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An organism’s body size tells us a lot about how it makes a living, suggesting that
body size is a key parameter in evolution. We outline three large-scale trends in
body size evolution. Bergmann’s Rule is the tendency for warm-blooded species
at high latitudes to be larger than their close relatives nearer the equator. The
Island Rule is the trend for small species to become larger, and large species
smaller, on islands. Cope’s Rule, which we discuss in much more detail, is the
tendency for lineages to increase in size over evolutionary time. Trends are best
studied by combining data on evolutionary relationships among species with
fossil information on how characters have changed through time. After highlighting
some methodological pitfalls that can trap unwary researchers, we summarise
evidence that Cope’s Rule, while not being by any means universal, has
operated in some very different animal groups – from microfauna (single-celled
Foraminifera) to megafauna (dinosaurs) - and we discuss the possibility that
natural selection and clade selection may pull body size in opposite directions.
Despite size’s central importance, there is little evidence that body size differences
among related groups affect their evolutionary success: careful comparisons
rarely reveal any correlation between size and present-day diversity. We end
by touching on human impacts, which are often more severe on larger species. |
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