Genetic, demographic and ecological factors contributing to the evolution of Anolis lizard populations

Geographic and environmental factors can be catalysts of evolutionary processes and central drivers of genetic and phenotypic differentiation. Characteristic patterns of genetic and phenotypic variation elucidate whether predominately adaptive or non-adaptive processes are involved in shaping variation among populations. Such genetic and phenotypic signatures can provide insight in how specific extrinsic factors relate to adaptive and non-adaptive processes. This dissertation aims to investigate whether and how disturbance events, intraspecific competition and secondary contact after a species introduction contribute to genetic and phenotypic differentiation among populations of Anolis lizard. In chapter 1, I tested whether patterns of genetic diversity and differentiation of island populations in the Bahamas were consistent with disturbance-driven demographic fluctuations and strong genetic drift. I used ten microsatellite markers to measure genetic diversity and population differentiation. Population genetic structure and differentiation were consistent with expectations of strong genetic drift after founder events. Genetic bottlenecks were prevalent across the island populations and coalescent-based demographic models supported a colonization scenario for most islands, rather than scenarios for population bottlenecks or relative stability of populations. Low rates of gene flow among island populations have likely preserved the genetic signatures of colonization and extinction events. This study provides evidence that disturbance events such as hurricanes are catalysts of long-term, non-adaptive evolution when gene flow is limited. In chapter 2, I examined whether intraspecific competition is a potential driver of directional morphological change in male and female lizards (Anolis sagrei) among island populations in the Bahamas. I used measures of population density and injuries, as a proxy for aggressive encounters, to test whether body size and head size vary in the direction predicted by natural selection. I found that lizards in high-density populations had a higher proportion of injuries as compared to low-density populations, suggesting that the former experience higher levels of intraspecific aggression. Additionally, lizards in these higher density populations had larger heads (longer and wider, corrected for body size). In both sexes, relative head size and frequency of injury increased with population density, suggesting that females may play an active role in intraspecific competition. In chapter 3, I examined whether the introduction of the Cuban green anole (Anolis porcatus) to South Florida has resulted in local hybridization with its closely related sister species, the native green anole (Anolis carolinensis), or whether reproductive barriers prevent gene exchange across species boundaries. I used one mtDNA marker and 18 microsatellite loci to test for cyto-nuclear discordance, which is indicative of hybridization, and whether population-genetic patterns are consistent with recent or historic gene flow. I found mtDNA haplotypes of two species in South Miami, local A. carolinensis and A. porcatus from West Cuba. Contrary to expectations of recent hybridization, I did not find intermediate nuclear genotypes. Instead, the population in South Miami forms a separate, genetically homogeneous cluster, which is differentiated from both parental species. A historic gene flow analysis confirms that ~33% of the nuclear ancestry of the South Miami population is derived from West Cuban A. porcatus. Thus, reproductive isolation between A. porcatus and A. carolinensis is weak or absent, despite considerable divergence time in allopatry, which reinforces a proposal to revise the taxonomy of A. carolinensis and A. porcatus from West Cuba according to guidelines of the biological species concept. In summary, my dissertation evaluated how demographic and ecological factors relate to patterns of genetic and phenotypic variation in populations of Anolis lizards. My research shows that hurricanes can act as evolutionary catalysts by altering non-adaptive evolutionary processes when gene flow is limited. The same island populations vary morphologically in the direction predicted by natural selection as a consequence of intraspecific competition. Lastly, historic hybridization resulting from secondary contact between formerly allopatric introduced and native species can locally erode species boundaries and alter the genetic composition of native populations. By characterizing patterns of genetic and phenotypic variation, my research provides insight in to how extrinsic factors and long-term evolutionary processes are interconnected and potentially shape future trajectories of these populations