COPES RULES, SEXUAL SELECTION, AND THE LOSS OF ECOLOGICAL PLASTICITY

The history of diverse mammal taxa is characterized by evolutionary increase in size and by relatively higher rates of extinction among species of larger size (Cope's rules). I suggest that sexual selection accounts for both observations because: (1) the degree of sexual dimorphism, indicative of strong sexual selection, is greater in larger species. and (2) elaboration of male secondary sexual traits coincided with advances in size. I hypothesize that sexual selection on males to control access to females favors larger size and other characters involved in male-male combat, especially as harem size increases. The greater the intensity of sexual selection, which is proportional to harem size. the greater the forfeiture of adaptedness to prevailing ecological conditions and consequently the greater the vulnerability to extinction. Long-term periodicities in the amount of incident solar radiation ensure that mammal taxa have been subjected to profound cycles of environmental change. Therefore, response to sexual selection pressures is envisaged to be ratchet-like with advances in size occurring during periods of environmental amelioration but with the evolutionary inertia of mating systems resisting morphological retreat when conditions deteriorate. Consequently, mean population fitness plummets when the environment deteriorates and extinction may result. Ecological field studies are necessary to document the principle that allocation of energy to sexually selected traits reduces both the adaptedness of populations to prevailing ecological conditions and their ability to track changes in the environment.