-
ing and elaborate black, white and carotenoid colored
patterns. It seems likely that many lineages have colonized
temperate habitats through long-distance migration, and
that these species have lost bright female coloration inde-
pendently (K. E. Omland, unpublished data). It also seems
likely that delayed plumage maturation is ancestral for the
genus Icterus, and that a few lineages may have lost it.
Studying the loss of delayed plumage maturation in these
species may provide unique insights into the evolution of
this paradoxical life history characteristic.
3 Discussion
New World orioles have proven to be an excellent
model group for phylogenetic studies of plumage evolu-
tion and speciation. Our mtDNA phylogeny has provided a
firm framework for these studies, showing that individual
feather areas and overall plumage patterns are evolving rap-
idly and convergently. Mitochondrial data are also inform-
ing us about speciation in Icterus, especially in the “north-
ern oriole” group. Baltimore and black-backed orioles have
speciated very recently, and provide a well-documented
Kevin E. OMLAND et al.: Speciation and phylogeny of plumages
Acta Zoologica Sinica
example of just how rapidly bird plumage coloration can
evolve. But what role has speciation played in plumage
divergence; and conversely, how has plumage divergence
helped drive speciation?
During the early years of the biological species
concept, many papers addressed the possible role of bird
plumage coloration in species recognition (e.g., Sibley, 1957;
Mayr, 1963). However, during the 1980s and 1990s this is-
sue was largely neglected, as studies focused on the role of
elaborate plumage in intra-specific mate choice (reviewed
in Andersson, 1994). More recently several comparative
studies have documented a correlation between various
indices of plumage coloration and species richness
(Barraclough et al., 1995; Owens et al., 1999; Panhuis et al.,
2001). Many of these studies used dichromatism as an in-
dex of plumage coloration (e.g., Barraclough et al., 1995;
Owens et al., 1999).
Yet such an index needs to be used with caution.
Dichromatism may work well in groups in which many spe-
cies are cryptic and monomorphic, and the most elaborately
ornamented species strongly dichromatic, such as the spe-
cies of Anas (Omland, 1997). In others, however, it may
work poorly, particularly those such as the New World ori-
oles with bright monomorphic species (e.g., Trail, 1990).
Many oriole species with dramatically contrasting plumage
colors are sexually monochromatic, such as the Altamira
oriole (I. gularis). As a result, orioles would probably have
a fairly low index score despite the fact that they are the
most speciose genus in the Icteridae. Therefore, there is a
conservative bias in the methodology of the index, which
would not account for the significant correlations some-
times documented.
Early studies that pointed out a correspondence be-
tween plumage ornamentation and species richness often
suggested that this relationship was driven by the need for
species recognition, and invoked reproductive character
displacement (e.g., Sibley, 1957). Slight differences that had
arisen in allopatry would be exaggerated through reinforce-
ment of isolating mechanisms in sympatry, thus contribut-
ing to the tremendous plumage diversity, for example, in
prairie regions where many Anas duck species breed sym-
patrically (Sibley, 1957; Mayr, 1963). Under this scenario,
reinforcement drives the evolution of plumage diversity.
However, cases of reproductive character displacement in
birds are not well established (cf. Saetre et al., 1997).
Rather than character displacement, it seems likely
that plumage differences could evolve entirely by sexual
selection in allopatric populations. The extent of these dif-
ferences could then play a prominent role in determining
whether such forms would remerge or not upon secondary
contact. Here plumage diversity helps drive speciation,
rather than the reverse. New World orioles provide several
case studies for considering the options involved. As dis-
cussed above, Baltimore and Bullock’s orioles are over 4%
different in mtDNA sequences, and differ by sixteen dis-
crete plumage differences. Although the two species form
an extensive hybrid zone, recent research indicates that the
zone is stable and quite ancient (Allen, 2002). Thus the two
species show no evidence of merging, and plumage prefer-
ences may play a role in keeping the species distinct
(reviewed in Allen, 2002).
In contrast, we have documented two mitochondrial
clades in the Common Raven (Corvus corax) that also dif-
fer by about 4% in mtDNA sequence (Omland et al., 2000).
Unlike orioles, however, these birds have no plumage
differences, nor any other phenotypic characters that we
know of, which would enable them to distinguish these two
cryptic clades. In this case, we have evidence that the “Cali-
fornia Clade” and “Holarctic Clade” exchange genes fre-
quently throughout the west and may be remerging
(unpublished data). The contrast between orioles and
ravens illustrates how it is still possible to find correlations
between speciation and plumage coloration (e.g.,
Barraclough et al., 1995; Owens et al., 1999) even in the
absence of reproductive character displacement (also see
Price, 1998; Price, this symposium).
It seems likely that sexual selection in allopatry may
indeed drive plumage divergence in Icterus, and that spe-
ciation is driven largely by geographic isolation. For example,
the Jamaican oriole (I. leucopteryx) is largely confined to
the island of Jamaica, where no other orioles are found.
