Social selection parapatry in Afrotropical sunbirds

The extent of range overlap of incipient and recent species depends on the type and magnitude of phenotypic divergence that separates them, and the consequences of phenotypic divergence on their interactions. Signal divergence by social selection likely initiates many speciation events, but may yield niche‐conserved lineages predisposed to limit each others’ ranges via ecological competition. Here, we examine this neglected aspect of social selection speciation theory in relation to the discovery of a nonecotonal species border between sunbirds. We find that Nectarinia moreaui and Nectarinia fuelleborni meet in a ～6 km wide contact zone, as estimated by molecular cline analysis. These species exploit similar bioclimatic niches, but sing highly divergent learned songs, consistent with divergence by social selection. Cline analyses suggest that within‐species stabilizing social selection on song‐learning predispositions maintains species differences in song despite both hybridization and cultural transmission. We conclude that ecological competition between moreaui and fuelleborni contributes to the stabilization of the species border, but that ecological competition acts in conjunction with reproductive interference. The evolutionary maintenance of learned song differences in a hybrid zone recommend this study system for future studies on the mechanisms of learned song divergence and its role in speciation.     

Lack of premating isolation at the base of a phylogenetic tree

Darwin's finches in the Galápagos archipelago are an unusual example of adaptive radiation in that the basal split separates two lineages of warbler finches (Certhidea olivacea and Certhidea fusca) believed until recently to be only one species. The large genetic difference between them contrasts with their similarity in plumage, size, shape, and courtship behavior. They differ in song, which is a key factor in premating isolation of other sympatric Darwin's finches. We conducted playback experiments to see whether members of the population of C. olivacea on Santa Cruz Island would respond to songs of C. fusca from two islands, Genovesa and Pinta, and songs of C. olivacea from another island (Isabela). Another set of experiments was performed, using the same playback tapes, with C. fusca on Genovesa. Some members of both populations responded to all playbacks; therefore, the hypothesis of complete premating isolation on the basis of song is rejected. Discrimination between songs of the two lineages was inconsistent. We conclude that premating barriers to interbreeding among the tested populations have not arisen in the 1.5-2.0 m.yr. of their geographical isolation on different islands. This contrasts with strong premating barriers between more recently derived sympatric species. Early learning of song associated with morphology is later used in mate recognition. This explains why sympatric species that are vocally and morphologically distinct yet genetically less differentiated than Certhidea do not interbreed, whereas the Certhidea lineages that are genetically well differentiated but vocally and morphologically similar have no apparent premating barrier. We discuss this unusual situation in terms of the forces that have produced similarities and differences in song, morphology, and ecology and their relevance to phylogenetic and biological species concepts. Neither principles nor details are unique to Darwin's finches, and we conclude by pointing out strong parallels with some continental birds.     

Behavioural response to song and genetic divergence in two subspecies of white‐crowned sparrows (Zonotrichia leucophrys)

Divergence in sexual signals may drive reproductive isolation between lineages, but behavioural barriers can weaken in contact zones. Here, we investigate the role of song as a behavioural and genetic barrier in a contact zone between two subspecies of white‐crowned sparrows (Zonotrichia leucophrys). We employed a reduced genomic data set to assess population structure and infer the history underlying divergence, gene flow and hybridization. We also measured divergence in song and tested behavioural responses to song using playback experiments within and outside the contact zone. We found that the subspecies form distinct genetic clusters, and demographic inference supported a model of secondary contact. Song phenotype, particularly length of the first note (a whistle), was a significant predictor of genetic subspecies identity and genetic distance along the hybrid zone, suggesting a close link between song and genetic divergence in this system. Individuals from both parental and admixed localities responded significantly more strongly to their own song than to the other subspecies song, supporting song as a behavioural barrier. Putative parental and admixed individuals were not significantly different in their strength of discrimination between own and other songs; however, individuals from admixed localities tended to discriminate less strongly, and this difference in discrimination strength was explained by song dissimilarity as well as genetic distance. Therefore, we find that song acts as a reproductive isolating mechanism that is potentially weakening in a contact zone between the subspecies. Our findings also support the hypothesis that intraspecific song variation can reduce gene flow between populations.     

