One proposed mechanism of speciation is divergent sexual selection, whereby divergence in female preferences and male signals results in behavioural isolation. Despite the appeal of this hypothesis, evidence for it remains inconclusive. Here, we present several lines of evidence that sexual selection is driving behavioural isolation and speciation among populations of an Amazonian frog (Physalaemus petersi ). First, sexual selection has promoted divergence in male mating calls and female preferences for calls between neighbouring populations, resulting in strong behavioural isolation. Second, phylogenetic analysis indicates that populations have become fixed for alternative call types several times throughout the species' range, and coalescent analysis rejects genetic drift as a cause for this pattern, suggesting that this divergence is due to selection. Finally, gene flow estimated with microsatellite loci is an average of 30 times lower between populations with different call types than between populations separated by a similar geographical distance with the same call type, demonstrating genetic divergence and incipient speciation. Taken together, these data provide strong evidence that sexual selection is driving behavioural isolation and speciation, supporting sexual selection as a cause for speciation in the wild.

Many animals advertise their chemical defense to predators with conspicuous coloration and unpalatability, but little is known about the information in these signal elements. To effectively avoid predation, is it more advantageous to invest in increased conspicuousness or greater noxiousness, or to allocate equally to both signal modalities? Using natural variation among poison frog species measured with spectral reflectance and toxicity assays, we tested the relative importance of warning signal components with predator-learning and avoidance experiments. We demonstrate that closely related species use alternative strategies: increasing either conspicuousness or toxicity affords equivalent avoidance by predators and provides protection to nontoxic mimic species. These equally effective predator avoidance tactics demonstrate different aposematic solutions for two potentially costly signal components, providing a mechanism for natural diversity in warning signals.aposematism ͉ chemical defense ͉ predation

No abstract

1 . Mimetic advantage is considered to be dependent on frequency because an increase in mimic abundance leads to breakdown of the warning signal 2,3 . Where multiple toxic species are available, batesian polymorphism 4 is predicted-that is, mimics diversify to match sympatric models. Despite the prevalence of batesian mimicry in nature 5 , batesian polymorphism is relatively rare 6 . Here we explore a poison-frog mimicry complex comprising two parapatric models and a geographically dimorphic mimic that shows monomorphism where models co-occur. Contrary to classical predictions, our toxicity assays, field observations and spectral reflectances show that mimics resemble the less-toxic and less-abundant model. We examine "stimulus generalization" 7 as a mechanism for this nonintuitive result with learning experiments using naive avian predators and live poison frogs. We find that predators differ in avoidance generalization depending on toxicity of the model, conferring greater protection to mimics resembling the lesstoxic model owing to overlap of generalized avoidance curves. Our work supports a mechanism of toxicity-dependent stimulus generalization 8 , revealing an additional solution for batesian mimicry where multiple models coexist.In batesian mimicry, an edible species co-opts a warning signal from an unpalatable species to gain advantage through predator deception 1 . If batesian mimics are too common, however, this advantage breaks down as predators learn to ignore the warning signal. Where more than one model species is available 4 , diversifying frequency-dependent selection predicts the evolution of polymorphism in which mimics diverge in appearance to resemble sympatric models 6,9,10 . Batesian polymorphism is suggested to distribute warning signal degradation over several defended model species, enabling the mimic to increase in abundance. Reported accounts of such mimetic polymorphism, however, are relatively rare 6 and unknown in vertebrate mimicry systems 11,12 .Here we investigate a mimicry system that is inconsistent with the predictions of frequency dependence. We examine a poison-frog mimicry complex composed of two parapatric models and a geographically varying mimic (Fig. 1). The model Ecuadorian poison frogs Epipedobates bilinguis and Epipedobates parvulus share a similar warning signal of a bright red-spotted dorsum but differ in axilla and groin colouration (Fig. 1b). Their phylogenetically distant relative 13 , Allobates zaparo, is geographically dimorphic, matching each warning signal where models are parapatric (Fig. 1b). Where the two models co-occur, however, the mimic resembles only a single model (E. bilinguis; Figs 1 and 2). Here we use spectral reflectances, toxicity assays, field abundance measurements and predator learning experiments to investigate mechanisms that may be contributing to this pattern in nature.Theoretical and empirical studies predict that coexistence of aposematic models may lead to (1) batesian polymorphism 6,9,10 , (2) evolution of a mimic phenotype intermed...

Defensive mechanisms, including noxious or toxic substances, are favored by predation-driven natural selection. The acquisition of noxious/toxic substances can be either endogenous, in which the substances are produced by the organism, or exogenous, in which the substances are produced by another organism and are sequestered. Evidence indicates that the defensive skin alkaloids of Neotropical poison frogs (Dendrobatidae) have an exogenous source: a diet of ants and other small alkaloid-containing arthropods, which we term the diet-toxicity hypothesis. A critical prediction of the diet-toxicity hypothesis is that independent origins of dietary specialization will be found to be correlated with independent origins of skin alkaloids. We tested this prediction in an integrated framework using comparative methods with new and published data on feeding ecology and chemical defense for 15 species of dendrobatids in five genera. We found a significant correlation between alkaloid profiles and degree of dietary specialization. This reveals a recurring association of dietary specialization and alkaloid sequestration in dendrobatids, which suggests parallel evolutionary trends in the origins of defensive mechanisms.