Nevertheless, this species has a highly distinctive
appearance, and differs from other oriole species in at least
seven plumage areas (Omland and Lanyon, 2000). In this
and other island orioles, there is no evidence that isolated
species lose their plumage differences, nor that continental
species sympatric with other oriole species are more diver-
gent in appearance, cf. Anas ducks (cf., Sibley, 1957; Omland,
1997). In fact, several similar-looking species with the
Altamira-type pattern are sympatric throughout much of
their ranges in Mexico and Central America (e.g., Altamira
oriole and hooded oriole).
Nevertheless, much more work is needed to clarify
the role of plumage in species recognition and reproductive
isolation. Furthermore, research needs to focus on whether
and how speciation drives plumage divergence, especially
when considered in combination with other selective forces.
There are several mechanisms that effect evolution of elabo-
rate plumage ornamentation in birds, and there are studies
that support each of them: 1) sexual selection by female
choice, which has been documented in a large number of
bird species (Andersson, 1982; reviewed in Andersson,
1994), including orchard orioles (Enstrom, 1993); 2) sexual
selection for status signaling through male-male aggres-
sive competition, which has never seriously been doubted,
although the number of careful studies that document it is
surprisingly small (e.g., Peek, 1972; Roskraft and Rohwer,
1987; Sorenson and Derrickson, 1994); and 3) the specia-
tion process itself, which has also been supported by a few
studies (Sibley, 1957), although there is really only one well-
documented case of reproductive character displacement
(Saetre et al., 1997). Predator avoidance and other processes
may also play a role (Dumbacher et al., 1992; Götmark, 1992).
Studies are needed that consider the continuum of
mate choice decisions, from relative choices between oth-
erwise acceptable conspecifics to threshold choices against
unacceptable conspecifics, and choices that include indi-
viduals of other populations, races, or species (see Ryan,
1990). All such studies should be careful to emphasize indi-
vidual fitness; there are cases when choosing to mate with
heterospecifics may make the best of a bad situation (e.g.,
Nuechterlein and Buitron, 1998; Veen et al., 2001), or may
actually lead to increased offspring fitness (Grant and Grant,
1996). Ultimately it will be valuable to know for at least some
individual species, what roles female choice, male-male
aggression, species recognition, and other processes have
had in driving and maintaining the evolution of plumage
and other signals. Similarly, it will be helpful to know the
percentage of birds in which female choice, male-male ag-
gression or species recognition has played the dominant
role, and whether different mechanisms prevail over one
another and in what circumstances. These are ambitious
and long range goals, but now that there is good evidence
for each of the mechanisms, more knowledge of their rela-
tive importance is needed, and of the interactions between
Acknowledgements The US National Science Foundation
provided funding for this research (DEB-0004400). R. C.
Fleischer, R. Greenberg and S. M. Lanyon provided logistical
and financial support for tissue collection. We thank the
faculty and students at Museo de Zoología, UNAM,
Mexico, for assistance in collecting black-backed orioles.
Many other museums, institutions and individuals have
provided tissue loans for our oriole research projects. Jeff
Peters and Chris Hofmann provided helpful comments on
the manuscript. We thank Ian Owens and Trevor Price for
inviting us to participate in this symposium.
Allen ES, 2002. Long-term Hybridization and the Maintenance of
Species Identity in Orioles (Icterus). Unpublished PhD Thesis.
Bloomington, USA: Indiana University.
Amundsen T, 2000. Why are female birds ornamented? Trends Ecol.
Evol. 15: 149–155.
Andersson M, 1994. Sexual Selection. Princeton, New Jersey:
Princeton University Press.
Andersson MB, 1982. Female preference selects for extreme tail
length in a widowbird. Nature 299: 818–820.
Arbogast BS, Slowinski JB, 1998. Pleistocene speciation and the
mitochondrial DNA clock. Science 282: 1955a.
Avise JC, 2000. Phylogeography: the History and Formation of
Species. Cambridge, Mass.: Harvard University Press.
Avise JC, Ankney CD, Nelson WS, 1990. Mitochondrial gene trees
and the evolutionary relationship between mallard and black
ducks. Evolution 44: 1 109–1 119.
Avise JC, Walker D, 1998. Pleistocene phylogeographic effects on
avian populations and the speciation process. Proc. Roy. Soc.
Lond. B 265: 457–463.
Barraclough TG, Harvey PH, Nee S, 1995. Sexual selection and
taxonomic diversity in passerine birds. Proc. Roy. Soc. Lond. B
259: 211–215.
Beecher WJ, 1950. Convergent evolution in the American orioles.
Wilson Bull. 62: 51–86.
Broadsky LM, Ankney CD, Nelson WS, 1988. Social experience
influences preferences in black ducks and mallards. Can. J. Zool.
67: 1 434–1 438.
Cunningham CW, Omland KE, Oakley TH, 1998. Reconstructing
ancestral character states: a critical reappraisal. Trends Ecol.
Evol. 13: 361–366.
