From species to populations: Genetic and eco-physiological divergence across Puerto Rican Anolis lizards
McElroy, Matthew Thomas
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Species pairs of Puerto Rican anoles inhabit distinct thermal environments and are locally adapted to those environments with regard to physiological traits. In chapter 1, I analyzed multilocus nuclear genetic data and genome-wide SNP data to test for simultaneous divergence and gene flow between physiologically divergent species pairs. I utilized new model-based methods that rigorously test for shared evolutionary history by jointly estimating divergence times for multiple species while implementing models of divergence with and without gene flow, and compared results from Sanger and SNP datasets to determine which dataset is best suited for comparative phylogeographic analyses. These analyses indicate that ‘xeric’ adapted lizards may have evolved synchronously, but in general, recent speciation of Puerto Rican anoles proceeded idiosyncratically. Recent models show that tropical ectotherms may respond to increasing temperatures by undergoing range shifts, acclimatizing, or becoming locally extinct. These models often overlook standing genetic variation and assume that tropical ectotherms will not be able to adapt rapidly enough to track global climate change. In chapter 2, I used multilocus nuclear genetic data and genome-wide SNP data to investigate phylogeographic patterns in A. cristatellus across Puerto Rico, and to test whether genetic variation correlates with environmental climate layers. These analyses identified multiple genetic populations of A. cristatellus that correlate with thermal environment indicating that selection on physiological traits may drive population divergence. Surprisingly, I found asymmetrical gene flow from xeric to mesic populations. Genes flowing ‘out’ of xeric habitats fits a pattern in which selection against ‘warm-adapted’ genotypes in cool habitats is weak, whereas selection against ‘cool-adaptive’ genotypes in warm habitats is relatively strong. Phylogeographic approaches identify geographic and ecological features that generate and maintain genetic diversity in co-distributed taxa. I analyzed genome-wide SNP data for three species of widespread Puerto Rican anoles to test the relative contributions of geography and ecology in driving population divergence. While only one species (A. cristatellus) had phylogeographic structure associated with ecological transitions, all three (A. cristatellus, A. pulchellus, and A. stratulus) shared concordant population breaks and simultaneous divergence across the San Loiza Basin in eastern Puerto Rico. These results are consistent with phylogeographic work on other Puerto Rican vertebrates including frogs, indicating the San Loiza Basin as an important geographic feature driving population divergence across diverse ecological groups on Puerto Rico. Divergent selection and local adaptation are recognized as primary drivers of reproductive isolation. However, thermoregulatory behavior may buffer divergent selection acting on thermal physiology, resulting in high gene flow across thermal gradients. I tested this prediction by analyzing genome-wide SNP data to investigate genetic clines along elevation gradient for populations of A. cristatellus that differ in operative temperatures and body temperatures. I also ran ecological association analysis of SNPs to test for signatures of selection resulting from temperature and precipitation. I found sharp genetic clines, despite thermoregulatory behavior that should buffer natural selection, indicating that precipitation may be the main agent of selection along the elevational gradient in southern Puerto Rico.
- Biology