EVOLUTION IN PERIPHERAL ISOLATED POPULATIONS: CARPODACUS FINCHES ON THE CALIFORNIA ISLANDS.

Abstract

The effect of being isolated on an island is seen as an important factor in the evolution of insular populations. An island population may be very different phenetically and genetically from a presumed conspecific population living in a similar environment on the mainland, whereas conspecific populations on a mainland may be very similar to one another even though occurring in widely distant localities and under different ecological conditions (Mayr, 1954). The reason for this is in part that rapid evolution may occur in small peripheral populations (Carson, 1959; Cook, 1961). However, an opposite effect, centrifugal speciation, has also been described wherein peripheral forms of a species may show more primitive traits, not having received the advanced character states which spread from centers where a species is best adapted and where neighboring forms come together and increase genetic heterogeneity (Brown, 195 7). A related problem is that of genetic variation in marginal and peripheral populations. Soule (1973) has reviewed data on reduction in the diversity of coadapted gene combinations in novel environments, the advantages of preserving allelic polymorphism, and the loss of alleles at geographic margins because of genetic drift, and reduced immigration. Recently, the importance of geographic isolation, and especially of populations on the periphery of a species' range, also appears in arguments supporting the view that speciation is primarily by punctuated equilibria (Eldredge and Gould, 1972; Gould and Eldredge, 1977). If our understanding of the way evolution works is correct, there is a great potential for peripherally isolated populations to break from the genetic cohesiveness of the parent species and become genetically independent. A potential for speciation and genetic distinction can be demonstrated when relatively large phenetic differences exist among populations separated by short distances but imposing physical barriers. This situation is often found with populations on fringing islands. The present paper is an analysis of phenetic similarity among populations of the house finch (Carpodacus mexicanus) from the California Islands and the western U.S. (Fig. 1). I want to concentrate on barriers to dispersal and the accumulation of morphological differences among populations. Details of character variation, systematics, and population variability on the islands will be included in later papers (e.g., Power, in press). The house finch is a widespread passerine species of western North America. The genus Carpodacus originated in the Old World; most of the adaptive radiation (about 18 species) has been in Asia. Three species-the house finch (or linnet), purple finch (C. purpureus), and Cassin's.,finch (C. cassini)-are in the New World. Purple and Cassin's finches prefer forested areas, whereas the house finch breeds in desert and scrub vegetation from British Columbia south to central Mexico (Salt, 1952). It is also common in suburbs and around farm buildings, gardens, and orchards. Range expansion into eastern Oregon, eastern Washington, and Oklahoma, and the successful, man-aided establishment of this species in Hawaii (Hirai, 1975) and on the east coast of the U.S. (Gill and Lanyon, 1965) may be due to the presence of human alteration of the habitat, but it also demonstrates the bird's