Dumbacher JP, Beehler BM, Spande TF, Garraffo HM, Daly JW,
1992. Homobatrachotoxin in the genus Pitohui: chemical de-
fense in birds? Science 258: 799–801.
Enstrom DA, 1993. Female choice for age-specific plumage in the
orchard oriole: implications for delayed plumage maturation.
Anim. Behav. 45: 435–442.
Fleischer RC, McIntosh CE, Tarr CL, 1998. Evolution on a volcanic
conveyor belt: using phylogeographic reconstructions and K-
Ar-based ages of the Hawaiian Islands to estimate molecular
evolutionary rates. Molec. Ecol. 7: 533–545.
Freeman S, Zink RM, 1995. A phylogenetic study of the blackbirds
based on variation in mitochondrial DNA restriction sites. Syst.
Biol. 44: 409–420.
Friesen VL, Congdon BC, Kidd MG, Birt TP, 1999. Polymerase
chain reaction (PCR) primers for the amplification of five nuclear
introns in vertebrates. Molec. Ecol. 8: 2 147–2 149.
Götmark F, 1992. Anti-predator effect of conspicuous plumage in a
male bird. Anim. Behav. 44: 51–55.
Grant BR, Grant PR, 1996. High survival of Darwin’s finch hybrids:
effects of beak morphology and diets. Ecology 77: 500–509.
Harvey PH, Pagel MD, 1991. The comparative method in evolu-
tionary biology. Oxford: Oxford University Press.
Hillis DM, Mable BK, Moritz C, 1996. Applications of molecular
systematics: the state of the field and a look to the future. In:
Hillis DM, Moritz C, Mable BK ed. Molecular Systematics, 2nd
edn. Sunderland, MA: Sinauer Assoc., 515–543.
Kessler LG, Avise JC, 1984. Systematic relationships among water-
fowl (Anatini) inferred from restriction endonuclease analysis
of mitochondrial DNA. Syst. Zool. 33: 370–380.
Klicka J, Zink RM, 1997. The importance of recent ice ages in
speciation: a failed paradigm. Science 227: 1 666–1 669.
Lanyon SM, 1993. Phylogenetic frameworks: towards a firmer foun-
dation for the comparative approach. Biol. J. Linn. Soc. 49: 45–
Mayr E, 1963. Animal Species and Evolution. Cambridge, Mass.:
Belknap Press.
Moore WS, 1995. Inferring phylogenies from mtDNA variation:
mitochondrial-gene trees versus nuclear-gene trees. Evolution
49: 718–726.
Nuechterlein GL, Buitron D, 1998. Interspecific mate choice by
late-courting male western grebes. Behav. Ecol. 9: 313–321.
Omland KE, 1997. Examining two standard assumptions of ances-
tral reconstructions: repeated loss of dimorphism in dabbling
ducks (Anatini). Evolution 51: 1 636–1 646.
Omland KE, 1999. The assumptions and challenges of ancestral
state reconstructions. Syst. Biol. 48: 604–611.
Omland KE, Lanyon SM, 2000. Reconstructing plumage evolution
in orioles (Icterus): repeated convergence and reversal in patterns.
Evolution 54: 2 119–2 133.
Omland KE, Lanyon SM, Fritz SJ, 1999. A molecular phylogeny of
the New World Orioles (Icterus): the importance of dense taxon
sampling. Molec. Phylog. Evol. 12: 224–239.
Omland KE, Tarr CL, Boarman WI, Marzluff JM, Fleischer RC,
2000. Cryptic genetic variation and paraphyly in ravens. Proc.
Roy. Soc. London B 267: 2 475–2 482.
Owens IPF, Bennett PM, Harvey PH, 1999. Species richness among
birds: Body size, life history, sexual selection or ecology? Proc.
Royal Soc. Lond. B 266: 933–939.
Palumbi SR, Cipriano F, Hare MP, 2001. Predicting nuclear gene
coalescence from mitochondrial data: the three-times rule. Evo-
lution 55: 859–868.
Panhuis TM, Butlin R, Zuk M, Tregenza T, 2001. Sexual selection
and speciation. Trends Ecol. Evol. 16: 364–371.
Peek FW, 1972. An experimental study of the territorial function of
vocal and visual displays in the male red-winged blackbird
Kevin E. OMLAND et al.: Speciation and phylogeny of plumages
Acta Zoologica Sinica
(Agelaius phoeniceus). Anim. Behav. 20: 112–118.
Price T, 1998. Sexual selection and natural selection in bird speciation.
Phil. Trans. R. Soc. Lond. B 353: 251–260.
Price T, Pavelka M, 1996. Evolution of a colour pattern: history,
development and selection. J. Evol. Biol. 9: 451–470.
Rising JD, Flood NJ, 1998. Baltimore Oriole (Icterus galbula). In:
Poole A, Gill F ed. The Birds of North America Philadelphia, PA:
The Birds of North America, Inc.
Roskraft E, Rohwer S, 1987. An experimental study of the function
of the red epaulettes and the black body colour of male red-
winged blackbirds. Anim. Behav. 35: 1 070–1 077.
