MACROEVOLUTION OF WHALES AND THE DYNAMICS OF MORPHOLOGICAL DISPARITIES

Cetaceans are the poster child of macroevolution, exhibiting enormous morphological disparities relative to their terrestrial cousins. Most notably, the largest cetaceans are up to a hundred times larger than the largest terrestrial mammals. However, little is known about the macroevolutionary processes involved. We present new results suggesting that a constrained diffusion process is sufficient to explain patterns of size evolution. First, we show that the distribution of extant cetacean body sizes can be predicted, with no adjustable parameters, from a diffusion model fitted to terrestrial body size evolution. Second, we show that the dynamics of cetacean body-size disparity observed in the fossil record over the past 55.8 My can be predicted from the same model. More generally, this model predicts a two-phase process for morphological macroevolutionary dynamics. The first phase is radiative, in which disparity is dominated by morphological diffusion and is uncoupled from taxonomic diversity. For cetaceans, this period ended in the late Eocene or early Oligocene, which coincided with the emergence of the suborders Mysticeti and Odontoceti. The second phase, which is ongoing for cetaceans, represents equilibrium, where disparity is controlled by diversity and thus presumably by intraspecific competition. In this phase, the largest morphological disparities should co-occur with diversity maxima. We conclude by suggesting that this two-phase model may be generally characteristic of macroevolutionary dynamics.