According to a recent paradigm of the speciation process presented by Ayala (1975), geographic speciation often occurs in two stages. In the first stage isolated populations accumulate genetic differences which may reduce the genetic compatibility between populations. In the second stage the isolated populations regain sympatry. If hybrids have reduced fitness, selection favors the development of premating isolating mechanisms which prevent wasteful hybridization. This isolating mechanism is often ethological or sexual in nature (Mayr, 1963). This paradigm of the speciation process should be appropriate for many groups of organisms, but the relative importance of changes occurring in each of the two stages may differ considerably between species groups. For many pond and lake dwelling crustaceans, for example, stage two, the sympatric development of premating isolating mechanisms following geographic separation, appears to be rare. The degree to which populations differentiate during stage one of geographic speciation depends on the amount of gene flow between populations and on the differences in the selective regimes encountered by different populations. The disjunct nature of ponds and lakes should facilitate the genetic differentiation of aquatic populations by restricting gene flow and by offering many different selective regimes (Hebert, 1974). Studies of population differentiation suggest that gene flow between aquatic populations may be quite restricted. Baldi (1946), for example, found morphological differentiation between closely situated populations of both calanoid and cyclopoid copepods;