Energy and the Tempo of Evolution

The Evolutionary speed hypothesis (ESH) proposed by Rench (1959) and extended by Rohde (1978, 1992) directly links the genetic responses of animals to environmental change. This hypothesis could potentially explain patterns of biodiversity at the global level, with a decrease in species numbers along the climate gradient from the tropics to the poles. The cause of this global pattern has been the subject of much speculation. Rohde’s hypothesis suggests a link between higher temperature and solar radiation in lower latitude and high rates of speciation in equatorial regions, via two major processes. Warmer ecological environments characterized by high levels of solar radiation could have a direct mutagenic effect on DNA. In turn, this might be expected to result in higher mutation rates and consequently faster rates of molecular evolution. Increased mutation rates are argued to lead to rapid ‘reproductive isolation’ between populations and eventually to the

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promotion of rapid speciation (Rohde 1978, 1992; Wright et al. 2006). This would then be expected to lead to increased levels of divergence between species, at the molecular level. Second, it has been hypothesized that higher temperatures might increase individual growth rates, decreasing the generation times, increasing the speed at which the selection operates and elevating rates of speciation (Rohde 1978, 1992) (Figure 8.2). A shorter generation times have been associated with faster molecular evolution (Martin & Palumbi 1993, Bromham et al. 1996; Bromham 2002), this pathway could also potentially promoting rapid molecular evolution and speciation towards lower latitudes.

An alternative explanation for faster rates of DNA evolution in warmer climates is that, because of putatively smaller population size at low latitudes, warmer climate species might be subject to greater rates of nearly neutral genetic drifts (Stevens 1989; Ohta 1992). However this theory is not supported by recent empirical studies (Wright et al. 2006, 2009; Gillman et al. 2009).

There is a growing body of evidence that support the Rohde’s hypothesis for plants, marine foraminifera, terrestrial mammals, birds, amphibians and fishes (Gillman et al. 2010; Davies et al. 2004; Wright et al. 2006; Allen et al. 2006; Gillman et al. 2009; Bleiweiss 1998; Wright et al. 2010; Wright et al. 2011). However this positive relationship between the rate of genetic evolution and ambient temperature was not found among microbial thermophiles and mesophiles (Drake 2009; Swami 2009). It has been demonstrated that in extremely high temperature, thermophiles show a reduction in genetic substitution rates. This suggests that the slower genetic evolution in microbial thermophiles is a result of mutation control mechanisms in extreme thermal regimes.

Furthermore, Bromham and Cardillo (2003), using congeneric pairs of birds have detected no significant difference in evolutionary rates between high and low latitude avian species (Bromham & Cardillo 2003). Wright and his colleagues (2006) argued that the power of this study was affected by including species pairs with overlapping distributions (up to 100%) when the seasonal migration is taken into analysis (Wright et al. 2006). They suggested that species that experience tropical or subtropical winter climates cannot be defined as temperate in terms of their temperature regime. They explained that this feature is more related to their incursions and migration into higher latitudes in summer and low latitudes in winter (Wright et al. 2006). Therefore, the result of study by Bromham and Cardillo (2003) could not potentially reject the correlation between molecular evolution

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and ambient temperature because these congeneric bird species are migrating between high and low latitude seasonally.

Figure 8.2: The climate-speciation hypothesis (Rohde, 1978, 1992) links higher temperatures and solar radiation levels at low latitudes with higher speciation rate, via two pathways. First, higher temperatures may increase mutation rates, and subsequently the rate of molecular evolution. The latter might increase the rate of speciation. Secondly, higher temperatures and more solar radiation might increase growth rates; decrease generation time and potentially increasing the speciation rates. The gray lines indicate the links between generation time, evolutionary rates and speciation that were not explicitly discussed by Rohde. The figure is re-drawn by the Author of the thesis.

The alternative hypothesis to ESH is the tropical conservatism hypothesis (TCH) that assumes the rates of molecular evolution and diversification of clades are independent of latitudes and elevations (Wiens et al. 2006). Consistent with the THC, and in

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contradiction to the ESH, Wiens et al. (2006) found no evidence that diversification rates (number of species) for Hylidae (tree frogs) were greater in clades that are located in lower latitudes. Wright et al. (2010) argued that these results suffer from limitations in latitudinal breadth of clade distribution (Algar et al. 2009; Wright et al. 2010). They explained that seven out of the 11 clades examined by Wiens et al. (2006) span the equator to north and south and extended into temperate latitudes. There is only one clade in their study with entirely extra-tropical distribution and this clade actually shows the lowest diversification rate they reported. Another limitation for this study is the method used by Wiens et al. (2006). They assume that rates of DNA divergence can be used to estimate time since divergence and to subsequently estimate the speciation rates. The latter was then compared across latitudes. This approach assumes that the rate of molecular evolution is not related to changes in latitude and climate. However, Wright et al. (2010) tested for rate heterogeneity in molecular DNA evolution in Hylidae. In contrast, they suggested that species of Hylidae in warmer climates have undergone faster DNA evolution than species in cooler climates. Therefore, the slower rates of molecular evolution in Hylidae species in cooler climate indicates that the age of cooler temperature Hylidae clades have been underestimated by Wiens et al. (2006) and subsequently the diversification rates overestimated.