Convergence and Divergence from Populations to Species: Phenotypic Evolution of Caribbean Anolis Lizards

This dissertation is primarily focused on understanding the generation and maintenance of phenotypic diversity through ecological selection. Natural selection acts on the phenotype because the phenotype lies at the interface between an organisms genotype and its environment. Thus, the study of phenotypic diversity is central to our understanding of evolution. I have primary addressed questions of phenotypic evolution using the radiations of Caribbean Anolis lizards. Chapter I of my dissertation examines if claw and toepad morphology formed an integrated phenotype correlated with microhabitat partitioning in Greater Antillean anoles. The Greater Antilles forms an exceptional system to study convergent evolution due to the repeated evolution of structural microhabitat specialist ‘ecomorphs’. As locomotor traits associated with clinging performance, claw and toepad traits are likely highly relevant to partitioning of vertical arboreal habitat. I found that correlative selection did drive the evolution of claw and toepads as an integrated phenotype to optimize performance on specific microhabitats. Additionally, rates of evolution were coupled between claws and toepads across most species, except twig specialists which experienced a dramatic reduction in claw structure. Thus, this study provides a convincing case study for correlative selection facilitating ecological divergence between species. Chapter II studies claw and toepad integration in the context of the species depauperate Lesser Antillean anole fauna. These anoles represent broad habitat generalists with only one or two species per island. Broadly, claw and toepad morphology correlated with overall island habitat availability on one species islands, but not two species islands. Here, I found that Anolis bimaculatus series anoles closely-related to the Greater Antillean adaptive radiations followed predictable patterns of divergence on two species islands in that they partitioned vertical habitat space and showed associated phenotypic integration between claws and toepads. However, the distantly-related A. roquet series anoles did not show predictable patterns of divergence on two species islands and a decoupling of claw and toepad traits. These results suggest that phylogenetic history may influence the capacity for adaptive divergence. Chapter III tests for parallel intraspecific patterns of divergence across environmental gradients in nine species of A. bimaculatus series anoles inhabiting the northern Lesser Antilles. Species within the A. bimaculatus series are broadly generalist and occur widely throughout their native islands. Thus, each encounters a wide range of environmental variation ranging from xeric coastal scrub to montane rainforests. I found that patterns of morphological divergence across these environmental gradients were idiosyncratic. It is possible that due to many-to-one mapping of form to function, adaptation to environmental gradients can be achieved through different ecologically-relevant traits. However, strong patterns of convergence did emerge for some traits, notably dorsal coloration. Chapter IV examines the phenomenon of species richness-genetic diversity in the context of island biogeography and extends this framework to include phenotype diversity. Specifically, I examined the relative roles of island size, distance from mainland, environmental variance, and topographic complexity across the Lesser Antilles. I partitioned biogeographical effects across species, phenotypic, and genetic diversity using both native and introduced herpetofauna. I found that island size correlates with both species richness and genetic diversity in all contexts. Secondary island characteristic effects on species and genetic diversity differed between groups likely due to differences in their natural history. Phenotypic diversity was not correlated with tested island variables in all cases suggesting that macroecological forces which shape species and genetic diversity are decoupled from phenotypic diversity.

Advisor: Wang, Ian J