Witt and Hebert 2000

Witt JDS, Hebert PDN. 2000. Cryptic species diversity and evolution in the amphipod genus Hyalella within central glaciated North America: a molecular phylogenetic approach. Canadian Journal of Fisheries and Aquatic Sciences 57: 687-698.

These authors examined mtDNA and nuclear (allozyme) markers in a species of amphipod that is a strong candidate for cryptic diversification. Hyalella azteca has a vast geographic range, stretching from Panama to north of the Arctic circle and from the Atlantic to Pacific coasts of North America. It is found in a wide range of different freshwater habitats, including streams, rivers, ponds and lakes up to and including the Laurentian Great Lakes. These freshwater habitats are also highly disjunct in geographic distribution. All of these factors together present the large number of populations of this species with a huge range of selective pressures and long times since many populations last were in contact.

The goals of this study were first to extend earlier work that was suggestive of diversity within this putative species, and second to determine the ages of lineages with Hyalella azteca and compare those ages with major geological events such as the Pleistocene glaciations and the formation of the Isthmus of Panama. The subgenus, Hyalella, to which H. azteca belongs was previously thought to have originated after the formation of the Isthmus, by colonization from the genus’ center of origin in South America.

Seven mtDNA lineages were detected that differed from each other by between 9 and 28% at the gene COI. This exceeds many divergence values between congeneric or occasionally confamilial species. These authors recommend that these seven lineages be considered cryptic species, based on this and some of the biogeographic distribution evidence, and furthermore suggest that Clade 7, apparently divergent also in morphology, be included in the species Hyalella inermis, a taxon that was included into H. azteca early in the 20th century but should be resurrected.

Patterns of allozyme differences within Clade 5 and Clade 6 were consistent with a scenario of divergence during isolation in distinct glacial refugia during the Pleistocene. Other patterns of divergence strongly suggest the subgenus originated long before the Isthmus of Panama formed. These authors estimated a divergence date for the subgenus of 11 million years ago, while the Isthmus formed approximately 3 million years ago.

From a practical point of view, this paper is very important to me. First, it provides an example of a comparison of phylogeographic patterns with major geological events and inferred patterns of dispersal and colonization. Second, many of the collection locations described in this paper are near my proposed route of collection and travel from Guelph to Winnipeg, planned for the summer of 2008. As I intend to collect specimens of Hyalella azteca (or, indeed, of species within this apparent species complex), recollections from some of the same locations as this study are desirable.