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Geographic range size : speciation, extinction and what happens in-between Waldron, Anthony Simon

Abstract

I studied the impact of geographic range size on clade diversification rates. Recent studies have suggested that, although geographic range size shows phylogenetic signal, this signal may be a statistical artefact. I created two models of range size evolution to determine the expected division of range size at speciation and to model the subsequent evolution of range size in sister species. Range size "symmetry" (the degree of similarity between sister species' range sizes) was then compared to these expectations. The range size of sister species of birds both show phylogenetic signal and are more similar than expected under the model, suggesting that range size is heritable. I then show that range size has a positive relationship with diversification rate in young clades of primates, but that the relationship may become asymptotic or even negative at very large range sizes. This is the first evidence of a non-linear relationship between range size and diversification rate, and may also be evidence of a non-linear relationship between range size and speciation rate. Finally, I test the novel hypothesis that clades which can tolerate the extinction risks associated with range restriction will diversify more quickly than intolerant clades. I find that risk-tolerant primate clades do have higher diversification rates. I also find, surprisingly, that the biological correlates of extinction risk tolerance are habitat specialisation and small geographic range size. "Rare" species (i.e. those with narrow geographic distributions or small population sizes) may therefore be characterised by their tolerance of extinction risk, rather than being risk-prone as is widely thought.

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