Acoustic signals in insects: A reproductive barrier and a taxonomic character

In singing insects, the song is an important component of the specific mate recognition system (SMRS). In communities of sympatric singing species, there is a partitioning of communication channels, the so-called “acoustic niches.” Within one community, the songs of different species always differ in temporal or frequency characters, i.e. occupy different acoustic niches. However, conspecific songs do not always act as an interspecific reproductive barrier, despite always being a SMRS component. The species that do not communicate acoustically due to allopatry, different timing of vocalization, inhabiting different biotopes, or unmatched food specializations can produce similar songs while forming reproductively isolated communities. Individuals of different sexes need not only to recognize a conspecific mate but also to evaluate its “quality.” The close-range signal (courtship song) provides more opportunities for choosing the “best” male than does the distant signal (calling song). In many species of Orthoptera, courtship includes not only acoustic but also vibrational, visual, chemical, and mechanical signals. An analysis of cricket songs showed the courtship songs to be on average more elaborate and variable than the calling songs. At the same time, due to the difference in mating behavior between the two groups, the acoustic component of courtship is used for mate quality evaluation to a greater extent in grasshoppers than in crickets. The courtship songs of grasshoppers are generally more elaborate in temporal structure than cricket songs; moreover, they may be accompanied by visual displays such as movements of various body parts. Thus, song evolution in grasshoppers is more strongly driven by sexual selection than that in crickets. According to the reinforcement hypothesis, the premating barrier between hybridizing species becomes stronger in response to reduced hybrid fitness. However, our behavioral experiments with two groups of hybridizing grasshopper species did not confirm the reinforcement hypothesis. We explain this, firstly, by a low level of genetic incompatibility between the hybridizing species and secondly, by high hybrid fitness when attracting a mate. A high competitive capability of hybrids may be accounted for by attractiveness of new elements in hybrid courtship songs. When we divide similar forms based on their songs, we in fact distinguish biological species using the criterion of their reproductive isolation. Acoustic differences between species are usually greater than morphological ones. Therefore, song analysis allows one to determine the real status of doubtful species-rank taxa, to distinguish species in a medley of sibling forms, and to reveal cryptic species in the cases when morphological studies fail to provide a univocal result. At the same time, songs are subject to intraspecific variation the range of which is different in different groups. Therefore, it is necessary to study which degree of difference corresponds to the species level before interpreting the status of some forms based on song comparisons. Besides, song similarities cannot indicate conspecificity of acoustically isolated forms; on the other hand, song differences between these forms prove that they are full-rank species.     

Reinforced postmating reproductive isolation barriers in Neurospora,an Ascomycete microfungus

Maladaptive hybridization promotes reinforcement, selection for stringent reproductive isolation barriers during speciation. Reinforcement is suspected when barriers between sympatric populations are stronger than allopatric barriers, and particularly when stronger barriers evolve in the species and sex suffering the greatest costs of hybridization. Canonically, reinforcement involves premating barriers. Selection for postmating barriers is controversial, but theoretically possible. We examined geographical patterns in reproductive isolation barriers between Neurospora crassa and Neurospora intermedia, fungi with pheromone‐mediated mate recognition and maternal care. We find that isolation is stronger between sympatric populations than allopatric populations, and stronger barriers are associated with the species (N. crassa) and mating role (maternal) suffering the greater costs of hybridization. Notably, reinforced isolation involves a postmating barrier, abortion of fruitbodies. We hypothesize that fruitbody abortion is selectively advantageous if it increases the likelihood that maternal Neurospora individuals successfully mate conspecifically after maladaptive hybrid fertilization.     

Behavioral responses of hybrid lacewings (Neuroptera: Chrysopidae) to courtship songs

The study of hybrid courtship songs and the behavioral responses of hybrids and parental individuals to hybrid songs can be useful in understanding the origin of reproductive isolation among species that differ mainly in their courtship songs. Here we test the hypotheses (a) that hybrid lacewings prefer hybrid songs to either of the parental songs from a cross betweenChrysoperla plorabunda andC. johnsoni, and (b) that parental individuals prefer their own song over those of hybrids. Analysis of songs showed that most features of hybrid songs are intermediate between the two parents. Hybrids organize their songs with a series of simple volleys like those ofC. plorabunda. Female hybrids from two reciprocal crosses and females and males of the parental species were presented with choices of hybrid and parental songs. Hybrids responded more to recordings of hybrid songs than to recordings ofC. plorabunda but did not differ in the responses given toC. johnsoni and hybrid songs. In contrast, males and females of both parental lines preferred to duet with recordings of their own song types and did not respond to hybrid songs. Our results demonstrate that hybrids would be at a disadvantage in nature, because neitherC. plorabunda norC. johnsoni will respond to their songs.     

