ECOLOGICAL ADAPTATION AND SPECIES RECOGNITION DRIVES VOCAL EVOLUTION IN NEOTROPICAL SUBOSCINE BIRDS

The causes and evolutionary consequences of mixed singing in two hybridizing songbird species (Luscinia spp.)

Bird song plays an important role in the establishment and maintenance of prezygotic reproductive barriers. When two closely related species come into secondary contact, song convergence caused by acquisition of heterospecific songs into the birds’ repertoires is often observed. The proximate mechanisms responsible for such mixed singing, and its effect on the speciation process, are poorly understood. We used a combination of genetic and bioacoustic analyses to test whether mixed singing observed in the secondary contact zone of two passerine birds, the Thrush Nightingale (Luscinia luscinia) and the Common Nightingale (L. megarhynchos), is caused by introgressive hybridization. We analysed song recordings of both species from allopatric and sympatric populations together with genotype data from one mitochondrial and seven nuclear loci. Semi-automated comparisons of our recordings with an extensive catalogue of Common Nightingale song types confirmed that most of the analysed sympatric Thrush Nightingale males were ‘mixed singers’ that use heterospecific song types in their repertoires. None of these ‘mixed singers’ possessed any alleles introgressed from the Common Nightingale, suggesting that they were not backcross hybrids. We also analysed songs of five individuals with intermediate phenotype, which were identified as F₁ hybrids between the Thrush Nightingale female and the Common Nightingale male by genetic analysis. Songs of three of these hybrids corresponded to the paternal species (Common Nightingale) but the remaining two sung a mixed song. Our results suggest that although hybridization might increase the tendency for learning songs from both parental species, interspecific cultural transmission is the major proximate mechanism explaining the occurrence of mixed singers among the sympatric Thrush Nightingales. We also provide evidence that mixed singing does not substantially increase the rate of interspecific hybridization and discuss the possible adaptive value of this phenomenon in nightingales.

Experience dependence of neural responses to different classes of male songs in the primary auditory forebrain of female songbirds

There is both extensive species-specificity and critical experience-dependence in the recognition of own species songs in many songbird species. For example, female zebra finches Taeniopygia guttata raised by their parents show behavioral preferences for the songs of the father over unfamiliar conspecific males and for unfamiliar songs of conspecifics over heterospecifics. Behavioral discrimination between different species' songs is also displayed by females raised without exposure to any male songs but it is diminished in females raised by heterospecific foster parents. We tested whether neural responses in the female auditory forebrain paralleled each of these known behavioral patterns in song-class discrimination. We analyzed spike rates, above background levels, recorded from single units in the L2a subregion of the field L complex of female zebra finches. In subjects raised by genetic parents, spike rates were similar to songs of fathers and unfamiliar male zebra finches, and higher to unfamiliar conspecific over unfamiliar heterospecific songs. In females raised in isolation from male songs, we also found higher spike rates to unfamiliar conspecific over heterospecific songs. In females raised by heterospecific foster parents, spike rates were similar in response to songs of the foster father and unfamiliar males of the foster species, similar between unfamiliar songs of conspecifics and the heterospecific foster species, and higher to unfamiliar songs of the foster species over a third finch species. Thus, in parallel to the experience-dependence of females' behaviors in response to different male song classes, differences in social experiences can also alter neural response patterns to male song classes in the auditory forebrain of female zebra finches.

Ecological speciation along an elevational gradient in a tropical passerine bird?

