Species-specific song responses emerge as a by-product of tuning to the local dialect

Oscine birds preferentially respond to certain sounds over others from an early age, which focuses subsequent learning onto sexually relevant songs.^1,2,3 Songs vary both across species and, due to cultural evolution, among populations of the same species. As a result, early song responses are expected to be shaped by selection both to avoid the fitness costs of cross-species learning⁴ and to promote learning of population-typical songs.⁵ These sources of selection are not mutually exclusive but can result in distinct geographic patterns of song responses in juvenile birds: if the risks of interspecific mating are the main driver of early song discrimination, then discrimination should be strongest where closely related species co-occur.⁴ In contrast, if early discrimination primarily facilitates learning local songs, then it should be tuned to songs typical of the local dialect.^5,6,7 Here, we experimentally assess the drivers of song discrimination in nestling pied flycatchers (Ficedula hypoleuca). We first demonstrate that early discrimination against the songs of the closely related collared flycatcher (F. albicollis) is not strongly affected by co-occurrence. Second, across six European populations, we show that nestlings' early song responses are tuned to their local song dialect and that responses to the songs of collared flycatchers are similarly weak as to those of other conspecific dialects. Taken together, these findings provide clear experimental support for the hypothesis that cultural evolution, in conjunction with associated learning predispositions, drives the emergence of pre-mating reproductive barriers.

Secondary contact rather than coexistence-Erebia butterflies in the Alps

Secondary contact zones are ideal systems to study the processes that govern the evolution of reproductive barriers, especially at advanced stages of the speciation process. An increase in reproductive isolation resulting from selection against maladaptive hybrids is thought to contribute to reproductive barrier buildup in secondary contact zones. Although such processes have been invoked for many systems, it remains unclear to which extent they influence contact zone dynamics in nature. Here, we study a very narrow contact zone between the butterfly species Erebia cassioides and Erebia tyndarus in the Swiss Alps. We quantified phenotypic traits related to wing shape and reproduction as well as ecology to compare the degree of intra- and interspecific differentiation. Even though only very few first-generation hybrids occur, we find no strong indications for current reinforcing selection, suggesting that if reinforcement occurred in our system, it likely operated in the past. Additionally, we show that both species differ less in their ecological niche at the contact zone than elsewhere, which could explain why coexistence between these butterflies may currently not be possible.

Female mate preferences do not predict male sexual signals across populations

New species can arise when female preferences and male sexual signals diverge across populations and thereby reduce mating between populations. Under this hypothesized mechanism for speciation, mate preferences and sexual signals should be correlated, but divergent, across populations. We evaluated this prediction using spadefoot toads (Spea multiplicata). We measured a sexually selected male signal (call rate) for which female preferences are known to vary across populations in response to the risk of hybridizing with another species. Contrary to expectation, we found no correlation between male call rate and female preferences across populations. We discuss possible mechanisms of this pattern, including the effect of gene flow from heterospecifics on male call rate. Our results suggest that, even when populations vary in mating traits, the independent evolution of female preferences and male sexual signals might impede reproductive isolation between populations.

Geographic contact drives increased reproductive isolation in two cryptic Empidonax flycatchers

Geographic contact between sister lineages often occurs near the final stages of speciation, but its role in speciation's completion remains debated. Reproductive isolation may be essentially complete prior to secondary contact. Alternatively, costly interactions between partially reproductively isolated species - such as maladaptive hybridization or competition for resources - may select for divergence, increasing reproductive isolation and driving speciation toward completion. Here, we use coalescent demographic modelling and whole-genome data sets to show that a period of contact and elevated hybridization between sympatric eastern North American populations of two cryptic bird species preceded a major increase in reproductive isolation between these populations within the last 10,000 years. In contrast, substantial introgression continues to the present in a western contact zone where geographic overlap is much narrower and probably of more recent origin. In the sympatric eastern region where reproductive isolation has increased, it is not accompanied by character displacement in key morphometric traits, plumage coloration, or ecological traits. While the precise trait and underlying mechanism driving increased reproductive isolation remains unknown, we discuss several possibilities and outline avenues for future research. Overall, our results highlight how demographic models can reveal the geographic context in which reproductive isolation was completed, and demonstrate how contact can accelerate the final stages of speciation.

