PLUMAGE AND SONG DIFFERENCES MEDIATE SPECIES RECOGNITION BETWEEN INCIPIENT FLYCATCHER SPECIES OF THE SOLOMON ISLANDS

Population variation in signaling behavior and contest outcome in the jacky dragon

Being aggressive and by extension, dominant, is an important mechanism for determining access to resources such as mates or territories. While predictors of contest outcome and dominance are increasingly studied, we have a poor understanding of how they vary across populations. Here, I use the widely distributed Australian agamid lizard, the Jacky dragon (Amphibolurus muricatus), to quantify variation in features predicting contest outcome among males of different populations. I measured physical attributes, maximal physiological performance capacity (sprint speed, endurance, bite force) and visual displays during staged encounters. I found that morphology, performance capacity and the type and frequency of visual displays used during agonistic interactions varied significantly across populations. Contest winners from the Cann River State Forest population favored tail-flicks and push-up/body-rocks, while those from Royal National Park were more likely to chase and individuals from Yarratt State Forest performed more bite-lunges than other populations. The losers of contests also differed in their displays. Individuals from the Cann River population were dominant over the others based on behavioral attributes (i.e., aggressive visual displays, chases and bite-lunges). I suggest that population differences in signal form and function could have implications for range dynamics as populations come into contact in an era of rapid environmental change.

Accelerated body size evolution in upland environments is correlated with recent speciation in South American freshwater fishes

Speciation rates vary greatly among taxa and regions and are shaped by both biotic and abiotic factors. However, the relative importance and interactions of these factors are not well understood. Here we investigate the potential drivers of speciation rates in South American freshwater fishes, the most diverse continental vertebrate fauna, by examining the roles of multiple biotic and abiotic factors. We integrate a dataset on species geographic distribution, phylogenetic, morphological, climatic, and habitat data. We find that Late Neogene-Quaternary speciation events are strongly associated with body-size evolution, particularly in lineages with small body sizes that inhabit higher elevations near the continental periphery. Conversely, the effects of temperature, area, and diversity-dependence, often thought to facilitate speciation, are negligible. By evaluating multiple factors simultaneously, we demonstrate that habitat characteristics associated with elevation, as well as body size evolution, correlate with rapid speciation in South American freshwater fishes. Our study emphasizes the importance of integrative approaches that consider the interplay of biotic and abiotic factors in generating macroecological patterns of species diversity.

Selective sweeps on different pigmentation genes mediate convergent evolution of island melanism in two incipient bird species

Insular organisms often evolve predictable phenotypes, like flightlessness, extreme body sizes, or increased melanin deposition. The evolutionary forces and molecular targets mediating these patterns remain mostly unknown. Here we study the Chestnut-bellied Monarch (Monarcha castaneiventris) from the Solomon Islands, a complex of closely related subspecies in the early stages of speciation. On the large island of Makira M. c. megarhynchus has a chestnut belly, whereas on the small satellite islands of Ugi, and Santa Ana and Santa Catalina (SA/SC) M. c. ugiensis is entirely iridescent blue-black (i.e., melanic). Melanism has likely evolved twice, as the Ugi and SA/SC populations were established independently. To investigate the genetic basis of melanism on each island we generated whole genome sequence data from all three populations. Non-synonymous mutations at the MC1R pigmentation gene are associated with melanism on SA/SC, while ASIP, an antagonistic ligand of MC1R, is associated with melanism on Ugi. Both genes show evidence of selective sweeps in traditional summary statistics and statistics derived from the ancestral recombination graph (ARG). Using the ARG in combination with machine learning, we inferred selection strength, timing of onset and allele frequency trajectories. MC1R shows evidence of a recent, strong, soft selective sweep. The region including ASIP shows more complex signatures; however, we find evidence for sweeps in mutations near ASIP, which are comparatively older than those on MC1R and have been under relatively strong selection. Overall, our study shows convergent melanism results from selective sweeps at independent molecular targets, evolving in taxa where coloration likely mediates reproductive isolation with the neighboring chestnut-bellied subspecies.

