Multi-locus genetic evidence for rapid ecologically based speciation in Daphnia

The process of speciation involves the divergence of two or more subpopulat ions of a parent species into independent evolutionary trajectories. To stu dy this process in natural populations requires a detailed knowledge of the genetic and ecological characteristics of the parent species and an unders tanding of how its populations can lose evolutionary cohesion. The cosmopol itan and speciose genus Daphnia provides many of these features by existing in multiple freshwater habitat types, particularly permanent lakes and tem porary ponds, each of which presents distinct ecological challenges. We ass ayed the genetic composition of 20 temporary pond populations of members of the Daphnia pulex species complex in north-western Oregon and compared the m to published data on related lake and pond populations. We collected mole cular genetic data from 13 allozyme loci, from six microsatellite loci, and from the control region of the mitochondrial DNA. By assaying over 400 ind ividual Daphnia for these data, we were able to compile composite genotypes not only of individual Daphnia but of each pond population as a whole. In these ponds, we discovered two distinct genotypic constellations, one which bears resemblance to the lake-dwelling taxon D, pulicaria, and one which b ears resemblance to the pond-dwelling taxon, D. pulex. Using published gene tic data from these and other species as a frame of reference, we character ized 13 of these ponds as being 'pond-like', three as being 'lake-like', an d four as being 'mixed'. Unlike studies performed elsewhere, however, these ponds do not exhibit high probabilities of interspecific hybridization. Ov er 95% of all individuals have either a lake-like or a pond-like genotype a t all three genetic systems, suggesting the two forms do not represent hybr idized vs, nonhybridized genotypes. Because both types can be found in the same ponds at the same time in gametic disequilibrium, we also discount the possibility that they are two extremes of a single species that is highly genetically subdivided. With these genetic data, and with supporting life-h istory and ecological data previously gathered on these pond populations, w e conclude that the most likely description of this system is of a taxon ca ught in the act of speciating, with new pond-adapted populations periodical ly stemming from lake-adapted sources during river flooding events.