Asymmetric reinforcement in Lucania killifish: assessing reproductive isolation when both sexes choose

Reinforcement can occur when maladaptive hybridization in sympatry favors the evolution of conspecific preferences and target traits that promote behavioral isolation (BI). In many systems, enhanced BI is due to increased female preference for conspecifics. In others, BI is driven by male preference, and in other systems both sexes exert preferences. Some of these patterns can be attributed to classic sex-specific costs and benefits of preference. Alternatively, sex differences in conspecific preference can emerge due to asymmetric postzygotic isolation (e.g., hybrid offspring from female A × male B have lower fitness than hybrid offspring from female B × male A), which can lead to asymmetric BI (e.g., female A and male B are less likely to mate than female B and male A). Understanding reinforcement requires understanding how conspecific preferences evolve in sympatry. Yet, estimating conspecific preferences can be difficult when both sexes are choosy. In this study, we use Lucania killifish to test the hypothesis that patterns of reinforcement are driven by asymmetric postzygotic isolation between species. If true, we predicted that sympatric female Lucania goodei and sympatric male L. parva should have lower levels of BI compared with their sympatric counterparts, as they produce hybrid offspring with the highest fitness. To address the problem of measuring BI when both sexes are choosy, we inferred the contribution to BI of each partner using assays where one sex in the mating pair comes from an allopatric population with potentially low preference, whereas the other comes from a sympatric population with high preference. For one hybrid cross direction, we found that both female L. parva and male L. goodei have high contributions to BI in sympatry. In the other hybrid cross direction, we found that only female L. goodei contribute to BI. Sympatric male L. parva readily engaged in hybrid spawnings with allopatric L. goodei females. These results indicate that both asymmetric postzygotic isolation and the traditional sex-specific costs to preference likely affect the nature of selection on conspecific preferences and target traits.

Heterospecific mating interactions as an interface between ecology and evolution

Reproductive interference (costly interspecific sexual interactions) is well-understood to promote divergence in mating-relevant traits (i.e. reproductive character displacement: RCD), but it can also reduce population growth, eventually leading to local extinction of one of the species. The ecological and evolutionary processes driven by reproductive interference can interact with each other. These interactions are likely to influence whether the outcome is coexistence or extinction, but remain little studied. In this paper, we first develop an eco-evolutionary perspective on reproductive interference by integrating ecological and evolutionary processes in a common framework. We also present a simple model to demonstrate the eco-evolutionary dynamics of reproductive interference. We then identify a number of factors that are likely to influence the relative likelihoods of extinction or RCD. We discuss particularly relevant factors by classifying them into four categories: the nature of the traits responding to selection, the mechanisms determining the expression of these traits, mechanisms of reproductive interference and the ecological background. We highlight previously underappreciated ways in which these factors may influence the relative likelihoods of RCD and local extinction. By doing so, we also identify questions and future directions that will increase our holistic understanding of the outcomes of reproductive interference.

Time spent together and time spent apart affect song, feather colour and range overlap in tinkerbirds

Most studies on the processes driving evolutionary diversification highlight the importance of genetic drift in geographical isolation and natural selection across ecological gradients. Direct interactions among related species have received much less attention, but they can lead to character displacement, with recent research identifying patterns of displacement attributed to either ecological or reproductive processes. Together, these processes could explain complex, trait-specific patterns of diversification. Few studies, however, have examined the possible effects of these processes together or compared the divergence in multiple traits between interacting species among contact zones. Here, we show how traits of two Pogoniulus tinkerbird species vary among regions across sub-Saharan Africa. However, in addition to variation between regions consistent with divergence in refugial isolation, both song and morphology diverge between the species where they coexist. In West Africa, where the species are more similar in plumage, there is possible competitive or reproductive exclusion. In Central and East Africa, patterns of variation are consistent with agonistic character displacement. Molecular analyses support the hypothesis that differences in the age of interaction among regions can explain why species have evolved phenotypic differences and coexist in some regions but not others. Our findings suggest that competitive interactions between species and the time spent interacting, in addition to the time spent in refugial isolation, play important roles in explaining patterns of species diversification.