Local adaptation of populations along elevational gradients is well known, but conclusive evidence that such divergence has resulted in the origin of distinct species in parapatry remains lacking. We integrated morphological, vocal, genetic and behavioural data to test predictions pertaining to the hypothesis of parapatric ecological speciation associated with elevation in populations of a tropical montane songbird, the Grey-breasted Wood-wren (Henicorhina leucophrys: Troglodytidae), from the Sierra Nevada de Santa Marta, Colombia. We confirmed that two distinct populations exist along the elevational gradient. Phylogenetic analyses tentatively indicate that the two populations are not sister taxa, suggesting they did not differentiate from a single ancestor along the gradient, but rather resulted from separate colonization events. The populations showed marked divergence in morphometrics, vocalizations and genetic variation in mitochondrial and nuclear loci, and little to no evidence of hybridization. Individuals of both populations responded more strongly to their own local songs than to songs from another elevation. Although the two forms do not appear to have differentiated locally in parapatry, morphological and vocal divergence along the elevational gradient is consistent with adaptation, suggesting a possible link between adaptive evolution in morphology and songs and the origin of reproductive isolation via a behavioural barrier to gene flow. The adaptive value of phenotypic differences between populations requires additional study.

Evolutionary divergence in acoustic signals: causes and consequences

Acoustic signals mediate mate choice, resource defense, and species recognition in a broad range of taxa. It has been proposed, therefore, that divergence in acoustic signals plays a key role in speciation. Nonetheless, the processes driving divergence of acoustic traits and their consequences in terms of speciation are poorly understood. A review of empirical and comparative studies reveals strong support for a role of sexual selection in acoustic divergence, but the possible concomitant influences of ecological context are rarely examined. We summarize a conceptual framework for testing the relative significance of both adaptive and neutral mechanisms leading to acoustic divergence, predictions for cases where these processes lead to speciation, and how their relative importance plays out over evolutionary time.

The role of ecological constraint in driving the evolution of avian song frequency across a latitudinal gradient

Just as features of the physical and biotic environment constrain evolution of ecological and morphological traits, they may also affect evolution of communication systems. Here we analyze constraints on rates of vocal evolution, using a large dataset of New World avian sister taxa. We show that species breeding in tropical forests sing at generally lower frequencies and across narrower bandwidths than species breeding in open habitats, or at high latitudes. We attribute these restrictions on birdsong frequency to the presence of high-frequency insect noise and greater degradation of high-frequency sounds in tropical forests. We fit Ornstein-Uhlenbeck models to show that recent evolution of song frequency has been more greatly constrained in tropical forests than elsewhere, that is, songs have shown less tendency to diverge over time in tropical forests, consistent with inferred acoustic restrictions. In addition, we find that song frequency has evolved more rapidly overall at high latitudes in both forest and open habitats. Besides a larger available sound window, other factors contributing to more rapid divergence at high latitudes may include an overall increased intensity of sexual selection, occupation of more divergent habitats, and the presence of fewer competing species.

Biogeography of species richness gradients: linking adaptive traits, demography and diversification

Here we review how adaptive traits contribute to the emergence and maintenance of species richness gradients through their influence on demographic and diversification processes. We start by reviewing how demographic dynamics change along species richness gradients. Empirical studies show that geographical clines in population parameters and measures of demographic variability are frequent along latitudinal and altitudinal gradients. Demographic variability often increases at the extremes of regional species richness gradients and contributes to shape these gradients. Available studies suggest that adaptive traits significantly influence demographic dynamics, and set the limits of species distributions. Traits related to thermal tolerance, resource use, phenology and dispersal seem to play a significant role. For many traits affecting demography and/or diversification processes, complex mechanistic approaches linking genotype, phenotype and fitness are becoming progressively available. In several taxa, species can be distributed along adaptive trait continuums, i.e. a main axis accounting for the bulk of inter-specific variation in some correlated adaptive traits. It is shown that adaptive trait continuums can provide useful mechanistic frameworks to explain demographic dynamics and diversification in species richness gradients. Finally, we review the existence of sequences of adaptive traits in phylogenies, the interactions of adaptive traits and community context, the clinal variation of traits across geographical gradients, and the role of adaptive traits in determining the history of dispersal and diversification of clades. Overall, we show that the study of demographic and evolutionary mechanisms that shape species richness gradients clearly requires the explicit consideration of adaptive traits. To conclude, future research lines and trends in the field are briefly outlined.

