12 - Effective evolutionary time and the latitudinal diversity gradient  pp. 169-180

Effective evolutionary time and the latitudinal diversity gradient

By Len N. Gillman and Shane D. Wright

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The relationship between climate and biodiversity is perhaps the most widely recognized and extensively studied pattern of nature on earth. Attempts to explain this relationship and the attendant latitudinal gradient in diversity began more than 200 years ago (von Humboldt, 1808; Wallace, 1878) and indeed the number of theories that attempt to address this question appears to be accumulating at an ever increasing rate. However, one theory which has received relatively little attention began with the observation by Rensch (1959) that animals living in warmer tropical climates have shorter generation times than those living at higher latitudes. He suggested that because natural selection accumulates change with each generation, shorter generation times found among tropical fauna might increase the pace of natural selection and thereby the pace at which evolution progresses. A faster evolutionary speed in the tropics would therefore lead to the evolution of more species there than at higher latitudes over an equivalent period of time.

Evidence suggesting that mutations can be induced by high temperatures prompted Rohde (1978, 1992) to predict that not only might rates of selection increase with increasing ambient energy towards the tropics, but that rates of mutation may also be greater in lower-latitude climates. It is predicted that the combined effects of faster rates of mutation and faster rates of selection will lead to greater rates of diversification. Over an equivalent period of time, regions experiencing generally faster rates of genetic evolution will therefore generate and accumulate more species and greater species richness than regions where genetic evolution is slower. Fundamental to this hypothesis is the precept that the incumbent diversity of species within communities is not at an equilibrium number set by contemporary environmental conditions. Instead, it suggests that communities continue to accumulate species at rates that depend on climatic variables. This theory is therefore quite different to those, such as the energy-richness or more individuals hypothesis (Hutchinson, 1959; Brown, 1981; Wright, 1983), that suggest that diversity is limited by energetic capacity of the environment and that species origination and extinction are therefore held in balance with climate.

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