Ryan MJ, 1990. Signals, species, and sexual selection. Amer. Sci. 78:
Ryan MJ, Fox JH, Wilczynski W, Rand AS, 1990. Sexual selection
for sensory exploitation in the frog, Physalaemus pustulosus.
Nature 343: 66–67.
Saetre GP, Moum T, Bures S, Kral M, Adamjan M, Moreno J, 1997.
A sexually selected character displacement in flycatchers rein-
forces premating isolation. Nature 387: 589–592.
Shields GF, Adams D, Garner G, Labelle M, Peitsch J, Ramsay M,
Schwartz C, Titus K, Williamson S, 2000. Phylogeography of
mitochondrial DNA variation in brown bears and polar bears.
Molec. Phylog. Evol. 15: 319–326.
Sibley CG, 1957. The evolutionary and taxonomic significance of
sexual dimorphism and hybridization in birds. Condor 59: 166–
Sorenson LG, Derrickson SR, 1994. Sexual selection in the northern
pintail (Anas acuta): the importance of female choice versus
male-male competition in the evolution of sexually selected
traits. Behav. Ecol. Sociobiol. 35: 389–400.
Theron E, Hawkins K, Bermingham E, Ricklefs RE, Mundy NI,
2001. The molecular basis of an avian plumage polymorphism
in the wild: a melanocortin-1-receptor mutation is perfectly
associated with the melanic plumage morph of the bananaquit
(Coerebra flaveola). Current Biology 11: 550–557.
Trail PW, 1990. Why should lek-breeders be monomorphic. Evolu-
tion 44: 1 837–1 852.
Veen T, Borge T, Griffith SC, Saetre GP, Bures S, Gustafsson L,
Sheldon BC, 2001. Hybridization and adaptive mate choice in
flycatchers. Nature 411: 45–50.
Zink RM, Avise JC, 1990. Patterns of mitochondrial DNA and
allozyme evolution in the avian genus Ammodramus. Syst. Zool.
... First, it could be related to species recognition, for example with these color differences evolving through reproductive character displacement (e.g., Saetre et al. 1997). Second, it could be that species that evolve color differences in allopatry are able to become sympatric (Omland and Kondo 2006). In the first mechanism, species recognition drives the evolution of color changes, whereas in the second mechanism, color changes that arise due to other causes such as sexual selection or drift, facilitate later sympatry (Omland and Kondo 2006). ...
... Second, it could be that species that evolve color differences in allopatry are able to become sympatric (Omland and Kondo 2006). In the first mechanism, species recognition drives the evolution of color changes, whereas in the second mechanism, color changes that arise due to other causes such as sexual selection or drift, facilitate later sympatry (Omland and Kondo 2006). The role of species recognition in driving color differences, or color differences facilitating species persistence should be pursued in future behavioral studies across caciques, or future comparative studies of Icterids and other birds. ...
Full-text available
Carotenoid-based colors have signaling roles in a range of animal taxa, yet little is known about how carotenoids vary across species or whether overall patterns may vary across genera. The caciques (Cacicus, Clypicterus and Ocyalus; family Icteridae), a group of passerine birds from Central and South America, appear to visually divide into discrete yellow and red color groups. Reflectance spectrometry supported this observation, showing widely separated short and long-wavelength groups corresponding with yellow and red-feathered taxa. Ancestral state reconstructions inferred a yellow ancestral state with two independent changes to red, and no reversals back to yellow. This pattern contrasts with that of another closely related icterid clade, the New World orioles (Icterus), which exhibit a full range of carotenoid colors and have a pattern of evolution that is continuous and extremely labile. To our knowledge, this study is the first to highlight the possibility of different modes of color evolution even among closely related clades. This study also emphasizes the importance of the methods and assumptions of ancestral state reconstruction. In particular, although coloration and other characters can be measured along a continuous scale, they should be reconstructed using discrete methods when data from extant taxa and underlying mechanisms suggest discrete changes.
... If the clades represent different adaptive lineages in our study area, then barriers to gene flow should exist and genetic divergence should parallel differences in phenotypic characters and individual fitness. Furthermore, differences in mating signals such as calls or behaviour could have accumulated as a result of genetic drift during a period of allopatry that might prevent interbreeding even among ecologically similar or identical groups (Mendelson & Shaw 2002;Mendelson 2003;Omland & Kondo 2006;Price 2008). In this study, we evaluated evidence for phenotypic character divergence paralleling divergence in mtDNA between Common Ravens with Holarctic and California clade mtDNA. ...
... The deep divergence of the mtDNA clades of the Common Raven suggests that the clades diverged between 3.5 and 1.7 Ma (Feldman & Omland 2005; see Price 2010), but remerging of the lineages suggests that any divergence experienced in isolation was not strong enough to cause substantial adaptive divergence. Signal traits may also contribute to the likelihood of speciation (Mendelson & Shaw 2002;Omland & Kondo 2006;Mendelson et al. 2007;Price 2008). Both vocalizations and plumage are much conserved across the genus Corvus (Madge & Burn 1994), with greater than 50% of the species being monomorphically black. ...
