The evolution of female polymorphism in a Neotropical radiation of lizards

Abstract

Color pattern polymorphism – the occurrence of multiple color patterns within populations – has provided excellent opportunities for the study of evolution. Recently, a renewed interest in polymorphism was sparked by the realization that variation among females may be more important to evolutionary processes than originally thought. Few species have been thoroughly studied with regards to female polymorphism (FP), and most studies have focused on single species, thus ignoring the broad effects of evolutionary history. I present anoles (Squamata: Polychrotidae) as a model for studying FP. This speciose lizard radiation contains several species with discontinuous variation in female dorsal patterns. My overall question addressed the origination and maintenance of FP in anoles. As recommended for studies of possible adaptive traits, both pattern and process of its evolution were addressed by combining phylogenetic and geographic analyses with population studies of the female polymorphic anole Norops humilis. Phylogentic signal was determined to be moderate, resulting from independent evolution of FP in ancestors of multiple anole clades as well as in individual species, as indicated by parsimony and maximum likelihood methods. Combining phylogenetic and geographic distribution of FP showed a dichotomy in the evolution of this trait. Among the basal radiation of anoles, island species were commonly polymorphic, while mainland species were not. The opposite pattern was seen in the rest of the anole radiation. Comparative analyses revealed similarities in habitat use and especially perch use among female polymorphic species, indicating that FP may have evolved in response to selective pressures typical in those environments. Based on these results and because the dorsal patterns appear cryptic against their background, I examined the commonly accepted but virtually untested hypothesis that this polymorphic trait evolved in response to predator pressure. Population level studies in Norops humilis examined predation on clay models, habitat selection, and survival of different female morphs in juveniles and adults. Possible predator associated mechanisms include frequency dependent predation (FDP) and morph specific microhabitat choice to reduce visibility to predators. Similar survival rates of morphs refuted FDP. Although a predation experiment indicated morph specific variation in predation depending on perch type, females were not found to choose perches in accordance with the lowest predation rate. I conclude that female dorsal patterns in Norops humilis at La Selva are not maintained by predation alone and I suggest alternative hypotheses.