Hybridization increases population variation during adaptive radiation

Adaptive radiations are prominent components of the world's biodiversity. They comprise many species derived from one or a small number of ancestral species in a geologically short time that have diversified into a variety of ecological niches. Several authors have proposed that introgressive hybridization has been important in the generation of new morphologies and even new species, but how that happens throughout evolutionary history is not known. Interspecific gene exchange is expected to have greatest impact on variation if it occurs after species have diverged genetically and phenotypically but before genetic incompatibilities arise. We use a dated phylogeny to infer that populations of Darwin's finches in the Galápagos became more variable in morphological traits through time, consistent with the hybridization hypothesis, and then declined in variation after reaching a peak. Some species vary substantially more than others. Phylogenetic inferences of hybridization are supported by field observations of contemporary hybridization. Morphological effects of hybridization have been investigated on the small island of Daphne Major by documenting changes in hybridizing populations of Geospiza fortis and Geospiza scandens over a 30-y period. G. scandens showed more evidence of admixture than G. fortis Beaks of G. scandens became progressively blunter, and while variation in length increased, variation in depth decreased. These changes imply independent effects of introgression on 2, genetically correlated, beak dimensions. Our study shows how introgressive hybridization can alter ecologically important traits, increase morphological variation as a radiation proceeds, and enhance the potential for future evolution in changing environments.

Divergence in female damselfly sensory structures is consistent with a species recognition function but shows no evidence of reproductive character displacement

Males and females transmit and receive signals prior to mating that convey information such as sex, species identity, or individual condition. In some animals, tactile signals relayed during physical contact between males and females before and during mating appear to be important for mate choice or reproductive isolation. This is common among odonates, when a male grasps a female's thorax with his terminal appendages prior to copulation, and the female subsequently controls whether copulation occurs by bending her abdomen to complete intromission. It has been hypothesized that mechanosensory sensilla on the female thoracic plates mediate mating decisions, but is has been difficult to test this idea. Here, we use North American damselflies in the genus Enallagma (Odonata: Coenagrionidae) to test the hypothesis that variation in female sensilla traits is important for species recognition. Enallagma anna and E. carunculatum hybridize in nature, but experience strong reproductive isolation as a consequence of divergence in male terminal appendage morphology. We quantified several mechanosensory sensilla phenotypes on the female thorax among multiple populations of both species and compared divergence in these traits in sympatry versus allopatry. Although these species differed in features of sensilla distribution within the thoracic plates, we found no strong evidence of reproductive character displacement among the sensilla traits we measured in regions of sympatry. Our results suggest that species-specific placement of female mechanoreceptors may be sufficient for species recognition, although other female sensory phenotypes might have diverged in sympatry to reduce interspecific hybridization.

Interspecific competition, hybridization, and reproductive isolation in secondary contact: missing perspectives on males and females

Research on sexual selection and hybridization has focused on female mate choice and male-male competition. While the evolutionary outcomes of interspecific female preference have been well explored, we are now gaining a better understanding of the processes by which male-male competition between species in secondary contact promotes reproductive isolation versus hybridization. What is relatively unexplored is the interaction between female choice and male competition, as they can oppose one another or align with similar outcomes for reproductive isolation. The role of female-female competition in hybridization is also not well understood, but could operate similarly to male-male competition in polyandrous and other systems where costs to heterospecific mating are low for females. Reproductive competition between either sex of sympatric species can cause the divergence and/or convergence of sexual signals and recognition, which in turn influences the likelihood for interspecific mating. Future work on species interactions in secondary contact should test the relative influences of both mate choice and competition for mates on hybridization outcomes, and should not ignore the possibilities that females can compete over mating resources, and males can exercise mate choice.