Flycatcher song in allopatry and sympatry--convergence, divergence and reinforcement

The theory of reinforcement predicts that natural selection against the production of unfit hybrids favours traits that increase assortative mating. Whether culturally inherited traits, such as bird song, can increase assortative mating by reinforcement is largely unknown. We compared songs of pied (Ficedula hypoleuca) and collared flycatchers (F. albicollis) from two hybrid zones of different ages with songs from allopatric populations. Previously, a character divergence in male plumage traits has been shown to reinforce premating isolation in sympatric flycatchers. In contrast, we find that the song of the pied flycatcher has converged towards that of the collared flycatcher (mixed singing). However, a corresponding divergence in the collared flycatcher shows that the species differences in song characters are maintained in sympatry. Genetic analyses suggest that mixed song is not caused by introgression from the collared flycatcher, but rather due to heterospecific copying. Circumstantial evidence suggests that mixed song may increase the rate of maladaptive hybridization. In the oldest hybrid zone where reinforcement on plumage traits is most pronounced, the frequency of mixed singing and hybridization is also lowest. Thus, we suggest that reinforcement has reduced the frequency of mixed singing in the pied flycatcher and caused a divergence in the song of the collared flycatcher. Whether a culturally inherited trait promotes or opposes speciation in sympatry may depend on its plasticity. The degree of plasticity may be genetically determined and accordingly under selection by reinforcement.     

A narrow hybrid zone between the grasshoppers Stenobothrus clavatus and Stenobothrus rubicundus: courtship song analysis

The closely related grasshopper species Stenobothrus rubicundus and Stenobothrus clavatus are known to hybridize in a very narrow contact zone on Mt. Tomaros in northern Greece. These species produce very different and complex courtship songs accompanied with visual display. We analyzed the courtship songs and underlying stridulatory movements of the hind legs in natural hybrids from Mount Tomaros. The two species were also hybridized in the laboratory and their songs were compared with the songs of the natural hybrids. Some hybrid songs were shown to have intermediate features between parental songs, whereas other hybrid songs comprised completely new elements. The clavatus‐like song elements were found to dominate in hybrid songs. These song features may influence the mating success of hybrid males in the contact zone. A comparison of hybrid songs with the song pattern of the north European S. rubicundus populations allowed us to suggest a scenario of S. rubicundus and S. clavatus origin. © 2012 The Linnean Society of London, Biological Journal of the Linnean Society, 2012, ?? , ??–??.     

Dominance effects strengthen premating hybridization barriers between sympatric species of grasshoppers (Acrididae,Orthoptera)

Sexual selection can lead to the rapid evolution of premating hybridization barriers and allows accelerated diversification and speciation within an evolutionary lineage. Especially during early stages of divergence, hybridization may impede further divergence, which strongly depends on the reproductive success of hybrids. Behavioural sterility of hybrids can limit or even prevent homogenizing gene flow. In this study, we investigated the attractiveness of male courtship songs for females of the grasshopper species Chorthippus biguttulus and C. brunneus and their interspecific F1 and F2 hybrids. Song preferences of females of both species are highly species specific and differ in three parameters: shape of the preference function, preference for syllable pattern and phrase duration. F1 hybrid females of both reciprocal crosses as well as F2 hybrid females resembled closely pure C. biguttulus females in respect of shape of the preference function and preference for syllable pattern, while preference for phrase duration showed an intermediate expression. This resulted in song preferences of hybrid females that closely resembled those of one parental species, that is C. biguttulus females. Such strong dominance effects were rarely reported so far. They represent an effective barrier limiting gene flow between the two species, since hybrid females will backcross to only one parental species and discriminate against hybrid males, which are behaviourally sterile. Such taxon‐specific modes of inheritance may have facilitated the rapid divergence of acoustically communicating grasshoppers of the species group of Chorthippus biguttulus. Our findings have novel implications on the expression of neuronal filters and the evolution of complex courtship signals.     

