Abstract

In the past it has been widely assumed that latitudinal diversity gradients in the marine realm must be underpinned in some way by a cline in evolutionary rates. Although there is some theoretical justification for believing that speciation is enhanced in the tropics and extinction towards the poles, this has not been borne out so far by empirical studies based on the fossil record. A detailed comparison of ten tropical and ten temperate molluscan clades showed that, although the former were, on average, three times as diverse as the latter, they had not radiated at a significantly faster rate. Many temperate clades, in both the marine and terrestrial realms, appear to have evolved comparatively rapidly; they are less diverse at the present day simply because they are geologically younger. Available evidence from the marine realm suggests that extinction is no more likely in cold than in warm waters. Explanations of large-scale biodiversity patterns may now reside more in regional rather than local phenomena. In particular, extant latitudinal taxonomic diversity gradients may reflect as much range fluctuation in response to Cenozoic climatic cycles as to any intrinsic properties of either tropical or polar biotas.

Introduction

In recent years there has been a growing realisation that the patterns of biodiversity we see today may be the product of a substantial historical legacy.