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Large changes in genotypic frequencies have been observed over a 2-year period in a population of Daphnia magna polymorphic for two alleles of malate dehydrogenase. These changes were not the result of migration or random drift. Genotypic differences in both parthogenetic and sexual egg production were observed. There was a strong correlation between the parthogenetic fecundities of the genotypes and their frequencies 1 month later. Sexual egg production was enhanced among low fitness genotypes. These observations strongly suggest that populations of Daphnia magna consist of a limited number of ecologically differentiated clones.

In temporary habitats populations of the cyclical parthenogen, Daphnia magna, are re-established each year from sexual eggs and reproduce partheno-genetically for two or at most three generations. The genetic effects of this breeding system have been investigated by analyzing allozyme frequencies in nineteen intermittent populations—Genotypic frequencies at polymorphic loci were ordinarily found to be in good agreement with Hardy-Weinberg proportions and disequilibria between loci were not observed. Although significant changes in gene frequencies were observed both during and between successive cycles, there was no evidence of the marked instability of genotype frequencies characteristic of permanent populations. The recombinational degradation of genotypes at the end of each annual cycle in temporaly habitats effectively prevents the genotypic structuring which develops when continued parthenogenesis is possible.

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

Natural populations of Daphnia magna have been found which are polymorphic for electrophoretic variants of supernatant malic dehydrogenase, esterase, and alkaline phosphatase. Using these enzyme variants as genetic markers it has been possible to demonstrate the absence of recombination during parthenogenetic reproduction. Genetic uniformity is expected within parthenogenetic clones derived from a single female.

Three zones of L( + ) lactate dehydrogenase (LDH) activity are apparent after electrophoresis of extracts prepared from well fed individuals of Daphnia magna. Phenotypes of individuals subjected to brief starvation are characterized by additional bands below each of the original zones of activity, while prolonged starvation results in the total disappearance of LDH activity in two of the three zones. Proteolysis of pre-existing LDH molecules is responsible for the appearance of the more anodally migrating components after brief starvation, but the cause of the phenotypic changes after prolonged starvation is uncertain. The possibility of the using LDH phenotypes to monitor changes in food availability within natural populations of D. magna is discussed.

SUMMARYThe frequencies of genetically determined electrophoretic variants of two enzyme systems in the parthenogenetic crustacean D. magna have been followed in two isolated populations. In both populations a marked excess of hetero-zygotes was found in the later samples. It is concluded that the observed changes in gene and genotypic frequencies are due to natural selection as both migration and genetic drift can be excluded.

MANY natural populations of Drosophila melanogaster are polymorphic for genetically determined electrophoretic variants of the enzyme alcohol dehydrogenase (ADH). Two alleles have been found, ADH-S (the slow variant) and ADH-F (the fast variant)1 and crude extracts of ADH-F homozygotes typically have a higher enzyme activity than extracts from ADH-S homozygotes2, 3.