Ecological character displacement in the face of gene flow: evidence from two species of nightingales

Background

Ecological character displacement is a process of phenotypic differentiation of sympatric populations caused by interspecific competition. Such differentiation could facilitate speciation by enhancing reproductive isolation between incipient species, although empirical evidence for it at early stages of divergence when gene flow still occurs between the species is relatively scarce. Here we studied patterns of morphological variation in sympatric and allopatric populations of two hybridizing species of birds, the Common Nightingale (Luscinia megarhynchos) and the Thrush Nightingale (L. luscinia).

Results

We conducted principal component (PC) analysis of morphological traits and found that nightingale species converged in overall body size (PC1) and diverged in relative bill size (PC3) in sympatry. Closer analysis of morphological variation along geographical gradients revealed that the convergence in body size can be attributed largely to increasing body size with increasing latitude, a phenomenon known as Bergmann's rule. In contrast, interspecific interactions contributed significantly to the observed divergence in relative bill size, even after controlling for the effects of geographical gradients. We suggest that the divergence in bill size most likely reflects segregation of feeding niches between the species in sympatry.

Conclusions

Our results suggest that interspecific competition for food resources can drive species divergence even in the face of ongoing hybridization. Such divergence may enhance reproductive isolation between the species and thus contribute to speciation.

Variation in the structure of bird nests between northern Manitoba and southeastern Ontario

Traits that converge in appearance under similar environmental conditions among phylogenetically independent lineages are thought to represent adaptations to local environments. We tested for convergence in nest morphology and composition of birds breeding in two ecologically different locations in Canada: Churchill in northern Manitoba and Elgin in southeastern Ontario. We examined nests from four families of passerine birds (Turdidae: Turdus, Parulidae: Dendroica, Emberizidae: Passerculus and Fringillidae: Carduelis) where closely related populations or species breed in both locations. Nests of American Robins, Yellow Warblers, and Carduelis finches had heavier nest masses, and tended to have thicker nest-walls, in northern Manitoba compared with conspecifics or congenerics breeding in southeastern Ontario. Together, all species showed evidence for wider internal and external nest-cup diameters in northern Manitoba, while individual species showed varying patterns for internal nest-cup and external nest depths. American Robins, Yellow Warblers, and Carduelis finches in northern Manitoba achieved heavier nest masses in different ways. American Robins increased all materials in similar proportions, and Yellow Warblers and Common Redpolls used greater amounts of select materials. While changes in nest composition vary uniquely for each species, the pattern of larger nests in northern Manitoba compared to southeastern Ontario in three of our four phylogenetically-independent comparisons suggests that birds are adapting to similar selective pressures between locations.

Asymmetric response patterns to subspecies-specific song differences in allopatry and parapatry in the gray-breasted wood-wren

Song divergence between closely related taxa may play a critical role in the evolutionary processes of speciation and hybridization. We explored song variation between two Ecuadorian subspecies of the gray-breasted wood-wren (Henicorhina leucophrys) and tested the impact of song divergence on response behaviors. Songs were significantly different between the two subspecies, even between two parapatric populations 10 km apart. Playback experiments revealed an asymmetric response pattern to these divergent subspecies specific songs; one subspecies responded more to songs of its own subspecies than to the other subspecies' songs, whereas the second responded equally strongly to songs of both subspecies. While song parameters revealed a mixed pattern of divergence between allopatric and parapatric populations, the majority of spectral characteristics showed increased divergence in parapatry, suggestive of character displacement. This increased song divergence in parapatry appeared to affect behavioral responses to playback as discriminating responses were most prominent in parapatry and against parapatric songs. The clear behavioral impact of subspecies-specific song differences supports a potential role for song as an acoustic barrier to gene flow. The asymmetric nature of the responses suggests that song divergence could affect the direction of gene flow and the position of the subspecies-specific transition.

