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Title: Likkle But Tallawah: Evolution and Phylogeography of a Caribbean Adaptive Radiation, The Anoline Lizards of Jamaica
Abstract: Understanding diversification is a major goal of evolutionary biology, and the study of adaptive radiations has been a key part of this pursuit – especially replicate adaptive radiations, the natural experiments that help us illuminate the course of evolution. The four replicate radiations of Greater Antillean Anolis lizards have become a classic example of the power of such natural experiments. Yet, despite all we know about these four faunas, much remains unknown about the series of events that gave rise to each of these assemblages.
In Chapter 1, I inferred brand new nuclear and mitochondrial phylogenies for the Jamaican Anolis to investigate the relationships among species and to test previously-proposed hypotheses suggesting differences between the two phylogenies. Even though both tree topologies received strong support by traditional metrics, I found discordance between the nuclear and mitochondrial phylogenies at almost every major node, along with low concordance between the underlying data and the consensus topologies, suggesting that the evolutionary history of the Jamaican anoles proceeded in a not-strictly-treelike fashion. These findings also hold implications for processes operating early in the history of the Jamaican adaptative radiation, including putative mitochondrial capture. Moreover, with higher sample sizes and more widespread geographic sampling relative to previous studies, I discovered new relationships in both trees. I closed this species-level analysis by describing broad scale patterns of genetic variation across the clade, revealing that some species harbor much higher diversity than others, including geographically-structured variation.
In Chapter 2, I studied patterns of intraspecific diversity suggested by the results of the first chapter, and searched for evidence of independently-evolving lineages within each of Jamaica’s five widespread anole species. Here, I uncovered evidence for deep, spatially-circumscribed structuring of genetic variation in two species, diffuse clustering in a third, and shallow variation in the last two. After characterizing intraspecific genetic clusters across the landscape, I examined patterns of genetic exchange between clusters to test whether intraspecific variation follows an isolation-by-distance (IBD) pattern, where major barriers to dispersal occur, and which processes underlie shared variation among lineages. Results of these analyses showed that species vary in their fit to an IBD model, and the two species with deep structuring also exhibited clear evidence of barriers to dispersal along the edges of intraspecific lineage ranges. In the three species with evident clustering, I found evidence for non-ILS processes which pointed to major areas of elevated genetic exchange between lineages – and to areas where such exchange was depressed. I used the patterns discovered in the above analyses to compare the spatial distribution of lineages between co-distributed species, and to test whether morphological variation differs along the same lines as genetic variation. Taken together, these results also made it possible to test subspecies classifications in two of Jamaica’s anole species, proposed over half a century ago. My findings indicated strong evidence for subspecies hypothesis in one species, and mixed evidence for those of the second.
In Chapter 3, I used phylogenetic network inference to test the hypothesis of reticulate evolution suggested by the results in Chapter 1. Complementing the finding of high discordance in the bifurcating tree (Chapter 1), I uncovered evidence in the nuclear dataset consistent with ancient hybridization: the first between the two lineages involved in putative mitochondrial capture, the second in the history of the radiation’s only remote species. To put these events in context, and to better understand the past processes leading to presently-observed patterns, I assembled a new geological timeline of Jamaica, inferred a time-calibrated tree, and performed model-based biogeographic history estimation. The above-water geology of Jamaica over the past ~20 million years involves the piecemeal assembly of the island from three major blocks. Across this shifting landscape, I inferred a mid-Miocene origin for the Jamaican anole clade in the most topographically-complex block of the three, which also emerged from the Caribbean Sea earliest: the Blue Mountain Block in the east. Biogeographic history estimation supported this finding and revealed two pulses of concurrent diversification in three species groups: first, late Miocene splits between lineages along the Blue Mountains’ growing elevational gradient, and second, Pliocene divergences between lowland lineages on either side of the mountains.
Taken together, these results shed light on the potential processes underlying the diversification of a classic Caribbean adaptive radiation and illuminate the patterns we see in the present day.
Committee: Jonathan Losos (Washington University), Scott Edwards, David Haig (Co-Advisors), and Robin Hopkins