Swifter, Higher, Stronger

Abstract

ECOLOGY/EVOLUTION Sexual selection, the evolutionary corollary of mate choice, is generally studied in organisms where direct matings (for example, internal fertilization) between individuals take place. The variance in male mating success that results when females choose, in particular, can lead to the evolution of showy and sometimes bizarre signals of male quality. However, the ancestral condition for sexual reproduction in animals is broadcast spawning and external fertilization—that is, the release of sperm and eggs by benthic marine organisms into the water column. Does sexual selection operate under these conditions? In an experimental study of reproduction in sea urchins, Levitan finds that sexual selection—as identified by the difference between males and females in the variance for fertilization success—does indeed occur, but only at intermediate population densities of males and females. At low and high densities, the variance in fertilization success did not differ between the sexes, because of sperm limitation at low density and sperm competition at high density. Hence, sexual selection in sea urchins is under control of the adult only in the sense of timing and quantity of gamete release; the rest is mediated by traits of the gametes themselves. — AMS Am. Nat. 164 , 298 (2004).