Selection and geographic isolation influence hummingbird speciation: genetic, acoustic and morphological divergence in the wedge-tailed sabrewing (Campylopterus curvipennis)

BACKGROUND

Mesoamerica is one of the most threatened biodiversity hotspots in the world, yet we are far from understanding the geologic history and the processes driving population divergence and speciation for most endemic taxa. In species with highly differentiated populations selective and/or neutral factors can induce rapid changes to traits involved in mate choice, promoting reproductive isolation between allopatric populations that can eventually lead to speciation. We present the results of genetic differentiation, and explore drift and selection effects in promoting acoustic and morphological divergence among populations of Campylopterus curvipennis, a lekking hummingbird with an extraordinary vocal variability across Mesoamerica.

RESULTS

Analyses of two mitochondrial genes and ten microsatellite loci genotyped for 160 individuals revealed the presence of three lineages with no contemporary gene flow: C. c. curvipennis, C. c. excellens, and C. c. pampa disjunctly distributed in the Sierra Madre Oriental, the Tuxtlas region and the Yucatan Peninsula, respectively. Sequence mtDNA and microsatellite data were congruent with two diversification events: an old vicariance event at the Isthmus of Tehuantepec (c. 1.4 Ma), and a more recent Pleistocene split, isolating populations in the Tuxtlas region. Hummingbirds of the excellens group were larger, and those of the pampa group had shorter bills, and lineages that have been isolated the longest shared fewer syllables and differed in spectral and temporal traits of a shared syllable. Coalescent simulations showed that fixation of song types has occurred faster than expected under neutrality but the null hypothesis that morphological divergence resulted from drift was not rejected.

CONCLUSIONS

Our phylogeographic analyses uncovered the presence of three Mesoamerican wedge-tailed sabrewing lineages, which diverged at different time scales. These results highlight the importance of the Isthmus of Tehuantepec and more recent Pleistocene climatic events in driving isolation and population divergence. Coalescent analyses of the evolution of phenotypic traits suggest that selection is driving song evolution in wedge-tailed sabrewings but drift could not be rejected as a possibility for morphological divergence.

A latitudinal gradient in rates of evolution of avian syllable diversity and song length

We ask whether rates of evolution in traits important for reproductive isolation vary across a latitudinal gradient, by quantifying evolutionary rates of two traits important for pre-mating isolation-avian syllable diversity and song length. We analyse over 2500 songs from 116 pairs of closely related New World passerine bird taxa to show that evolutionary rates for the two main groups of passerines-oscines and suboscines-doubled with latitude in both groups for song length. For syllable diversity, oscines (who transmit song culturally) evolved more than 20 times faster at high latitudes than in low latitudes, whereas suboscines (whose songs are innate in most species and who possess very simple song with few syllable types) show no clear latitudinal gradient in rate. Evolutionary rates in oscines and suboscines were similar at tropical latitudes for syllable complexity as well as for song length. These results suggest that evolutionary rates in traits important to reproductive isolation and speciation are influenced by latitude and have been fastest, not in the tropics where species diversity is highest, but towards the poles.

Songs of Darwin's finches diverge when a new species enters the community

Bird species sing different songs and as a result rarely breed with each other. Species are not static but can shift in acoustic and morphological space, yet maintain their distinctiveness. Investigating such a situation in a community of Darwin's finches sheds light on the origin and maintenance of premating barriers between species. Explanations for songs divergence generally invoke morphological changes to the sound-producing apparatus, environmental changes influencing transmitting properties of song, avoidance of acoustical interference with other species, and random processes including copying errors. We investigated changes in songs of Geospiza fortis (medium ground finch) and Geospiza scandens (cactus ground finch) from 1978 to 2010 on Daphne Major Island, Galápagos. The habitat did not change significantly; however, the finch community changed. The socially aggressive congener Geospiza magnirostris (large ground finch), singing in the same frequency band (2-4 kHz), colonized Daphne in 1983 and increased in numbers. Temporal features of the songs of G. fortis and G. scandens, especially trill rate and song duration, diverged from G. magnirostris songs as it became increasingly common. Changes in song were not a passive consequence of a change in beak morphology. Instead they arose as a bias during song imprinting and production. Sons of both G. fortis and G. scandens sang faster songs than their respective fathers and thereby differed more from G. magnirostris in their songs than did their fathers. Divergence from an aversive or confusing stimulus during learning illustrates a "peak shift" that may be a common feature of song evolution and speciation.

