Rapoport's Rule is a local phenomenon and cannot explain latitudinal gradients in species diversity
It is proposed to restrict the term Rapoport's Rule to an increase in latitudinal ranges of animals and plants with latitude, because the causes of correlations of geographical breadth, measured differently, with a point along a biogeographical gradient may be different. The latitudinal Rapoport's Rule is a strictly local phenomenon, restricted to the Palae- and Nearctic above a latitude of approximately 40-50°N. It has never been shown to apply to the tropics. Greater depth ranges of cold-water marine teleosts are not due to their greater ecological tolerances but to the more uniform temperature conditions over a larger depth range. Restriction of Rapoport's Rule to northern latitudes, where the effects of the Ice Ages have been greatest, suggests that it is due to selection for tolerance to greater temperature fluctuations during the Ice Ages (=extinction of less tolerant species). Rapoport's Rule, like other ecological factors, does not correlate well with species richness and cannot explain latitudinal gradients in species diversity. A shift of emphasis from an ecological to an evolutionary approach (from establishing correlations to experimental work) is necessary to find an explanation for the gradients.
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