LATITUDINAL GRADIENTS IN SPECIES-DIVERSITY - THE SEARCH FOR THE PRIMARY CAUSE

Hypotheses that attempt to explain latitudinal gradients in species diversity are reviewed. Some hypotheses are circular, i.e. they are based on the assumption that some taxa have greater diversity in the tropics. These include explanations assuming different degrees of competition, mutualism, predation, epiphyte load, epidemics, biotic spatial heterogeneity, host diversity, population size, niche width, population growth rate, environmental harshness, and patchiness at different latitudes. Other explanations are not supported by sufficient evidence, i.e. there is no consistent correlation between species diversity and environmental stability, environmental predictability, productivity, abiotic rarefaction, physical heterogeneity, latitudinal decrease in the angle of the sun above the horizon, area, aridity, seasonality, number of habitats, and latitudinal ranges. The ecological and evolutionary time hypotheses, as usually understood, also cannot explain the gradients, nor does the temperature dependence of chemical reactions permit predictions on species richness. Only differences in solar energy are consistently correlated with diversity gradients along latitude, altitude and perhaps depth. It is concluded that greater species diversity is due to greater "effective" evolutionary time (evolutionary speed) in the tropics, probably as the result of shorter generation times, faster mutation rates, and faster selection at greater temperatures. There is an urgent need for experimental studies of temperature effects on speed of selection.