GENETIC-VARIATION IN SIMOCEPHALUS (ANOMOPODA, DAPHNIIDAE) IN NORTH-AMERICA - PATTERNS AND CONSEQUENCES

The presumption of intercontinental distributions of many species of A nomopoda has been reinforced by their considerable morphological unifo rmity and ease of passive dispersal via ephippial eggs. To test the va lidity of this dogma, genetic variation among taxa in the cladoceran g enus Simocephalus was examined on a continental scale. Genetic variabi lity (percentage polymorphic loci = 8.8%, individual heterozygosity = 3.4%) was comparable to that determined on a local scale in Simocephal us and somewhat less than for other zooplankton groups. Four species c omplexes were distinguished allozymically with unique allelic substitu tions found for at least two loci between these taxa. Eight species in North America were differentiated within these complexes (Nei s genet ic distance, D > 0.30), all of which were clearly separated from two E uropean species. Both S. cf, vetulus and S. cf. serrulatus are broadly distributed in North America, and intraspecific divergence is relativ ely low. In contrast, S. cf. congener and S. cf. exspinosus represent species complexes, both consisting of several species, differentiated allozymically and morphologically. The concept of generalist species w ith moderate genetic variation throughout a broad range and specialist species, genetically depauperate, with restricted distributions, is e xplored.