Trade-offs direct the evolution of coloration in Galápagos land snails.

Increasingly, multiple selective factors are recognized as jointly contributing to the evolution of morphology. What is not clear is how these forces vary across communities to promote morphological diversification among related species. In this study of Galápagos endemic snails (genus Naesiotus), we test several hypotheses of colour evolution. We observe mockingbirds (genus Mimus) predating live snails and find that avian predation selects against conspicuous shells. The evolutionary outcome of this selection is a diversity of shell colours across snails of the archipelago, each closely matching local backgrounds. We also find that snails more regularly exposed to the hot, equatorial sun reflect more light than shells of species from shadier habitats, suggesting a role for thermoregulatory constraints directing colour evolution. The signature of thermoregulatory selection is most clear in comparatively young communities (on the youngest islands), while the signature of selection from predators is most evident in older communities (on the older islands). Together, our findings point to a scenario of shifting selective forces along island ontogeny and community maturity that lead to the distribution of snail coloration we observe in Galápagos. Complex selective regimes such as these may have more responsibility for morphological diversity than is currently recognized.

Batesian mimics influence the evolution of conspicuousness in an aposematic salamander.

Conspicuousness, or having high contrast relative to the surrounding background, is a common feature of unpalatable species. Several hypotheses have been proposed to explain the occurrence of conspicuousness, and while most involve the role of conspicuousness as a direct signal of unpalatability to potential predators, one hypothesis suggests that exaggerated conspicuousness may evolve in unpalatable species to reduce predator confusion with palatable species (potential Batesian mimics). This hypothesis of antagonistic coevolution between palatable and unpalatable species hinges on the 'cost of conspicuousness', in which conspicuousness increases the likelihood of predation more in palatable species than in unpalatable species. Under this mimicry scenario, four patterns are expected: (i) mimics will more closely resemble local models than models from other localities, (ii) there will be a positive relationship between mimic and model conspicuousness, (iii) models will be more conspicuous in the presence of mimics, and (iv) when models and mimics differ in conspicuousness, mimics will be less conspicuous than models. We tested these predictions in the salamander mimicry system involving Notophthalmus viridescens (model) and one colour morph of Plethodon cinereus (mimic). All predictions were supported, indicating that selection for Batesian mimicry not only influences the evolution of mimics, but also the evolution of the models they resemble. These findings indicate that mimicry plays a large role in the evolution of model warning signals, particularly influencing the evolution of conspicuousness.