The evolutionary history of freshwater zooplankton is still relatively unkn
own. However, studies of the microcrustacean Daphnia have revealed interest
ing patterns; the daphniids that dominate ponds and lakes in the northern h
emisphere may have recent origins, likely associated with the glacial advan
ces and retreats during the Pleistocene. Moreover, they form species comple
xes that actively engage in hybridization and introgression. The present st
udy examines the phylogenetic relationships among circumarctic members of t
he Daphnia pulex complex, through the analysis of sequence diversity in 498
nt of the ND5 mitochondrial gene. Our results suggest that the complex is
composed of three major clades, two of which are subdivided into at least e
ight different lineages. Clearly, species in the complex shown genetic disc
ontinuity. Many lineages originated during the Pleistocene, but at least th
ree lineages diverged during the Pliocene. Two taxa (D. pulex, D. pulicaria
), thought to be broadly distributed in the northern hemisphere, are shown
to be endemic to single continents. In general, the diversification of the
pulex complex is characterized by rapidly dispersed lineages spanning enorm
ous distances and also by endemism in temperate areas. Gene flow among line
ages from the temperate region of different continents are restricted to ra
re intercontinental migrations across a polar bridge followed by convergent
morphological evolution. (C) 1998 The Linnean Society of London.