Chestnut-bellied Monarchs (Monarcha castaneiventris ugiensis) from two archipelagos in the Solomon Islands have evolved entirely black plumage from a chestnut ancestor (Monarcha castaneiventris megarhynchus), a phenomenon known as island melanism. We obtain and analyze whole genome sequences using traditional summary statistics and new methods that combine inference of the ancestral recombination graph with machine learning. We find multiple lines of evidence for independent selective sweeps on the MC1R and ASIP genes, a receptor/ligand pair which regulates the production of melanin. Melanism on each archipelago is mediated by mutations in one of these two genes. Mutations in and around MC1R underwent a recent soft sweep experiencing strong selection on the islands of Santa Ana and Santa Catalina, whereas selection was also strong but comparatively older for ASIP on the island of Ugi. We show how melanism originated under positive selection on independent molecular targets, evolving convergently in taxa where coloration mediates reproductive isolation.

Color and morphological differentiation in the Sinaloa Wren (Thryophilus sinaloa) in the tropical dry forests of Mexico: The role of environment and geographic isolation

The role and the degree to which environment and geographic isolation contribute to phenotypic diversity has been widely debated. Here, we studied phenotypic variation (morphology and plumage reflectance) in the Sinaloa Wren, an endemic bird distributed throughout the tropical dry forest (TDF) on the Mexican pacific slope where a pronounced variability in environmental conditions has been reported. In particular, we aimed: 1) to characterize phenotypic variation between subspecies; 2) to analyze the relationship between phenotypic and environmental variation in the context of classic ecogeographic rules, such as Bergmann’s, Allen’s, Gloger’s, and Bogert’s, and to quantify the relative roles of environment and geographic isolation and their interaction in shaping phenotypic variation; and 3) to test for niche conservatism between subspecies. Our data revealed significant differences among subspecies morphology and plumage reflectance. The environment explained a higher proportion of the morphological variation, while geography explained a smaller proportion. However, variation in plumage reflectance was mainly explained by the joint effect of geography and environment. Our data did not support for Bergmann´s and Allen´s rule. However, longer tails and wings were positively associated with higher elevations, larger tarsus and culmens were positively related to higher latitudes and to greater tree cover, respectively. Our data partially supported Gloger´s rule, where darker plumages were associated with more humid environments. The effects of temperature on plumage coloration were more consistent with Bogert´s rule. In addition, we found darker plumages related to higher levels of UV-B radiation. Finally, niche divergence was detected between T. s. cinereus and T. s. sinaloa vs. T. s. russeus. In a continuously distributed ecosystem such as the TDF on the pacific slope of Mexico, the environmental conditions and geographic isolation have played an important role in promoting phenotypic differentiation in the Sinaloa Wren.

Faster evolution of a premating reproductive barrier is not associated with faster speciation rates in New World passerine birds

Why are speciation rates so variable across the tree of life? One hypothesis is that this variation is explained by how rapidly reproductive barriers evolve. We tested this hypothesis by conducting a comparative study of the evolution of bird song, a premating barrier to reproduction. Speciation in birds is typically initiated when geographically isolated (allopatric) populations evolve reproductive barriers. We measured the strength of song as a premating barrier between closely related allopatric populations by conducting 2339 field experiments to measure song discrimination for 175 taxon pairs of allopatric or parapatric New World passerine birds, and estimated recent speciation rates from molecular phylogenies. We found evidence that song discrimination is indeed an important reproductive barrier: taxon pairs with high song discrimination in allopatry did not regularly interbreed in parapatry. However, evolutionary rates of song discrimination were not associated with recent speciation rates. Evolutionary rates of song discrimination were also unrelated to latitude or elevation, but species with innate song (suboscines) evolved song discrimination much faster than species with learned song (oscines). We conclude that song is a key premating reproductive barrier in birds, but faster evolution of this reproductive barrier between populations does not consistently result in faster diversification between species.

Correlated evolution of distinct signals associated with increased social selection in female white-shouldered fairywrens

Conspicuous female signals have recently received substantial scientific attention, but it remains unclear if their evolution is the result of selection acting on females independently of males or if mutual selection facilitates female change. Species that express female, but not male, phenotypic variation among populations represents a useful opportunity to address this knowledge gap. White-shouldered fairywrens (Malurus alboscapulatus) are tropical songbirds with a well-resolved phylogeny where female, but not male, coloration varies allopatrically across subspecies. We explored how four distinct signaling modalities, each putatively associated with increased social selection, are expressed in two populations that vary in competitive pressure on females. Females in a derived subspecies (M. a. moretoni) have evolved more ornamented plumage and have shorter tails (a signal of social dominance) relative to an ancestral subspecies (M. a. lorentzi) with drab females. In response to simulated territorial intrusions broadcasting female song, both sexes of M. a. moretoni are more aggressive and more coordinated with their mates in both movement and vocalizations. Finally, M. a. moretoni songs are more complex than M. a. lorentzi, but song complexity does not vary between sexes in either population. These results suggest that correlated phenotypic shifts in coloration and tail morphology in females as well as song complexity and aggression in both sexes may have occurred in response to changes in the intensity of social selection pressures. This highlights increased competitive pressures in both sexes can facilitate the evolution of complex multimodal signals.