Full-text available
DNA sequence studies frequently reveal evidence of cryptic lineages in morphologically uniform species, many of which turn out to be evolutionarily distinct species. The Common Raven (Corvus corax) includes two deeply divergent mtDNA lineages: one lineage seems restricted to western North America and the other is Holarctic in distribution. These deep clades hint of the possibility of cryptic species in the western United States. We tested this hypothesis in a population consisting of an equal proportion of both mtDNA clades, by quantifying mating patterns and associated fitness consequences with respect to mtDNA. We also tested for morphological, behavioural and ecological correlates of sex and mtDNA clade membership. Mate pairings were random with respect to mtDNA clades, and there were no differences in reproductive success between assortatively and nonassortatively mated pairs. We found no differences in survival or resource use between clades. There were no differences in morphological or behavioural characters between mtDNA clades, except one clade trended towards greater mobility. These results suggest there are no barriers to gene flow between mtDNA clades and argue that the mtDNA clades have remerged in this population, likely due to a lack of ecological or signal differentiation between individuals in each lineage. Hence, in Common Ravens, phylogeographic structure in mtDNA is a reflection of likely past isolation rather than currently differentiated species.
... Among lineages, class Aves generally shows high levels of shared ancestry due to its strong dispersal ability 15 , high chromosomal stasis 16 , and relatively slow development of post-zygotic breeding barriers 17,18 . These characteristics of avian evolution can result in species complexes that readily hybridize, as well as create discord in genomic and morphological divergence patterns [9][10][11]19 . Taxa within these groups often share large ancestral portions of their genome, which is readily passed between hybridizing groups, while only few genomic regions under directional selection are responsible for maintaining species boundaries 11,20-22 . ...
Full-text available
Causes for genomic and morphological similarities among recently radiated species are often multifaceted and are further convoluted among species that readily interbreed. Here, we couple genomic and morphological trait comparisons to test the extent that ancestry and gene flow explain the retention of mallard-like traits within a sister species, the Mexican duck. First, we confirm that these taxa remain genetically structured, and that Mexican ducks exhibit an isolation-by-distance pattern. Despite the assumption of wide-spread hybridization, we found only a few late-stage hybrids, all from the southwestern USA. Next, assessing 23 morphological traits, we developed a genetically-vetted morphological key that is > 97% accurate in distinguishing across sex-age cohorts of Mexican ducks, mallards, and hybrids. During key development, we determined that 25% of genetically pure, immature male Mexican ducks of the northern population naturally displayed mallard-like traits in their formative plumage. In fact, applying this key to 55 museum specimens, we identified that only four of the 14 specimens originally classified as phenotypic hybrids were truly hybrids. We discuss how genomic and morphological comparisons shed light into the mechanism(s) underlying the evolution of complex phenotypic traits in recent radiations, and how misunderstanding the true morphological diversity within Mexican ducks resulted in taxonomic revisions that hindered conservation efforts.
... Even intergeneric hybrids have been recorded in many avian families [34]. As a result, cases of allopatric speciation followed by secondary contact and hybridization are prevalent in birds [35][36][37][38]. Even though the large southwestern doves are not each other's closest relatives [39,40], their recent extensive secondary contact following biogeographic histories of substantial allopatry may be producing opportunities for hybridization and introgression. ...
Full-text available
Introductions and invasions provide opportunities for interaction and hybridization between colonists and closely related native species. We investigate this phenomenon using the mitochondrial DNA COI and 81,416 base-pairs of overlapping nuclear variation to examine the evolutionary histories and signatures of hybridization among introduced feral Rock Pigeon and Eurasian Collared-Dove and native White-winged and Mourning doves in southwestern North America. First, we report all four species to be highly divergent across loci (overall pair-wise species ΦST range = 0.17–0.70) and provide little evidence for gene flow at evolutionary timescales. Despite this, evidence from multiple population genetics analyses supports the presence of six putative contemporary late-stage hybrids among the 182 sampled individuals. These putative hybrids contain various ancestry combinations, but all involve the most populous species, the Mourning Dove. Next, we use a novel method to reconstruct demographic changes through time using partial genome sequence data. We identify recent, species-specific fluctuations in population size that are likely associated with changing environments since the Miocene and suggest that these fluctuations have influenced the genetic diversity of each dove species in ways that may impact their future persistence. Finally, we discuss the importance of using multiple marker types when attempting to infer complex evolutionary histories and propose important considerations when analyzing populations that were recently established or of domestic origins.
... Orioles are a much more speciose group, however, with 25 recognized species in comparison to 11 recognized oropendola species ( Sibley and Monroe 1990), so more speciation and/or fewer extinctions have apparently occurred during oriole evolution. More speciation could be correlated with more evolutionary changes in both song and plumage ( Slabbekorn and Smith 2001;Omland and Kondo 2006), which in turn could have provided more opportunities for evolutionary convergence and reversal given a limited range of potential character states. Reconstructing the evolution of plumage patterns in oropendolas using methods similar to those used for orioles by Omland and Lanyon (2000) will be needed to see whether the differences we found in song evolution between these clades are found in the evolution of plumage patterns as well. ...
