Microgeographic analysis of enzyme genetic variation in populations in the littoral zone cladoceran Simocephalus in southwestern Ontario has disclosed very low levels of intrapopulation variability. The mean percentage of polymorphic loci per population was 10.5%, and individual heterozygosity averaged less than 5% over all populations. Population structure typifies that of randomly mating, outbred sexual species. Most populations were polyclonal, with a few common widespread clones and several rarer ones, often found in only one pond. There were no significant temporal patterns in genotypic frequencies, but spatial differentiation of populations suggested that both selection and founder effects have played roles in the evolutionary divergence of these cyclic parthenogens. Despite extremely low intrapopulation and moderate intraspecific variation, there was extensive differentiation between species. As a consequence of virtually complete allelic substitution at several loci and great similarity or identity of alleles at others, diagnostic loci were detected in three of the four species. Of the four distinct clusters of populations, differentiated at the species level, three represented species previously recognized in North America (S. cf. vetulus, S. cf. serrulatus, S. cf. exspinosus), and the fourth group was identified as S. cf. congener, related to a taxon occurring in Europe, but not reported from North America. The great amount of divergence between populations of S. cf. exspinosus suggests that a complex of at least two sibling species exists within this taxon.
RE-INTERPRETATION OF GENETIC VARIATION IN SIMOCEPHALUS (CLADOCERA, DAPHNIIDAE)
