Effective Evolutionary Time - Recent Studies

Recent Studies

A considerable number of recent studies support the hypothesis. Thus, diversity of marine benthos, interrupted by some collapses and plateaus, has risen from the Cambrian to the Recent, and there is no evidence that saturation has been reached. Rates of diversification per time unit for birds and butterflies increase towards the tropics. Allen et al. found a general correlation between environmental temperature and species richness for North and Central American trees, for amphibians, fish, Prosobranchia and fish parasites. They showed that species richness can be predicted from the biochemical kinetics of metabolism, and concluded that evolutionary rates are determined by generation times and mutation rates both correlated with metabolic rates which have the same Boltzmann relation with temperature. They further concluded that these findings support the mechanisms for latitudinal gradients proposed by Rohde. Gillooly et al. (2002) described a general model also based on first principles of allometry and biochemical kinetics which makes predictions about generation times as a function of body size and temperature. Empirical findings support the predictions: in all cases that were investigated (birds, fish, amphibians, aquatic insects, zooplankton) generation times are negatively correlated with temperature. Brown et al.(2004) further developed these findings to a general metabolic theory of ecology. Indirect evidence points to increased mutation rates at higher temperatures, and the energy-speciation hypothesis is the best predictor for species richness of ants. Finally, computer simulations using the Chowdhury eosystem model have shown that results correspond most closely to empirical data when the number of vacant niches is kept large. Rohde gives detailed discussions of these and other examples. Of particular importance is the study by Wright et al. (2006) which was specifically designed to test the hypothesis. It showed that molecular substitution rates of tropical woody plants are more than twice as large as those of temperate species, and that more effective genetic drift in smaller tropical populations cannot be responsible for the differences, leaving only direct temperature effects on mutation rates as an explanation. Gillman et al. (2009) examined 260 mammal species of 10 orders and 29 families and found that substitution rates in the cytochrome B gene were substantially faster in species at warm latitudes and elevations, compared with those from cold latitudes and elevations. A critical examination of the data showed that this cannot be attributed to gene drift or body mass differentials. The only possibilities left are a Red Queen effect or direct effects of thermal gradients (including possibly an effect of torpor/hibernation differentials). Rohde (1992, 1978) had already pointed out that “it may well be that mammalian diversity is entirely determined by the diversity of plants and poikilothermic animals further down in the hierarchy”, i.e., by a Red Queen effect. He also pointed out that exposure to irradiation including light is known to cause mutations in mammals, and that some homoiothermic animals have shorter generation times in the tropics, which - either separately or jointly - may explain the effect found by Gillman et al. Gillman et al. (2010) extended their earlier study on plants by determining whether the effect is also found within highly conserved DNA. They examined the 18S ribosomal gene in the same 45 pairs of plants. And indeed, the rate of evolution was 51% faster in the tropical than their temperate sister species. Furthermore, the substitution rate in 18S correlated positively with that in the more variable ITS. These result lend further very strong support to the hypothesis. Wright et al. (2010) tested the hypothesis on 188 species of amphibians belonging to 18 families, using mitochondrial RNA genes 12S and 16S, and found substantially faster substitution rates for species living in warmer habitats at both lower latitudes and lower elevations. Thus, the hypothesis has now been confirmed for several genes and for plants and animals.

Vázquez, D.P. and Stevens, R.D. (2004) conducted a metaanalysis of previous studies and found no evidence that niches are generally narrower in the tropics than at high latitudes. This can be explained only by the assumption that niche space was not and is not saturated, having the capacity to absorb new species without affecting the niche width of species already present, as predicted by the hypothesis.