Latitudinal gradients in species diversity and Rapoport's rule revisited: a review of recent work and what can parasites teach us about the causes of the gradients?

A review is given of recent work on latitudinal gradients in species diversity and their explanations, including Rapoport's rule. Energy input. measured by temperature or potential evapotranspiration, correlates best with the gradients. However, such a correlation does not "explain" them. It merely suggests explanations. which may be either different ceilings to diversity set by different energy levels under equilibrium conditions. recent historical events, or a gradient in effective evolutionary time (determined by speed of evolution directly driven by temperature, and by relative constancy of conditions over evolutionary time) under non-equilibrium conditions. Marine parasites are used to show that equilibrium conditions are the exception rather than the rule among animals. It is concluded that latitudinal gradients in species diversity result from a gradient in effective evolutionary time modulated by several other factors. Dispersal abilities of many marine invertebrates are likely to be greater at low than at high latitudes, suggesting an opposite Rappoport effect.