The Effects of Size and Phylogeny on Patterns of Covariation in the Life History Traits of Lizards and Snakes

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Abstract
Explanations of patterns of covariation in life history traits have been couched in terms of processes acting within species to produce adaptations to local habitats and mortality schedules. Analysis of the impact of size and lineage on patterns of covariation in the life history traits of reptiles suggests that microevolutionary explanations, while perhaps necessary, are not sufficient to account for the patterns in the data. Patterns of covariation are strongly influenced by classwide correlations with a single trait, average adult female length; they are further influenced by the effects of family and genus, but the differences between the 2 orders of reptiles analyzed are entirely accounted for by the correlations with size. Thus much of the tendency for traits to covary in the pattern originally thought to have been produced r- and K-selection can be explained by selection on size alone followed by coadaptive shifts in life history traits. The pattern described here is also consistent with an adaptationist interpretation that would assert that both size and life history traits have been shaped by the same environmental conditions. Causation cannot be inferred from correlations based on a static description. In the residual variation, following the removal of correlations with size, there are significant effects of higher taxonomic levels, suggesting that lineage-specific differences in life histories, either constrained by or coadapted with morphological differences, can explain some of the tendency for life histories to be found in certain patterns. These observations do not rule out a role for microevolution operating on local populations through differences in age-specific mortality schedules. They do suggest that such microevolution occurs within a framework of relatively ancient constraints that set limits on the types of covariation of life history traits that occur within a given lineage.