BIOLOGY OF ECOLOGICALLY MARGINAL POPULATIONS OF ANTHOXANTHUM ODORATUM. I. PHENETICS AND DYNAMICS

Justification for the evolutionary study of marginal populations can be provided from at least five points of view: (1) peripheral populations are postulated to play a central role in speciation phenomena but in very different ways (Carson, 1955; Brown, 1957); (2) they are attributed a major role in 'bursts' of adaptive radiation (Eldredge and Gould, 1972); (3) comparison with central populations yields information on selection processes (Prakash et al., 1969); (4) marginal situations may be stressful and may therefore isolate or accentuate phenomena not otherwise detectable, e.g., developmental instability, breeding system changes, gene flow effects (Mayr, 1963; Soule, 1973); and (5) peripheral populations provide suitable empirical units for assessing the nature of range extension processes. Investigation of comparative genetic properties of marginal and central populations began in the early 1950's with studies of chromosomal inversions in several Drosophila species, for example (1) D. robusta in the southern U.S. (e.g., Carson and Heed, 1964); (2) D. subobscura in Europe, Asia Minor, Turkey and Israel (e.g., Sperlich, 1964); (3) D. willistoni in the southern U.S. and S. America (e.g., da Cunha et al., 1959; Dobzhansky, 1957) and (4) D. pseudoobscura in Columbia (Dobzhansky et al., 1963). (For review see Soule, 1973 and Grant, 1974.) The consistent finding of these studies was that geographically marginal or isolated populations were genetically depauperate with respect to inversion polymorphisms when compared to central or continental populations of the same species. A. da Cunha et al. (1959) advanced the reasonable overall hypothesis that the degree of structural heterozygosity was directly related to the number of ecological 'niches' available. The general acceptance of this hypothesis in the 1950's and 1960's gave way to discordance with the development of electrophoretic techniques. Whereas Prakash et. al. (1969) using allozyme techniques confirmed work by Dobzhansky (1957) showing a drastic decrease in genetic variability in the isolated Bogota population, they also state that for three U.S. populations, ". . . there is clearly no significant effect of marginality on genic heterogeneity." In a similar study of allozymes in D. robusta, Prakash (1973) notes a slight increase in genetic variability in the marginal populations, in direct contrast to strong decreases in chromosomal inversion heterozygosity in this species (Carson, 1956; Carson and Heed, 1964). Ayala et al. (1971; 1972a; 1972b), working with D. willistoni and D. equinoxialis, found a definite decrease in both genic and structural heterozygosity in the marginal populations but the decrease was less marked for allozymes. Studies with other animals support the basic patterns of decrease in genetic variation near species margins (see review by Soule, 1973). Plant populations have been studied to a much lesser extent. Tigerstedt (1973) used isozyme techniques with Picea abies to examine the pattern of genetic variation in a broad geographical sense and found no decrease in variability at the species margin. Agnew (1968), studying variation on a more local scale, found a marked decrease in the phenotypic varieties of Lysimachia volkensii at ecological margins.