Experimental tests of selection against heterospecific aggression as a driver of avian colour pattern divergence

Signal divergence is thought to reduce the costs of co-occurrence for closely related species and may thereby be important in the generation and maintenance of new biodiversity. In birds, closely related, sympatric species are more divergent in their colour patterns than those that live apart, but the selective pressures driving sympatric divergence in colour pattern are not well-understood. Here, we conducted field experiments on naïve birds using spectrometer-matched, painted, 3D-printed models to test whether selection against heterospecific aggression might drive colour pattern divergence in the genus Poecile. We found that territorial male black-capped chickadees (P. atricapillus) are equally likely to attack sympatric and allopatric congeners, and wintering flocks are equally likely to visit feeders occupied by sympatric and allopatric congeners, despite sympatric congeners being more divergent in colour pattern. These results suggest that either the concerted evolution of additional traits (e.g. discrimination), or interactions in sympatry that promote learning, is required if colour pattern divergence among sympatric species is to reduce heterospecific aggression. Alternatively, colour pattern divergence among sympatric species may be caused by other selective pressures, such as selection against hybridization or habitat partitioning and secondary signal adaptation.

Asymmetric acoustic signal recognition led to asymmetric gene flow between two parapatric frogs

Correct discrimination between courtship signals could help to maintain genetic integrity between closely related species. However, asymmetric usage of signals might cause asymmetric gene flow across the contact zone. Buergeria choui and B. otai are sibling-species with a parapatric distribution pattern in Taiwan, having two narrow contact zones on the east and west sides of the island. Combining behavioural experiments with genome-wide RAD-seq analyses, we test whether the ability of signal recognition influences genetic introgression across their species boundary. The playback experiments show that all B. choui populations respond strongest to their own ‘cricket’ trills, while the western population of B. otai have evolved a strong level of reproductive character displacement by showing the inclusive usage of the unique ‘chicken’ signals. In contrast, the eastern B. otai population uses both ‘chicken’ and ‘cricket’ trills, and has a stronger preference for the latter. The weak reproductive character displacement in the eastern population has led to asymmetry genetic introgression from B. choui toward B. otai. Our results support the prediction that a more specialized signal-user, compared to its sibling, generalized signal-user, might have a higher probability of maintaining their genetic integrity in the secondary contact region.

Rapid speciation via the evolution of pre-mating isolation in the Iberá Seedeater

Behavioral isolation can catalyze speciation and permit the slow accumulation of additional reproductive barriers between co-occurring organisms. We illustrate how this process occurs by examining the genomic and behavioral bases of pre-mating isolation between two bird species (Sporophila hypoxantha and the recently discovered S. iberaensis) that belong to the southern capuchino seedeaters, a recent, rapid radiation characterized by variation in male plumage coloration and song. Although these two species co-occur without obvious ecological barriers to reproduction, we document behaviors indicating species recognition by song and plumage traits and strong assortative mating associated with genomic regions underlying male plumage patterning. Plumage differentiation likely originated through the reassembly of standing genetic variation, indicating how novel sexual signals may quickly arise and maintain species boundaries.

Integumentary Colour Allocation in the Stork Family (Ciconiidae) Reveals Short-Range Visual Cues for Species Recognition

The family Ciconiidae comprises 19 extant species which are highly social when nesting and foraging. All species share similar morphotypes, with long necks, a bill, and legs, and are mostly coloured in the achromatic spectrum (white, black, black, and white, or shades of grey). Storks may have, however, brightly coloured integumentary areas in, for instance, the bill, legs, or the eyes. These chromatic patches are small in surface compared with the whole body. We have analyzed the conservatism degree of colouration in 10 body areas along an all-species stork phylogeny derived from BirdTRee using Geiger models. We obtained low conservatism in frontal areas (head and neck), contrasting with a high conservatism in the rest of the body. The frontal areas tend to concentrate the chromatic spectrum whereas the rear areas, much larger in surface, are basically achromatic. These results lead us to suggest that the divergent evolution of the colouration of frontal areas is related to species recognition through visual cue assessment in the short-range, when storks form mixed-species flocks in foraging or resting areas.