Full-text available
Both song and color patterns in birds are thought to evolve rapidly and exhibit high levels of homoplasy, yet few previous studies have compared the evolution of these traits systematically using the same taxa. Here we reconstruct the evolution of song in the New World orioles (Icterus) and compare patterns of vocal evolution to previously reconstructed patterns of change in plumage evolution in this clade. Individual vocal characters exhibit high levels of homoplasy, reflected in a low overall consistency index (CI = 0.27) and retention index (RI = 0.35). Levels of lability in song are comparable to those found for oriole plumage patterns using the same taxa (CI = 0.31, RI = 0.63), but are strikingly dissimilar to the conservative patterns of change seen in the songs of oropendolas (Psarocolius, Ocyalus; CI = 0.82, RI = 0.87), a group closely related to the orioles. Oriole song is also similar to oriole plumage in exhibiting repeated convergence in overall patterns, with some distantly related taxa sounding remarkably similar. Thus, both song and plumage in orioles show repeated convergence in individual elements and in overall patterns across the clade, suggesting that both of these character classes are highly labile between taxa yet highly conserved within the genus. Our results provide new insights into the tempo and mode of evolution in sexually selected traits within and across clades.
... The Baltimore oriole and black-backed oriole differ in plumage as well as in migratory distance. Male plumage between these species is unusually divergent for species so closely related (Omland and Lanyon 2000;Kondo et al. 2004;Omland and Kondo 2006, also see Price et al. 2007), suggesting that there may have been strong selection on male black-backed oriole plumage after the founder event. If so, this would be another example of rapid phenotypic change following a change in migration and breeding location, as predicted by the model of speciation through loss of migration. ...
A widely accepted paradigm is that sedentary Neotropical bird species are a reservoir that gives rise to temperate-tropical migratory species. Recently, an alternative theory has been proposed, that developmental plasticity can allow some individuals within a migratory species to establish a disjunct breeding range through loss of migration, thus facilitating the founding of a new sedentary species. We used mtDNA and two nuclear introns to perform coalescent analyses for two closely related New World oriole species, one a long-distance temperate-tropical migrant and the other a short-distance intratropical migrant. Our results suggest that the short-distance migrant recently diverged from the long-distance migrant via a founder event. In this species pair, the widely accepted paradigm is not supported. These results are consistent with a model of speciation through reduction of migratory distance.
Sexual selection has been hypothesized to promote speciation, but evidence relating sexual selection to differences in speciation rates among taxa is equivocal. We note that evolutionary changes in ornaments are the link connecting sexual selection to speciation, and that ornament evolution is influenced by many factors so that its relationship with the strength of sexual selection may not be linear. We test if the evolution of ornamental coloration in Carduelis finches is related with speciation and if more ornamented lineages speciate more. We found that coloration evolves with a speciational pattern, but we found no evidence that the evolutionary changes associated with speciation are predominantly gains in ornamentation. The speciational pattern was found for both carotenoid- and melanin-based coloration, suggesting that traits putatively under stronger sexual selection by female choice (carotenoid coloration) are not the sole ones facilitating reproductive isolation. We conclude that in the genus Carduelis the evolutionary lability of ornaments influences speciation more than the strength of sexual selection, and we suggest that ornament lability should be considered as a possible causal factor in studies comparing cladogenesis among taxa.
Although phylogenetic reconstruction of ancestral character states is becoming an increasingly common technique for studying evolution, few researchers have assessed the reliability of these reconstructions. Here I test for congruence between a phylogenetic reconstruction and a widely accepted scenario based on independent lines of evidence. I used Livezey's (1991) phylogeny to reconstruct ancestral states of plumage dichromatism in dabbling ducks (Anatini). Character state mapping reconstructs monochromatic ancestors for the genus Anas as well as most of its main clades. This reconstruction differs strongly from the widely accepted scenario of speciation and plumage evolution in the group (e.g., Delacour and Mayr 1945; Sibley 1957). This incongruence may occur because two standard assumptions of character state reconstruction are probably not met in this case. Violating either of these two assumptions would be a source of error sufficient to create misleading reconstructions. The first assumption that probably does not apply to ducks is that terminal taxa, in this case species, are monophyletic. Many of the widespread dichromatic species of ducks may be paraphyletic and ancestral to isolated monochromatic species. Three lines of evidence support this scenario: population-level phylogenies, biogeography, and vestigial plumage patterns. The second assumption that probably does not apply to duck plumage color is that gains and losses of character states are equally likely. Four lines of evidence suggest that dichromatic plumage might be lost more easily than gained: weak female preferences for bright male plumage, biases toward the loss of sexually dichromatic characters, biases toward the loss of complex characters, and repeated loss of dichromatism in other groups of birds. These seven lines of evidence support the accepted scenario that widespread dichromatic species repeatedly budded off isolated monochromatic species. Drift and genetic biases probably caused the easy loss of dichromatism in ducks and other birds during peripatric speciation. In order to recover the accepted scenario using Livezey's tree, losses of dichromatism must be five times more likely than gains. The results of this study caution against the uncritical use of unordered parsimony as the sole criterion for inferring ancestral states. Detailed population-level sampling is needed and altered transformation weighting may be warranted in ducks and in many other groups and character types with similar attributes.
