Is the rate of molecular evolution inversely related to body size?

The wealth of molecular data generated over the past two decades has led to the documentation of highly disparate rates of molecular evolution among different taxa. Among the more intriguing studies is that of Martin and Palumbi (1993), who presented data (their Table 2) derived from a variety of vertebrate taxa purporting to show that the rate of molecular evolution is inversely correlated with body size (and its correlates: metabolic rate, generation time, etc.). Their hypothesis has been cited as a possible explanation for variation in rate of molecular evolution among different taxa (e. We argue, however, that no such relationship is evidenced from Martin and Palumbi's data and that the problem lies in the fact that their distance-based estimates for the rate of molecular evolution were un-dercorrected for multiple substitutions. In studies designed to estimate the rate of molecular evolution based on DNA sequence distance data, it is critical that the distances be adequately corrected for superimposed substitutions. Otherwise, estimates of the rate of molecular evolution will show a decline through time. In their study, Martin and Palumbi (1993) sampled a series of estimated rates (their Table 2) of mito-chondrial DNA (mtDNA) evolution for various vertebrate taxa from the literature. The rates they sampled were based on restriction fragment length polymorphism (RFLP) distances corrected for multiple substitutions by use of equation 8 or 9 from Nei and Li (1979). These equations provide distances that correct for multiple substitutions under the Jukes–Cantor model (JC; Jukes and Can-tor, 1969) of nucleotide substitution, which assumes that every type of substitution is equally likely. Brown et al. (1979), however, had shown that RFLP distances treated in this manner increase curvilinearly through time rather than linearly, demonstrating that these distances are undercorrected for superimposed substitutions. From an examination of Martin and Palumbi's Table 2, it became clear to us that the large-bodied pairs of taxa they sampled tended to be associated with older divergence times (linear regression of body size on time: homeotherms, P = 0.016; poikilo-therms, P = 0.029). This led us to suspect that the inverse relationship between body size and substitution rate observed by Mar-tin and Palumbi might simply reect the propensity for rates of evolution based on JC distances to decrease through time. Brown et al. (1979) argued that the major reason for the time-dependence of JC-based rates of evolution is that the JC model ignores variation among nucleotide sites …