Linking macro-trends and micro-rates: re-evaluating micro-evolutionary support for Cope’s rule

Cope's rule, wherein a lineage increases in body size through time, was originally motivated by macro-evolutionary patterns observed in the fossil record. More recently, some authors have argued that evidence exists for generally positive selection on individual body size in contemporary populations, providing a micro-evolutionary mechanism for Cope's rule. If larger body size confers individual fitness advantages as the selection estimates suggest, thereby explaining Cope's rule, then body size should increase over micro-evolutionary time scales. We test this corollary by assembling a large database of studies reporting changes in phenotypic body size through time in contemporary populations, as well as studies reporting average breeding values for body size through time. Trends in body size were quite variable with an absence of any general trend, and many populations trended toward smaller body sizes. Although selection estimates appear to support Cope's rule, our results suggest tha...