ENZYME VARIABILITY IN NATURAL POPULATIONS OF DAPHNIA MAGNA I. POPULATION STRUCTURE IN EAST ANGLIA.

Abstract

The patchy distribution of freshwater ponds would seem to provide ideal opportunities for deme formation and enhancement of evolutionary rates due to selective and stochastic exploration of genetic variation (Wright, 1940, 1960). In actual fact, however, pond dwelling organisms show evidence of slow evolutionary differentiation. The zoogeographical uniformity of pond life, particularly of those groups which rely on passive dispersal, has long been recognized (Darwin, 1859; Belt, 1874); global distributions are frequent at the generic level, while species usually have large ranges over which they show little evidence of subspeciation. Bayley (1967), for example, points out that many of the cyclopoid copepods found in Australia are phenotypically indistinguishable from North American species. This apparent lack of genetic differentiation has generally been related to high vagility (Mayr, 1963). The cladoceran crustacean, Daphnia magna, possesses many of the characteristics typical of passively dispersing pond organisms. The range of the species is extensive: D. magna occurs throughout the Palearctic, over much of the Nearctic and in the southern portion of the Ethiopian region as well (Richard, 1896; Sars, 1916). Over this area the species is subject to little phenotypic variation. Brooks (1957) mentions that European and North American populations can be distinguished only with difficulty. If gene flow is responsible for maintaining phenotypic uniformity over this large range, then one might expect microgeographical differentiation of gene frequencies to be minimal. Yet in an