Reinforcement: Origin, Dynamics, and Fate of an Evolutionary Hypothesis

Abstract

Abstract Species concepts and definitions have provided fertile ground for disagreements among biologists for more than 100 years (Darwin, 1859; Jordan, 1905; Du Rietz, 1930; Dobzhansky, 1937; Mayr, 1942, 1963, Burma, 1954; Mecham, 1961; Paterson, 1978, 1980, 1981, 1982; Donoghue, 1985; Coyne et al., 1988; Cracraft, 1989; Templeton, 1989). Yet most biologists would agree that in sexually reproducing organisms barriers to gene exchange exist between distinct species, and these barriers can be bro ken down into two classes: prezygotic and postzygotic. Included among prezygotic barriers are seasonal and habitat differences, long-distance signaling and courtship differences, and differences in genitalic structures. Included among postzygotic barriers are embryo inviability, hybrid inviability, hybrid sterility, and hybrid breakdown (for a more complete description of reproductive barriers see Mayr, 1963, and Dobzhan sky, 1970). Prezygotic barriers prevent individuals from wasting their gametes in the for mation of unfit hybrid individuals; hence such barriers are susceptible to enhancement by natural selection (Dobzhansky, 1940; Mayr, 1963). On the other hand, it is difficult to envision how natural selection could operate to strengthen postzygotic barriers such as hybrid inviability or sterility (Darwin, 1859; Mayr, 1963), except under fairly restrictive circumstances (Coyne, 1974). The potential involvement of natural selection in the formation of prezygotic barriers to gene exchange is what sets this group of barriers apart from postzygotic barriers. The process by which prezygotic barriers to gene exchange are improved by natural selection is known as reinforcement.