GENETIC VARIABILITY IN EDAPHICALLY RESTRICTED AND WIDESPREAD PLANT SPECIES.

Abstract

The intuitively appealing notion that there are adaptive relationships between the genetic variability and ecological amplitude of populations is a persistent theme in evolutionary biology (Van Valen, 1965; Dobzhansky, 1970; Sammeta and Levins, 1970; Thoday, 1972). Adaptation to spatial and temporal heterogeneity of the environment can be achieved in several ways (Levins, 1969), including the employment of multiple genotypes specialized to exploit different subniches and the maintenance of purpose genotypes (Baker, 1965) providing phenotypic flexibility (Thoday, 1959) and plasticity (Bradshaw, 1965). Genetic aspects of the latter mode of adaptation are poorly understood (Cook and Johnson, 1968; Marshall and Jain, 1968), but it is well established theoretically (Levins, 1968; Maynard Smith, 1970) and experimentally (Beardmore, 1970; Powell, 1971) that disruptive (diversifying) selection in heterogeneous environments can maintain genetic polymorphism. One method of detecting possible ecogenetic relationships is to compare levels of genetic variability in closely related species differing in ecological versatility. For animals it is generally difficult to specify variation in ecological amplitude in an unequivocal manner. Although animal parasites differing in degree of host specificity and certain groups of monoand polyphytophagous insects may prove to be useful in the analysis ofthe problem, plants in general provide better material because the spatial limits of species and races often can be described in terms of the distribution of specific environmental factors. The