Song discrimination suggests premating isolation among sympatric indigobird species and host races

Indigobirds (Vidua spp.) are host-specific brood parasites thathave diversified in a recent radiation apparently driven byhost colonization. Behavioral imprinting of both male and femaleindigobirds on host song is thought to promote rapid speciationbecause it results in assortative mating between indigobirdsassociated with a particular host. We conducted a song playbackexperiment to test whether male indigobirds discriminate amongpotential competitors based on song. Of particular interestwas the behavior of two sympatric host races of the Cameroonindigobird Vidua camerunensis that differ only in host songmimicry and other components of their vocal repertoires. Territorialmales of the two V. camerunensis host races and Vidua raricola,a morphologically distinct indigobird species, were tested withplaybacks of each other's songs. Males of all three groups respondedmost aggressively to songs of their own species and/or hostrace, as evidenced by strong and highly significant differencesin a variety of response variables. This differential territorialdefense suggests that an intruding male with different songsdoes not represent a competitive threat and is expected if femalesmate assortatively with respect to song. Thus, our results provideevidence of premating reproductive isolation among recentlyevolved indigobird species and host races.     

Asymmetric Response of Costa Rican White-Breasted Wood-Wrens (Henicorhina leucosticta) to Vocalizations from Allopatric Populations

Divergence in song between allopatric populations can contribute to premating reproductive isolation in territorial birds. Song divergence is typically measured by quantifying divergence in vocal traits using audio recordings, but field playback experiments provide a more direct way to behaviorally measure song divergence between allopatric populations. The White-breasted Wood-Wren (Henicorhina leucosticta; hereafter “WBWW”) is an abundant Neotropical species with four mitochondrial clades (in Central America, the Darién, the Chocó and the Amazon) that are deeply divergent (~5–16% sequence divergence). We assessed the possibility that the WBWW as currently defined may represent multiple biological species by conducting both statistical analysis of vocal characters and field playback experiments within three clades (Central America, Chocó and Amazon). Our analysis of vocal traits revealed that Central American songs overlapped in acoustic space with Chocó songs, indicating vocal similarity between these two populations, but that Central American songs were largely divergent from Amazonian songs. Playback experiments in the Caribbean lowlands of Costa Rica revealed that Central American WBWWs typically responded aggressively to songs from the Chocó population but did not respond to playback of songs from the Amazonian population, echoing the results of the vocal trait analysis. This marked difference in behavioral response demonstrates that the songs of Central American and Amazonian WBWWs (but not Central American and Chocó WBWWs) have diverged sufficiently that Central American WBWWs no longer recognize song from Amazonian WBWWs as a signal to elicit territorial defense. This suggests that significant premating reproductive isolation has evolved between these two populations, at least from the perspective of the Central American population, and is consistent with the possibility that Central American and Amazonian populations represent distinct biological species. We conclude by advocating for the further use of field playback experiments to assess premating reproductive isolation (and species limits) between allopatric songbird populations, a situation where behavioral systematics can answer questions that phylogenetic systematics cannot.     

Acoustic but no genetic divergence in migratory and sedentary populations of blackcaps,Sylvia atricapilla

In songbirds, territorial songs are key regulators of sexual selection and are learned from conspecifics. The cultural transmission of songs leads to divergence in song characteristics within populations, which can ultimately lead to speciation. Many songbirds also migrate, and individual differences in migratory behaviours can influence population genetic structure and local song differentiation. Blackcaps, Sylvia atricapilla, exhibit versatile territorial songs and show diversity in migration behaviours. They therefore comprise a good model for investigating the relationships between migratory patterns, song variation, and genetic diversity. We studied a migratory population (two groups near Paris) and a sedentary population (three groups in Corsica). All of the birds were ringed and blood sampled to investigate genetic relatedness using 17 microsatellite loci. A detailed song analysis showed that this species has a complex repertoire (> 100 syllables), which required the development of a semi‐supervised method to classify different categories of syllables and compare sequences of syllables. Our analysis showed no genetic structuring among populations: individuals belonging to the same group were not genetically closer than those from different groups. However, we found a strong wingsize difference between sedentary and migratory populations. We also showed that geographical variations in songs rely at least on both syllable and sequence content. Unexpectedly, despite a higher turnover of individuals, migratory groups share as many syllables and sequences as sedentary groups, which raises interesting issues on song learning and the maintenance of dialects in migratory birds.     

