Effects of Competition on Rates of Evolution, With Special Reference to Bivalve Mollusks and Mammals

Stanley, Steven M. (Department of Earth and Planetary Sciences, The Johns Hopkins University, Baltimore, Maryland 21218) 1974. Effects of competition on rates of evolution, with special reference to bivalve mollusks and mammals. Syst. Zool. 22: 486–506.—Rates of adaptive radiation and taxonomic turnover are much higher for mammals than for bivalve mollusks, largely because characteristic intensity of competition between species is different for the two groups. The high degree of interspecific competition typical of mammals is associated with specialized feeding habits, frequent limitation of population densities by food supply, highly developed mobility and behavior, aggressive interactions, and territoriality. Features associated with weak interspecific competition among most coexisting bivalve species are primitive behavior patterns, sedentary modes of life, generalized feeding habits, ability to endure long periods of near-starvation, and widespread limitation of population densities by physical disturbances and intense predation. By providing what Mayr has termed a “centrifugal force” in evolution, competition has produced rapid divergence in the Mammalia. Higher taxa of mammals have tended to maintain distinct adaptive zones, and ecologic displacement of one group by another has been common. Rate of appearance of new families has declined since early in the group's Cenozoic radiation because established groups have tended to preempt portions of the overall adaptive zone. In contrast, competition has played a minor role in bivalve evolution and divergence has been gradual. Adaptive zones of subtaxa have tended to overlap, and displacement has been rare. Despite the success of advanced bivalve taxa, many primitive groups have persisted for hundreds of millions of years, and various seemingly primitive groups have arisen quite recently. Competition has also contributed to higher rates of taxonomic turnover among mammals than among bivalves by accelerating rates of extinction. The anomalously high evolutionary rates of reef-building rudist bivalves resulted from competition for space. Similarly, reef-building scleractinian corals have radiated much more rapidly than have nonreef-building forms. The nonmammalian vertebrate classes, the trilobites, and the ammonites have all been characterized by competitive traits like those of the mammals and have also undergone notably rapid adaptive radiations.