Stereotypic vocalizations in harvest mice (Reithrodontomys): Harmonic structure contains prominent and distinctive audible, ultrasonic, and non-linear elements

Reports of audible vocalizations are rare in adult muroid rodents, animals generally very small in body size and under strong predation pressure. By contrast, communication using high, often ultrasonic vocal frequencies is relatively common. There are anecdotal reports of audible vocalizations for some harvest mice (genus Reithrodontomys), however none have been recorded or analyzed. Several species of harvest mice are studied, representing the subgenera Reithrodontomys (R. fulvescens, R. sumichrasti) and Aporodon (R. creper, R. mexicanus, R. spp.), as part of a larger phylogenetic analysis of stereotypical vocal communication. Only R. mexicanus produced stereotyped vocalizations on a regular basis. Acoustic signals of R. mexicanus contain prominent harmonics bridging both the audible and ultrasonic range, with evidence of non-linear distribution of energy within and between notes of an individual call. Harmonic emphasis varies, making the carrier frequency difficult to locate. These Reithrodontomys vocalizations are compared with members of their phylogenetic sister group Onychomys+Peromyscus, genera whose stereotypic calls exclusively occupy the audible or ultrasonic spectrum, respectively. It is hypothesized that the stereotypic signals of harvest mice represent announcement calls. Given the number of anecdotal reports of stereotyped 1, 2, and 3-note calls among Reithrodontomys species, further interspecific comparisons are warranted.

Song divergence by sensory drive in Amazonian birds

Visual signals are shaped by variation in the signaling environment through a process termed sensory drive, sometimes leading to speciation. However, the evidence for sensory drive in acoustic signals is restricted to comparisons between highly dissimilar habitats, or single-species studies in which it is difficult to rule out the influence of undetected ecological variables, pleiotropic effects, or chance. Here we assess whether this form of sensory drive-often termed "acoustic adaptation"-can generate signal divergence across ecological gradients. By studying avian communities in two Amazonian forest types, we show that songs of 17 "bamboo-specialist" bird species differ in predictable ways from their nearest relatives in adjacent terra firme forest. We also demonstrate that the direction of song divergence is correlated with the sound transmission properties of habitats, rather than with genetic divergence, ambient noise, or pleiotropic effects of mass and bill size. Our findings indicate that acoustic adaptation adds significantly to stochastic processes underlying song divergence, even when comparing between habitats with relatively similar structure. Furthermore, given that song differences potentially contribute to reproductive isolation, these findings are consistent with a wider role for sensory drive in the diversification of lineages with acoustic mating signals.

Character displacement from the receiver's perspective: species and mate recognition despite convergent signals in suboscine birds

Many social animals use long-distance signals to attract mates and defend territories. They face the twin challenges of discriminating between species to identify conspecific mates, and between individuals to recognize collaborators and competitors. It is therefore often assumed that long-distance signals are under strong selection for species-specificity and individual distinctiveness, and that this will drive character displacement when closely related species meet, particularly in noisy environments. However, the occurrence of signal stereotypy and convergence in rainforest species seems to contradict these ideas, and raises the question of whether receivers in these systems can recognize species or individuals by long-distance signals alone. Here, we test for acoustically mediated recognition in two sympatric antbird species that are known to have convergent songs. We show that male songs are stereotyped yet individually distinctive, and we use playback experiments to demonstrate that females can discriminate not only between conspecific and heterospecific males, but between mates and strangers. These findings provide clear evidence that stereotypy and convergence in male signals can be accommodated by fine tuning of perceptual abilities in female receivers, suggesting that the evolutionary forces driving divergent character displacement in animal signals are weaker than is typically assumed.

