CATASTROPHIC SELECTION AS A FACTOR IN SPECIATION

Speciation is generally considered to be a slow process involving gradual change in gene frequencies. At the same time, some patterns of variation and relationship among many groups of organisms seem explicable only on the basis of rapid genetic reorganization. Mayr (1954), for example, has suggested that the conspicuous differences between island populations of many birds and their mainland relatives may represent rapid evolution as a consequence of "genetic revolution" following drastic alteration of genetic background in newly founded peripherally isolated populations. Among flowering plants, patterns of differentiation are found that suggest a rapid process of speciation at the diploid level in ecologically peripheral populations within continental areas. For example, a study of species relationships in the genus Clarkia (Onagraceae) has led me to the conclusion that rapid speciation, involving an abrupt shift in adaptive mode and accompanied by barriers to gene exchange, represents the normal mode of speciation. Furthermore, the available evidence strongly suggests that speciation normally involves catastrophic selection in ecologically marginal populations.