An accurately resolved gene tree may not be congruent with the species tree because of lineage sorting of ancestral polymorphisms. DNA sequences from the mitochondrially encoded genes (mtDNA) are attractive sources of characters for estimating the phylogenies of recently evolved taxa because mtDNA evolves rapidly, but its utility is limited because the mitochondrial genes are inherited as a single linkage group (haplotype) and provide only one independent estimate of the species tree. In contrast, a set of nuclear genes can be selected from distinct chromosomes, such that each gene tree provides an independent estimate of the species tree. Another aspect of the gene-tree versus species-tree problem, however, favors the use of mtDNA for inferring species trees. For a three-species segment of a phylogeny, the branching order of a gene tree will correspond to that of the species tree if coalescence of the alleles or haplotypes occurred in the internode between the first and second bifurcation. From neutral theory, it is apparent that the probability of coalescence increases as effective population size decreases. Because the mitochondrial genome is maternally inherited and effectively haploid, its effective population size is one-fourth that of a nuclear-autosomal gene. Thus, the mitochondrial-haplotype tree has a substantially higher probability of accurately tracking a short internode than does a nuclear-autosomal-gene tree. When an internode is sufficiently long that the probability that the mitochondrial-haplotype tree will be congruent with the species tree is 0.95, the probability that a nuclear-autosomalgene tree will be congruent is only 0.62. If each of k independently sampled nuclear-gene trees has a probability of congruence with the species tree of 0.62, then a sample of 16 such trees would be required to be as confident of the inference based on the mitochondrial-haplotype tree. A survey of mtDNA-haplotype diversity in 34 species of birds indicates that coalescence is generally very recent, which suggests that coalescence times are typically much shorter than internodal branch lengths of the species tree, and that sorting of mtDNA lineages is not likely to confound the species tree. Hybridization resulting in transfer of mtDNA haplotypes among branches could also result in a haplotype tree that is incongruent with the species tree; if undetected, this could confound the species tree. However, hybridization is usually easy to detect and should be incorporated in the historical narrative of the group, because reticulation, as well as cladistic events, contributed to the evolution of the group.
To evaluate the potential of mitochondrial DNA (mtDNA) analysis for avian systematics, we have assayed mtDNA differences among 13 species of waterfowl in the genera Anas and Aythya (Anseriformes: Anatidae). Purified mtDNA was digested with each of 15 different type II restriction endonucleases which cleave at five- or six-base recognition sequences. Side-by-side comparisons of digestion profiles permitted the estimation of levels of fragment homology and nucleotide sequence divergence (p). Among nine Anas species, mean sequence divergence (p) was 0.062 (range 0.004-0.088); among four Aythya species, 0.034 (0.025-0.043); between selected species belonging to separate genera, 0.109. Phylogenetic trees and dendrograms were constructed from qualitative and quantitative data bases by a variety of procedures including undirected parsimony (Penny and Wagner algorithms), undirected compatibility (Estabrook algorithms), and phenetic clustering. These trees were highly concordant with one another, and with traditional phylogenies derived from independent sources of information. Previously published evidence from mammals has suggested a "saturation effect" on level of mtDNA differentiation: for p less than 0.15-0.20, mtDNA distances are reportedly linearly related to time since common ancestry, but for larger p values the relationship becomes curvilinear as differentiation approaches an observed plateau at a p of approximately 0.30. Our estimates of mtDNA sequence divergence among congeneric waterfowl fall in a broad range well within the expected linear portion of the curve. This observation, coupled with the general concordance of mtDNA-generated trees with those derived from independent information, demonstrates that the restriction fragment approach to mtDNA analysis should provide an important new molecular technique for studying evolutionary relationships among lower taxonomic levels in Aves.
