BODY SIZE EVOLUTION ON A COOLING PLANET

Many taxa seem to have increased in body size over time in accordance with Cope's Rule. The causes invoked to explain such trends typically involve either universal ecological benefits of large size, or generic dynamics of diversifying clades. Rather little attention has been paid to the environmental context of body size evolution, which is surprising since body size in geographically separated modern populations often varies consistently with environmental parameters. One of the most notable of these regularities is Bergmann's Rule, the inverse correlation between body size and temperature, which holds across a wide range of endothermic and ectothermic taxa. For over 50 years, there have been scattered suggestions that Cope's Rule may be an evolutionary manifestation of Bergmann's Rule during intervals of climatic cooling, but this suggestion has never been formally tested.

Here we use a rich 40 My fossil record of body size in the deep-sea ostracode genus Poseidonamicus, along with a published Cenozoic bottom-water temperature curve based on Mg/Ca thermometry (Lear et al., 2000) to test the link between climate change and body size evolution. In particular, we test three predictions of the Cope-Bergmann hypothesis: (1) body size should track temperature change; (2) body size changes should occur within lineages; and (3) the magnitude of size increase over time should be commensurate with the extent of cooling, given observed temperature – body size slopes in related modern populations.

Body size in Poseidonamicus has increased by ~50% in the last 40 My, coincident with marked cooling (~10ºC) of deep ocean waters. The details of these trends support the predictions of the Cope-Bergmann hypothesis: size gains are largest when cooling is most extreme, these size gains occur anagenetically within multiple lineages, and the magnitude of size change per degree of cooling is consistent with size – temperature trends in related modern species. These results suggest that evolutionary trends in body size may be caused by directional changes in environmental or climatic variables, rather than any context-independent advantages of larger body size. Moreover, substantial size change can occur as a consequence of within-lineage microevolutionary forces, and need not result from higher-level sorting among species.