Little evidence for Cope's rule from Bayesian phylogenetic analysis of extant mammals.

According to Cope's rule, lineages tend to evolve towards larger body size, possibly because of selective advantages of being large. The status of Cope's 'rule' remains controversial as it is supported in some but not all large-scale fossil studies. Here, we test for Cope's rule by Bayesian analyses of average body masses of 3253 extant mammal species on a dated phylogenetic tree. The data favour a model that does not assume Cope's rule. When Cope's rule is assumed, the best estimate of its strength is an average ancestor-descendant increase in body size of only 0.4%, which sharply contrasts with the 9% bias estimated from fossil mammals. Thus, we find no evidence for Cope's rule from extant mammals, in agreement with earlier analyses of existing species, which also did not find support for Cope's rule.