Geographic variation in the songs of neotropical singing mice: testing the relative importance of drift and local adaptation

Patterns of geographic variation in communication systems can provide insight into the processes that drive phenotypic evolution. Although work in birds, anurans, and insects demonstrates that acoustic signals are sensitive to diverse selective and stochastic forces, processes that shape variation in mammalian vocalizations are poorly understood. We quantified geographic variation in the advertisement songs of sister species of singing mice, montane rodents with a unique mode of vocal communication. We tested three hypotheses to explain spatial variation in the song of the lower altitude species, Scotinomys teguina: selection for species recognition in sympatry with congener, S. xerampelinus, acoustic adaptation to different environments, and stochastic divergence. Mice were sampled at seven sites in Costa Rica and Panamá; genetic distances were estimated from mitochondrial control region sequences, between-site differences in acoustic environment were estimated from climatic data. Acoustic, genetic and geographic distances were all highly correlated in S. teguina, suggesting that population differentiation in song is largely shaped by genetic drift. Contrasts between interspecific genetic-acoustic distances were significantly greater than expectations derived from intraspecific contrasts, indicating accelerated evolution of species-specific song. We propose that, although much intraspecific acoustic variation is effectively neutral, selection has been important in shaping species differences in song.

The secondary contact phase of allopatric speciation in Darwin's finches

Speciation, the process by which two species form from one, involves the development of reproductive isolation of two divergent lineages. Here, we report the establishment and persistence of a reproductively isolated population of Darwin's finches on the small Galápagos Island of Daphne Major in the secondary contact phase of speciation. In 1981, an immigrant medium ground finch (Geospiza fortis) arrived on the island. It was unusually large, especially in beak width, sang an unusual song, and carried some Geospiza scandens alleles. We followed the fate of this individual and its descendants for seven generations over a period of 28 years. In the fourth generation, after a severe drought, the lineage was reduced to a single brother and sister, who bred with each other. From then on this lineage, inheriting unusual song, morphology, and a uniquely homozygous marker allele, was reproductively isolated, because their own descendants bred with each other and with no other member of the resident G. fortis population. These observations agree with some expectations of an ecological theory of speciation in that a barrier to interbreeding arises as a correlated effect of adaptive divergence in morphology. However, the important, culturally transmitted, song component of the barrier appears to have arisen by chance through an initial imperfect copying of local song by the immigrant. The study reveals additional stochastic elements of speciation, in which divergence is initiated in allopatry; immigration to a new area of a single male hybrid and initial breeding with a rare hybrid female.

The genetics and ecology of reinforcement: implications for the evolution of prezygotic isolation in sympatry and beyond

Reinforcement, the evolution of prezygotic reproductive barriers by natural selection in response to maladaptive hybridization, is one of the most debated processes in speciation. Critics point to "fatal" conceptual flaws for sympatric evolution of prezygotic isolation, but recent theoretical and empirical work on genetics and ecology of reinforcement suggests that such criticisms can be overcome. New studies provide evidence for reinforcement in frogs, fish, insects, birds, and plants. While such evidence lays to rest the argument over reinforcement's existence, our understanding remains incomplete. We lack data on (1) the genetic basis of female preferences and the links between genetics of pre- and postzygotic isolation, (2) the ecological basis of reproductive isolation, (3) connections between prezygotic isolation between species and within-species sexual selection (potentially leading to a "cascade" of effects on reproductive isolation), (4) the role of habitat versus mate preference in reinforcement, and (5) additional detailed comparative studies. Here, we review data on these issues and highlight why they are important for understanding speciation.