Figure 7.2 Geographic variation in development rate (egg to adult) (negative latitudes are in the North) for a variety of insect species. Source: Data compiled by Addo-Bediako (2001).
development rates within a given area suggest that a consistent trend might be difficult to detect. Broad overviews of insect voltinism, which should provide greater insight into variation in generation time across space, unfortunately do little more to clarify the situation. Although multivoltinism in tropical species is probably common, some species behave as if the environment is highly seasonal, thus resulting in broad variety of strategies in the tropics (Wolda 1988).
Given these findings, and geographic variation in insect body size that regularly shows no consistent trends between groups and is often the consequence of latitudinal changes in the dominance of higher taxonomic groups (Hawkins and Lawton 1995), the actual form of the relationship between latitude and the number of generations seems remarkably difficult to determine. Thus, although these patterns do not resolve the question of the likelihood of declining generation times towards the tropics, especially owing to the complexities of interactions between size, generation time, voltinism, and feeding strategy (Chown et al. 2002a), they do suggest that the generation time assumption of the evolutionary rates hypothesis might be more complicated than previously thought. Intriguingly, Fig. 7.2 suggests that generation time actually increases towards the tropics. Moreover, metabolic rates show a decline towards the tropics (Addo-Bediako et al. 2002), suggesting that levels of DNA-damaging metabolites, which increase mutation rates, are unlikely to be higher in the tropics than elsewhere. Thus, more rapid evolution in the tropics, via mutation rates elevated by metabolic damage (Martin and Palumbi 1993), is unlikely. These findings are broadly in keeping with a major study on latitudinal variation in the rates of molecular evolution in birds, which found no support for the evolutionary rates idea (Bromham and Cardillo 2003).
The investigation of insect development rates in this context raises several points that are of more general importance. Most significant among these are the interactions between the determinants of body size and the consequences for spatial variation in development rate and generation time (Chown et al. 2002a), and the spatial extent of the available data. In Fig. 7.2 the latter appears limited, especially for the tropics.
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