A review of conspecific attraction for habitat selection across taxa

Many species across taxa select habitat based on conspecific presence, known as conspecific attraction. Studies that document conspecific attraction typically provide social information (i.e., cues that indicate the presence of a given species) and then determine if a given species is more likely to settle at locations where the social information is provided compared to those locations that do not. Although the number of studies examining conspecific attraction has grown in recent years, a comprehensive review has not yet been undertaken. Here, we conducted a review of the literature and found 151 studies investigating conspecific attraction across eight taxa. We found that conspecific attraction is widespread with between 80% and 100% of studies, depending on taxa, documenting positive associations between habitat selection and the presence of conspecific cues. Conspecific attraction has been documented more frequently in bird and fish species with less attention given to invertebrate and mammal species. We use the patterns we found to (a) provide an overview of the current state of research on conspecific attraction and (b) discuss how important factors, such as cue characteristics and life history traits, may play a role in shaping conspecific attraction patterns within and across taxa.

Vocal recognition suggests premating isolation between lineages of a lekking hummingbird

Species with genetically differentiated allopatric populations commonly differ in phenotypic traits due to drift and/or selection, which can be important drivers of reproductive isolation. Wedge-tailed sabrewing (Campylopterus curvipennis) is a species complex composed of three genetically and acoustically differentiated allopatric lineages that correspond to currently recognized subspecies in Mexico: C. c. curvipennis (Sierra Madre Oriental), C. c. pampa (Yucatán Peninsula), and C. c. excellens (Los Tuxtlas). Although excellens is taxonomically recognized as a distinct species, there is genetic evidence that lineages excellens and curvipennis have diverged from each other later than pampa. In this study, we experimentally tested C. c. curvipennis song recognition as a major factor in premating reproductive isolation for lineage recognition. To this end, we conducted a song playback experiment to test whether territorial males of one C. c. curvipennis lek discriminate among potential competitors based on male songs from the three lineages. Males of curvipennis responded more aggressively to songs of their own lineage and excellens, than to songs of the most divergent lineage pampa, as evidenced by significant differences in a variety of intensity and latency response variables. This indicate that the pampa male song does not represent a competitive threat as curvipennis and excellens songs, in which divergence and song recognition represent premating reproductive isolation between these isolated lineages. However, the acoustic limits between curvipennis and excellens might be attenuated by gene flow in case of secondary contact between them, despite the strong and relatively rapid divergence of their sexually selected song traits.

Song playbacks demonstrate slower evolution of song discrimination in birds from Amazonia than from temperate North America

Genetic data indicate differences in speciation rate across latitudes, but underlying causes have been difficult to assess because a critical phase of the speciation process is initiated in allopatry, in which, by definition, individuals from different taxa do not interact. We conducted song playback experiments between 109 related pairs of mostly allopatric bird species or subspecies in Amazonia and North America to compare the rate of evolution of male discrimination of songs. Relative to local controls, the number of flyovers and approach to the speaker were higher in Amazonia. We estimate that responses to songs of relatives are being lost about 6 times more slowly in Amazonia than in North America. The slow loss of response holds even after accounting for differences in song frequency and song length. Amazonian species with year-round territories are losing aggressive responses especially slowly. We suggest the presence of many species and extensive interspecific territoriality favors recognition of songs sung by sympatric heterospecifics, which results in a broader window of recognition and hence an ongoing response to novel similar songs. These aggressive responses should slow the establishment of sympatry between recently diverged forms. If male responses to novel allopatric taxa reflect female responses, then premating reproductive isolation is also evolving more slowly in Amazonia. The findings are consistent with previously demonstrated slower recent rates of expansion of sister taxa into sympatry, slower rates of evolution of traits important for premating isolation, and slower rates of speciation in general in Amazonia than in temperate North America.

Symmetrical discrimination despite weak song differentiation in 2 suboscine bird sister species

Song mediates territorial competition and mate choice in birds and population divergence in this signal can have important evolutionary consequences. For example, divergent songs can act in specific recognition and limit gene flow and, hence, have a fundamental role on the origin and/or integrity of evolutionary lineages. Especially interesting systems to test the role of song in specific recognition are species pairs that present small structural differences in this signal. Here, we perform song play-back experiments on males of a long-diverged sister pair of Neotropical Suboscine species, the squamate antbird (Myrmoderus squamosus) and the white-bibbed antbird (Myrmoderus loricatus), which occur in parapatry in the Atlantic Forest and that overlap extensively in song variation. Previous evidence indicates that genetic introgression between these species is either absent or negligible, suggesting that vocal discrimination or other mechanisms function as effective barriers to gene flow. Our results show that responses to heterospecific songs were symmetrical and intermediary compared with responses to conspecific songs in both species. A stronger response to conspecific territorial songs suggests that conspecific individuals pose greater competitive threat than heterospecifics. An important implication of our study is that even small song differences can play an important role in specific recognition.

