Effective Evolutionary Time - Historical Aspects

Historical Aspects

Some aspects of the hypothesis are based on earlier studies. Bernhard Rensch, for example, stated that evolutionary rates also depend on temperature: numbers of generation in poikilotherms, but sometimes also in homoiotherms (homoiothermic), are greater at higher temperatures and the effectiveness of selection is therefore greater. Ricklefs refers to this hypothesis as "hypothesis of evolutionary speed" or "higher speciation rates". Genera of Foraminifera in the Cretaceous and families of Brachiopoda in the Permian have greater evolutionary rates at low than at high latitudes. That mutation rates are greater at high temperatures has been known since the classical investigations of Timofeev-Ressovsky et al. (1935), although few later studies have been conducted. Also, theses findings were not applied to evolutionary problems.

The hypothesis of effective evolutionary time differs from these earlier approaches as follows. It proposes that species diversity is a direct consequence of temperature-dependent processes and the time ecosystems have existed under more or less equal conditions. Since vacant niches into which new species can be absorbed are available at all latitudes, the consequence is accumulation of more species at low latitudes. All earlier approaches remained without basis without the assumption of vacant niches, as there is no evidence that niches are generally narrower in the tropics, i.e., an accumulation of species cannot be explained by subdivision of previously utilized niches (see also Rapoport's rule). The hypothesis, in contrast to most other hypotheses attempting to explain latitudinal or other gradients in diversity, does not rely on the assumption that different latitudes or habitats generally have different "ceilings" for species numbers, which are higher in the tropics than in cold environments. Such different ceilings are thought to be, for example, determined by heterogeneity or area of the habitat. But such factors, although not setting ceilings, may well modulate the gradients.