Song evolution in the coal tit Parus ater

Since in oscine birds song divergence often leads to premating reproductive isolation, we asked which factors drive the evolution of song in a widespread species, the coal tit Parus ater. Based on an almost taxon‐complete phylogeny we correlated song divergence with morphometric (ecology), genetic (time), and geographic distances (separation). We found eight well‐supported mitochondrial lineages, one of which consists of melanolophus (usually treated as a separate species) and whose relationships remain generally unresolved. Due to intense song variability even sophisticated sonametric analyses failed to separate sub‐specific units. We tested for the role of song in reproductive isolation by playing songs from China and the Himalayas to Central European males. They recognize them as conspecific, but react to their playbacks less aggressively than to local songs. While morphological divergence is correlated with neither time nor separation, song divergence coincides more with geographic than genetic distance. This is mainly driven by differences along longitude and is especially prominent in the southern part of the species’ range, where six lineages persisted during the last glaciation while the north was subsequently re‐occupied by two lineages.     

Simulating secondary contact in allopatric speciation: an empirical test of premating isolation

Allopatric populations of Geospiza difficilis, the Sharp‐beaked Ground Finch, differ morphologically in association with different habitats to an extent unrivalled by any other species of Darwin's finch. The question arises as to whether they have diverged so much that they would not interbreed if they became sympatric; in other words, have they become separate species while remaining allopatric? In other species of Darwin's finches, it is known that a sexual imprinting‐like process based on early learning of song constrains breeding to conspecifics in sympatry. Therefore we used song playback experiments on Isla Genovesa to test the potential of G. difficilis to respond to songs from two other populations of the species on other, ecologically similar, islands. We found strong responses by males to songs of their own population, and heterogeneous but overall weaker responses to the structurally similar songs of G. difficilis from Isla Darwin. Tested birds did not respond to G. difficilis songs from Isla Wolf, songs of G. fuliginosa from Isla Pinta and control Cassin's finch songs. Female responses were infrequent and weak, apparently inhibited by the presence of responding males in most instances. Thus, assuming that females exercise similar discriminations to those of males, the Genovesa population of G. difficilis appears to be well advanced along the path of speciation: reproductively isolated from the Wolf population by a premating barrier to gene exchange that is culturally inherited, but not reproductively isolated from the Darwin population. We discuss the implications of imprinting for the process of speciation, the reasons for divergence of songs in allopatry, and the outcome of a hypothetical secondary contact in terms of coexistence, competition and interbreeding. © 2002 The Linnean Society of London, Biological Journal of the Linnean Society, 76 , 545–556.     

Rapid,complete reproductive isolation in two closely related Zosterops White‐eye bird species despite broadly overlapping ranges*

Examining what happens when two closely related species come into secondary contact provides insight into the later stages of the speciation process. The Zosteropidae family of birds is one of the most rapidly speciating vertebrate lineages. Members of this family are highly vagile and geographically widespread, raising the question of how divergence can occur if populations can easily come into secondary contact. On the small island of Kolombangara, two closely related nonsister species of white‐eyes, Zosterops kulambangrae and Zosterops murphyi, are distributed along an elevational gradient and come into secondary contact at mid‐elevations. We captured 134 individuals of both species along two elevational transects. Using genotyping‐by‐sequencing data and a mitochondrial marker, we found no evidence of past hybridization events and strong persistence of species boundaries, even though the species have only been diverging for approximately 2 million years. We explore potential reproductive barriers that allow the two species to coexist in sympatry, including premating isolation based on divergence in plumage and song. We also conducted a literature review to determine the time it takes to evolve complete reproductive isolation in congeneric avian species/subspecies in secondary contact (restricted to cases where congeneric taxa are parapatric or have a hybrid zone), finding our study is one of the youngest examples of complete reproductive isolation studied in a genomic context reported in birds.     

Socially flexible female choice and premating isolation in field crickets (Teleogryllus spp.)

Social influences on mate choice are predicted to influence evolutionary divergence of closely related taxa, because of the key role mate choice plays in reproductive isolation. However, it is unclear whether females choosing between heterospecific and conspecific male signals use previously experienced social information in the same manner or to the same extent that they do when discriminating among conspecific mates only. We tested this using two field cricket sister species (Teleogryllus oceanicus and Teleogryllus commodus), in which considerable information is known about the role of male calling song in premating isolation, in addition to the influence of acoustic experience on the development of reproductive traits. We manipulated the acoustic experience of replicate populations of both species and found, unexpectedly, that experience of male calling song during rearing did not change how accurate females were in choosing a conspecific over a heterospecific male song during playback trials. However, females with acoustic experience were considerably less responsive to male song compared with naïve females. Our results suggest that variation in the acoustic environment affects mate choice in both species, but that it may have a limited impact on premating isolation. The fact that social flexibility during interspecific mate discrimination does not appear to operate identically to that which occurs during conspecific mate discrimination highlights the importance of considering the context in which animals exercise socially flexible mating behaviours. We suggest an explanation for why social flexibility might be context dependent and discuss the consequences of such flexibility for the evolution of reproductive isolation.     