Mate choice in a changing world

Human activities by altering environmental conditions are influencing the mate choice of animals. This is by impacts on: (i) the production and expression of traits evaluated by mate choosers; (ii) the transmission of information about potential mates to choosers; (iii) the reception and processing of the information by choosers; and (iv) the final mate choice. Here, I first discuss how these four stages of the mate-choice process can be altered by environmental change, and how these alterations, in turn, can influence individuals, populations, and communities. Much evidence exists for human-induced environmental changes influencing mate choice, but the consequences for the fitness of courters and choosers are less well known, and even less is known about the impact on population dynamics, species interactions and community composition. More evidence exists for altered mate-choice systems influencing interspecific matings and thereby community composition and biodiversity. I then consider whether plastic adjustments and evolutionary changes can rescue adaptive mate-choice systems, and reflect on the possibility of non-adaptive mate-choice systems becoming less maladaptive under environmental change. Much evidence exists for plastic adjustments of mate-choice systems, but whether these are adaptive is seldom known, as is the contribution of genetic changes. Finally, I contemplate the possibility of mate-choice systems rescuing populations from decline in changing environments. I explain how this is context dependent with both positive and negative outcomes possible. In summary, while much evidence exists for human-induced environmental changes influencing mate-choice systems, less is known about the consequences for ecological and evolutionary processes. Considering the importance that mate choice plays in determining individual fitness and population viability, the effects of environmental change on mate-choice systems should be considered in studies on the ecological and evolutionary consequences of human disturbances to habitats.

Scary clowns: adaptive function of anemonefish coloration

Clownfishes, with their showy coloration, are well known for their symbiosis with sea anemones and for their hierarchical reproductive system, but the function of their coloration is unclear. We used a phylogeny of 27 clownfish species to test whether fish coloration (i) serves a protective function that involves their anemone hosts, or (ii) signals species identity in species with overlapping host ranges that can potentially share the same host. We tested for an association between fish colour pattern traits, host morphology and host toxicity and examined coloration in relation to host sharing and geographic proximity. Fish with fewer stripes occupied fewer anemone species, and hosts with shorter tentacles, than fish with multiple stripes. There was a negative relationship between anemone toxicity and tentacle length and these protective traits together were correlated with the evolution of stripes. Host sharing or range overlap was not associated with coloration divergence. We propose that ancestral anemonefishes had multiple stripes that served for hiding/camouflage among the hosts' long tentacles, whereas increased specialization towards fewer and more toxic hosts (with shorter tentacles) led to the use of coloration as an aposematic signal. The intriguing notion that an aposematic signal could advertise the defence of another species may reflect the unique symbiotic relationship between anemonefishes and their hosts.

Mutations in different pigmentation genes are associated with parallel melanism in island flycatchers

The independent evolution of similar traits across multiple taxa provides some of the most compelling evidence of natural selection. Little is known, however, about the genetic basis of these convergent or parallel traits: are they mediated by identical or different mutations in the same genes, or unique mutations in different genes? Using a combination of candidate gene and reduced representation genomic sequencing approaches, we explore the genetic basis of and the evolutionary processes that mediate similar plumage colour shared by isolated populations of the Monarcha castaneiventris flycatcher of the Solomon Islands. A genome-wide association study (GWAS) that explicitly controlled for population structure revealed that mutations in known pigmentation genes are the best predictors of parallel plumage colour. That is, entirely black or melanic birds from one small island share an amino acid substitution in the melanocortin-1 receptor (MC1R), whereas similarly melanic birds from another small island over 100 km away share an amino acid substitution in a predicted binding site of agouti signalling protein (ASIP). A third larger island, which separates the two melanic populations, is inhabited by birds with chestnut bellies that lack the melanic MC1R and ASIP allelic variants. Formal FST outlier tests corroborated the results of the GWAS and suggested that strong, directional selection drives the near fixation of the MC1R and ASIP variants across islands. Our results, therefore, suggest that selection acting on different mutations with large phenotypic effects can drive the evolution of parallel melanism, despite the relatively small population size on islands.

