Dissertations, Theses, and Capstone Projects

Date of Degree

2-2015

Document Type

Dissertation

Degree Name

Ph.D.

Program

Biology

Advisor

Frank T. Burbrink

Subject Categories

Bioinformatics | Evolution | Systems Biology

Keywords

biogeography, macroevolution, phylogenomics, ratsnakes, systematics

Abstract

Understanding the evolutionary processes that create the spectacular diversity of organisms, both in species numbers and form, is a primary goal for biologists. Global ratsnakes are a species-rich assemblage with high morphological and ecological diversity and a distribution that encompasses both the Old World (OW) and the New World (NW). To explore the mechanism leading to the divergence of the ratsnakes, I tested the hypotheses regarding the area of origin and global dispersal, and examined the patterns of diversification and trait evolution. Given adaptive radiation via ecological opportunity, a diversity-dependent diversification pattern and an early burst trait evolutionary pattern are expected with rapid divergence triggered by the appearance of new resources, extinction of competitors, colonization of new areas or the appearance of key innovations. Thus, I tested if the radiation of ratsnakes follows diversity-dependent diversification with an early burst in speciation and trait divergence and whether the variation in diversification is associated with OW-NW dispersal or changes in traits. Further, trait convergence between OW and NW lineages was investigated to determine, if given similar environmental conditions, rapid speciation via ecological opportunity is repeatable. To answer the questions mentioned above, a robust phylogenetic tree is fundamental. Due to potential gene tree/species discordance, hundreds of loci sampled across the entire genome were generated using the anchored hybrid enrichment approach and the multi-species coalescent methods were used to build the species phylogeny. Then, given this phylogenetic context, taxonomic changes were made to reflect named monophyletic groups and divergence time and ancestral areas were estimated to 1) infer the processes leading to the current ratsnake global distribution, 2) assess the best fitting diversification and trait evolution models, and 3) determine if ecomorphological convergence occurs with adaptive regimes of traits on the phylogeny. Among all of the inferred species trees, by comparing the extent of tree discordance and the gene tree errors, the species trees generated in the program MPEST with summary statistics of posterior probability gene trees was used for further analysis. First, it was determined that the traditional ratsnake genera Gonyosoma and Coelegnathus are excluded from the monophyletic ratsnake group, with the remaining monophyletic group defined as Coronellini. The reconstructed ancestral areas supported that ratsnakes originating in the OW Eastern Palearctic and with a single dispersal to the NW via Bergingia. Two subclades each defined by a single genus, Lampropeltis and Elaphe, were found to have exclusively elevated species diversification and trait evolutionary rates. As the rate accelerations were only in the recent divergent lineages, colonization to the NW and rapid speciation of the NW lineages were decoupled. A general diversity-dependent radiation pattern in both OW and NW lineages was supported with a recent sharp diversification elevation about 6.5 Ma mainly within the genera Lampropeltis and Elaphe. Three morphological convergence events were detected among OW and NW lineages, corresponding to the previously defined morphological taxonomies (i.e., Elaphe and Pantherophis), indicating without a robust molecular phylogeny, morphological convergence positively misleads taxonomy. This research demonstrates the advantages and challenges of phylogenetic inference using genome scale dataset, highlights the importance of incorporating the biogeographic history and trait evolution in studies of diversification and indicates that oversimplified models are insufficient to describe the complexity of processes shaping the diversity in a species-rich assemblage.

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