The limits of evolutionary convergence in sympatry: reproductive interference and developmental constraints leading to local diversity in aposematic signals

Mutualistic interaction between defended species is a striking case of evolutionary convergence in sympatry, driven by the increased protection against predators brought by mimicry. However, such convergence is often limited: sympatric defended species frequently display different or imperfectly similar warning traits. The phylogenetic distance between sympatric species may indeed prevent evolution towards the exact same signal. Moreover, warning traits are also implied in mate recognition, so that trait convergence might result in heterospecific courtship and mating. Here, we investigate the strength and direction of convergence in warning trait in defended species with different ancestral traits, using a mathematical model. We specifically determine the effect of phenotypic distances among ancestral traits of converging species, and costs of heterospecific sexual interactions on imperfect mimicry and trait divergence. Our analytical results confirm that reproductive interference limits the convergence of warning trait, leading to either imperfect mimicry or complete divergence. More surprisingly, our model pinpoints that reproductive interference can change the direction of convergence depending on the relative species densities. We also show that reproductive interference can generate imperfect mimicry only between species with different ancestral traits. Our model therefore highlights that convergence triggered by Mullerian mimicry not only depends on relative defence levels, but that relative species densities, heterospecific sexual interactions and ancestral traits interfere in the direction and strength of convergence between species.