Mating status correlates with dorsal brightness in some but not all poison frog populations

Sexual signals are important for intraspecific communication and mate selection, but their evolution may be driven by both natural and sexual selection, and stochastic processes. Strawberry poison frogs (Oophaga pumilio) show strong color divergence among populations, but coloration also varies among individuals of the same population. The importance of coloration for female mate choice has been studied intensely, and sexual selection seems to affect color divergence in strawberry poison frogs. However, the effect of coloration on mating success under field conditions has received very little attention. Furthermore, few studies examined how phenotypic variation among individuals of the same color morph affects mate selection under natural conditions. We measured the spectral reflectance of courting and noncourting individuals and their background substrates in three geographically separated populations. In one population (Sarapiquí, Costa Rica), we found that naturally occurring courting pairs of males and females had significantly brighter dorsal coloration than individual males and females not engaged in courtship interactions. Our field observations suggest that, in the wild, females prefer brighter males while the reason for the higher courtship activity of brighter females remains unclear. Overall our results imply that brightness differences among individuals of the same color morph may actually affect reproductive success in some populations of strawberry poison frogs.

Evolution and plasticity: Divergence of song discrimination is faster in birds with innate song than in song learners in Neotropical passerine birds

Plasticity is often thought to accelerate trait evolution and speciation. For example, plasticity in birdsong may partially explain why clades of song learners are more diverse than related clades with innate song. This "song learning" hypothesis predicts that (1) differences in song traits evolve faster in song learners, and (2) behavioral discrimination against allopatric song (a proxy for premating reproductive isolation) evolves faster in song learners. We tested these predictions by analyzing acoustic traits and conducting playback experiments in allopatric Central American sister pairs of song learning oscines (N = 42) and nonlearning suboscines (N = 27). We found that nonlearners evolved mean acoustic differences slightly faster than did leaners, and that the mean evolutionary rate of song discrimination was 4.3 times faster in nonlearners than in learners. These unexpected results may be a consequence of significantly greater variability in song traits in song learners (by 54-79%) that requires song-learning oscines to evolve greater absolute differences in song before achieving the same level of behavioral song discrimination as nonlearning suboscines. This points to "a downside of learning" for the evolution of species discrimination, and represents an important example of plasticity reducing the rate of evolution and diversification by increasing variability.

Phenotypic divergence during speciation is inversely associated with differences in seasonal migration

Differences in seasonal migration might promote reproductive isolation and differentiation by causing populations in migratory divides to arrive on the breeding grounds at different times and/or produce hybrids that take inferior migratory routes. We examined this question by quantifying divergence in song, colour, and morphology between sister pairs of North American migratory birds. We predicted that apparent rates of phenotypic differentiation would differ between pairs that do and do not form migratory divides. Consistent with this prediction, results from mixed effects models and Ornstein-Uhlenbeck models of evolution showed different rates of divergence between these groups; surprisingly, differentiation was greater among non-divide pairs. We interpret this finding as a result of variable rates of population blending and fusion between partially diverged forms. Ancient pairs of populations that subsequently fused are now observed as a single form, whereas those that did not fuse are observable as pairs and included in our study. We propose that fusion of two populations is more likely to occur when they have similar migratory routes and little other phenotypic differentiation that would cause reproductive isolation. By contrast, pairs with migratory divides are more likely to remain reproductively isolated, even when differing little in other phenotypic traits. These findings suggest that migratory differences may be one among several isolating barriers that prevent divergent populations from fusing and thereby increase the likelihood that they will continue differentiating as distinct species.

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.

Signal Diversity, Sexual Selection, and Speciation

Communication is ubiquitous. Developing a framework for the diversity of signals has important consequences for understanding alternative models of sexual selection and the processes contributing to speciation. In this article we review how models of neutral evolution in the perceptual space of signal perceivers provide a first step toward constructing a framework for signal diversity. We discuss how the distinction between additive and multiplicative effects of multimodal signaling represents a second step. We then assess how signal efficiency, reliability, and the aesthetics of perceivers provide distinct mechanisms for signals to be effective, thereby partly explaining signal diversity. Understanding the relative contribution of each of these mechanisms to the effectiveness of mate choice signals unravels the relative importance of alternative models of sexual selection. It can also help to distinguish whether divergence of communication is a driver or a consequence of speciation. Throughout the review we emphasize the importance of verification and learning in repeated interactions for understanding variation in signals.