UBC Theses and Dissertations

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UBC Theses and Dissertations

What drives biological diversification? detecting traits under species selection FitzJohn, Richard Gareth


Species selection - heritable trait-dependent differences in rates of speciation or extinction - may be responsible for variation in both taxonomic and trait diversity among clades. While initially controversial, interest in species selection has been revived by the accumulation of evidence of widespread trait-dependent diversification. In my thesis, I developed and applied a number of new likelihood-based methods for investigating species selection by detecting the association between species traits and speciation or extinction rates. These methods are explicitly phylogenetic and incorporate simple, but commonly used, models of speciation, extinction, and trait evolution; I assume throughout that speciation and extinction can be modelled as a birth-death process where rates depend in some way on one or more traits, and that these traits evolve under a Markov process. In particular, I extended the BiSSE (Binary State Speciation and Extinction) method to allow use with incompletely resolved phylogenies, and developed analogous methods for multi-state discrete traits or combinations of binary traits (MuSSE; Multi-State Speciation and Extinction) and quantitative traits (QuaSSE; Quantitative State Speciation and Extinction). I tested the statistical performance of the methods using simulations, investigating their performance with variation in tree size, degree of resolution, number of traits, and departure from the true model. I used each method to consider a different biological question; I found that sexual dimorphism was shortlived but associated with elevated rates of speciation in shorebirds; that solitariness and monogamy are associated with decreased speciation rates in primates (showing that a previous analysis was robust to treating both traits simultaneously); and that body size was a poor predictor of speciation rates in primates. In chapter 5, I extended this analysis of body size to all mammals, and investigated if within-lineage increases in body size (Cope's rule) were balanced by species selection against large bodied species. I found little support for this hypothesis, with clade-specific differences in the direction of species selection and idiosyncratic variation in speciation rates. Together, the methods I have developed allow testing of long-standing hypotheses about causes of variation in biological diversity.

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