A narrow hybrid zone between the grasshoppers Stenobothrus clavatus and Stenobothrus rubicundus (Orthoptera: Gomphocerinae): female preferences for courtship songs

Two grasshopper species, Stenobothrus rubicundus and Stenobothrus clavatus, were previously shown to hybridize in a narrow contact zone on Mount Tomaros in northern Greece. The species are characterized by complex and completely different courtship songs. In the present study, we investigated female preferences for the courtship songs of S. rubicundus, S. clavatus and hybrids in playback experiments. Playback of the courtship songs revealed assortative preferences in females of the parental species: they significantly more often preferred the songs of conspecific males. Hybrid females showed a lower selectivity than parental females, responding somewhat equally eager to playback of the songs of S. clavatus, S. rubicundus, and natural hybrid song, although less actively to the F₁ hybrid song. The results suggest that hybrid males may lose to males of parental species, whereas hybrid females would even have an advantage over parental females. Comparison of responses of females from allopatric populations and Mount Tomaros to different song types shows no evidence for reinforcement. Asymmetry found in female preferences may have implications for the structure of the hybrid zone. © 2013 The Linnean Society of London     

Strong selection on male plumage in a hybrid zone between a hybrid bird species and one of its parents

Homoploid hybrid speciation (HHS) requires reproductive barriers between hybrid and parent species, despite incomplete reproductive isolation (RI) between the parents. Novel secondary sexual trait values in hybrids may cause prezygotic isolation from both parents, whereas signals inherited by the hybrid from one parent species may cause prezygotic isolation with the other. Here we investigate whether differences in male plumage function as a premating barrier between the hybrid Italian sparrow and one of its parent species, the house sparrow, in a narrow Alpine hybrid zone. Italian sparrow male plumage is a composite mosaic of the parental traits, with its head plumage most similar to its other parent, the Spanish sparrow. We use geographical cline analysis to examine selection on three plumage traits, 75 nuclear single nucleotide polymorphisms (SNPs) and hybrid indices based on these SNPs. Several SNPs showed evidence of restricted introgression in the Alps, supporting earlier findings. Crown colour exhibited the narrowest plumage cline, representing a 37% (range 4–65%) drop in fitness. The cline was too narrow to be due to neutral introgression. Only crown colour was significantly bimodal in the hybrid zone. Bimodality may be due to RI or a major QTL, although fitness estimates suggest that selection contributes to the pattern. We discuss the implications with respect to HHS and the species status of the Italian sparrow.     

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.     

Geographic variation and the evolution of song in Mesoamerican rufous‐naped wrens Campylorhynchus rufinucha

Speciation may be influenced by geographic variation in animal signals, particularly when those signals are important in reproductive decisions. Here, we describe patterns of geographic variation in the song of rufous‐naped wrens Campylorhynchus rufinucha. This species complex is a morphologically variable taxon confined to tropical dry forest areas from Mexico to northwestern Costa Rica. Morphological and genetic analyses suggest that there are at least three partially isolated groups within the complex, including a secondary‐contact zone in coastal western Chiapas between the subspecies C. r. humilus and C. r. nigricaudatus. Based on recordings throughout their geographic range, we investigate the effects of historical isolation on song structure and analyze whether genetic differences or climatic conditions explain observed patterns of variation. Our findings, based on a culturally‐transmitted and sexually‐selected trait, support the hypothesis that three evolutionary units exist within this taxon. Our results suggest that song differences between genetic groups were influenced by historical isolation. We report a strong relationship between vocal dissimilarity and genetic distance, suggesting that differences in vocal characteristics are probably affected by the same factors that drive genetic divergence. We argue that the evolution of song in this taxon is influenced by vicariant events, followed by accumulation of changes in song structure due to several possible factors: cultural drift in song structure; genetic drift in features related to song production; or natural selection acting on features that influence songs, such as body and beak size.