EVOLUTIONARY TRENDS WITHOUT ECOLOGICAL DRIVERS

Many classic examples of evolutionary trends, such as the increase in body size described by “Cope’s Rule”, turn out to reflect increases in morphological and ecological variance upon closer inspection. Such trends frequently result from the initiation of a radiation by a single founder with a generalized morphology and ecology and (frequently) small body size. Although morphologies may become increasingly extreme during a radiation, the modal morphology changes little, and at least some species similar to the founding taxon may be found throughout the evolutionary history of the clade.

Trends resulting in an increase in variance do not require directional selection during a given radiation, but instead, paradoxically, result from strong selection before the radiation actually starts. Indeed, mass extinction may end up restricting the founding species to a very narrow part of the realizable morphology and ecology of the clade and predisposing a radiation to give rise to trends toward an increased variety of body sizes and ecological specializations. The common occurrence of some trends such as ‘increased body size’ owes much more to a bias in survivorship for small ancestors through mass extinctions than to any subsequent directional selection for large body size during the diversification process itself. We tend to describe these expansions in variance as trends when they are initiated from an extreme end member of the morphological or ecological spectrum but expansions in variance can be initiated from any part of morphospace. The ubiquity of some trends reflects the universality of selective extinction of specialists in everything from microplankton to elephants.