Three species of Darwin's finches (Geospiza fortis, G. scandens, and G. fuliginosa) hybridize rarely on the small Galapagos island of Daphne Major. Following the exceptionally severe El Nino event of 1982-1983, hybrids survived as well as, and in some cases better than, the parental species during dry seasons of potential food limitation. They also backcrossed to two of the parental species. This study was undertaken to compare the diets of hybrids with the diets of the parental species in order to assess possible reasons for the high hybrid survival. Diets of F hybrids and first generation backcrosses to G. fortis were intermediate between the diets of the respective parental species. Distinctiveness of the hybrid diets was most pronounced where the diets of the parental species differed most. A strong determinant was beak morphology; hybrids inherit beak traits from both parents, and, on average, have intermediate beak sizes. Among the combined groups of species and hybrids, and among the hybrids alone, dietary characteristics covaried with beak morphology. Hybrids that differ most from G. fortis in beak morphology, the G. fortis x G. scandens F hybrids, experience a feeding efficiency advantage when feeding on Opuntia echios seeds, commonly consumed in the dry season. These findings are used to interpret the higher survival of hybrids after 1983 than beforehand. The El Nino event that year led to an enduring (10-yr) change in the habitat and plant composition of the island. A decrease in absolute and relative abundance of large and hard seeds apparently caused relatively high mortality among G. scandens and the largest G. fortis individuals. Hybrids were favored by an abundance of small seeds. The high survival of G. fortis x G. scandens F hybrids may have been due, additionally, to a broad diet and to efficient exploitation of Opuntia seeds. The study demonstrates long-term ecological and evolutionary consequences of large-scale fluctuations in climate, and the role of ecological (food) factors in determining hybrid fitness.
We assayed restriction site differences in mitochondrial DNA (mtDNA) within and among allopatric populations of the Mallard (Anas platyrhynchos) and the American Black Duck (A. rubripes). The observed mtDNA clones grouped into two phylogenetically distinct arrays that we estimate differ by about 0.8% in nucleotide sequence. Genotypes in one clonal array were present in both species, while genotypes in the other array were seen only in Mallards. In terms of the mtDNA "gene tree," the assayed Mallards exhibit a paraphyletic relationship with respect to Black Ducks, meaning that genealogical separations among some extant haplotypes in the Mallard predate the species separation. Evidence is advanced that this pattern probably resulted from demographically based processes of lineage sorting, rather than recent, secondary introgressive hybridization. However, haplotype frequencies were most similar among conspecific populations, so the Mallard and Black Ducks cluster separately in terms of a population phenogram. The results provide a clear example of the distinction between a gene tree and a population tree, and of the distinction between data analyses that view individuals versus populations as operational taxonomic units (OTUs). Overall, the mtDNA data indicate an extremely close evolutionary relationship between Mallards and Black Ducks, and in conjunction with the geographic distributions suggest that the Black Duck is a recent evolutionary derivative of a more broadly distributed Mallard-Black ancestor.
HE problems of adaptive convergence in bird taxonomy present many fascinating and often neglected aspects. The present paper describes the evidence for convergence of 2 genera of American orioles that are scarcely distinct but apparently arise from opposite ends of the variable blackbird genus Agelaius, and show hitherto unsuspected evolutionary trends toward nearly exact resemblance. Sclater (1883), Ridgway (1902), and Hellmayr (1937) placed the orioles under a single genus (Icterus), and Hellmayr's nomenclature is followed here with indicated exceptions. However, evidence from functional anatomy and field study indicates that 2 phyletic lines are involved. It is proposed to retain the genus Icferus Brisson for the line to which the Baltimore Oriole belongs but a new name is needed for the line embracing the Orchard and Cayenne Orioles. The latter apparently arises virtually without plumage change from the black Agelaius thilius in the pampas region of South America. For this genus with its slender, nectar-adapted bill, the rather appropriate name Bananivorus Bonaparte seems to be the earliest available. Icterus, on the other hand, appears to arise in the same region with little plumage change from Xanthopsar-a yellow blackbird formerly included in Agelaius. It is primarily a fruit-eating genus with a straight, conical bill, though the occurrence in this line of forms secondarily adapted for nectar has caused much confusion in the above reviews. Convergence comes about when northern forms of Icterus reduce the amount of yellow while those of Bananivorus reduce the black. It is the principle aim of this paper to interpret this convergence in terms of selection pressure and environmental change. Osteological and anatomical specimens used in this investigation have been obtained from the collections of the United States National Museum, the American Museum of Natural History, the Museum of Vertebrate Zoology and, primarily, from the Chicago Natural History Museum. The bird skins used are entirely from the collection of the last museum. For use of the collec- tions in their care, for suggestions or services, I am deeply indebted to Alexander
Many authors have suggested that sexual selection by female choice may increase the speciation rate and hence generate taxonomic diversity. Using sister taxa comparisons, we find a significant positive correlation between the proportion of sexually dichromatic species within taxa of passerine birds, and the number of species in those taxa. Theory predicts this result if sexual dichromatism in passerines has evolved through the action of female choice.
In mixed populations of western and Clark's grebes, advertising calls by males and females play a critical role in mate choice and reproductive isolation. We conducted field playback experiments that tested whether courting western grebe males became less choosy in their responses to female Clark's grebe calls as the mating season progressed and mating opportunities diminished. Late-courting western grebe males were much more likely to answer and approach advertising calls of Clark's grebe females than were males courting earlier in the season. This change in responsiveness occurred as the operational sex ratio index of the population approached 3:1 male calls per female call. These and field census data support the hypothesis that late-season hybridization between these two closely related species may be a result not of species misidentification, but of active and adaptive mate choice by individuals with limited alternatives.