Biogeography of a widespread freshwater crustacean: Pseudocongruence and cryptic endemism in the North American Daphnia laevis complex

The lack of morphological variation in many freshwater invertebrates over v ast distances has been cited as evidence for their frequent, long-distance dispersal. This scenario implies that vicariance will be an insignificant d eterminant of species distributions or diversity. We carried out a phylogeo graphic and population genetics study of one widespread crustacean group, t he North American Daphnia laevis complex. Allozyme and sequence variation o f two mtDNA genes (12S and 16S rRNA) clearly indicates the existence of fiv e morphologically cryptic, largely allopatric groups (Daphnia dubia, D. lae vis laevis, D. laevis gessneri, D. magniceps magniceps, and D. magniceps pa cifica ssp. n.). Within each of these groups, there is weak or no genetic d ifferentiation over large geographic areas suggesting their recent long-dis tance dispersal. The present-day distributions and phylogeography of the re gional groups suggests the occurrence of both deep and shallow vicariance e vents. Although divergence times from mtDNA sequences do indicate both deep and shallow divergences, these estimates are incongruent with their propos ed vicariance times. The results show that even within closely related fres hwater invertebrates, a complex biogeography exists, whose analysis is made difficult by long-distance dispersal, cryptic endemism, and pseudocongruen ce.