Page last updated
27 December 2007

Palearctic Freshwater Mussels

In the West: From Nouvelle École to the Biological Species Concept

Critically appraising the taxonomy of Palearctic freshwater mussels is a huge task because of the daunting amount of raw taxonomic materials, especially the excess of available names. Over-description is a general problem for freshwater mussels (indeed, for freshwater mollusks), but it is especially so for the European assemblage. There, in the late 19th century, the French Nouvelle École ("New School") produced such malacological super-nominators as Bourguignat, Locard and Drouët. Today, according to our own on-going research, more than 1500 names are available for the European fauna of about 15 species (100:1 synonymy ration, on average), and that accounts for ca. 86% of all the names introduced for the entire Palearctic Region.

C.T. Simpson, at the turn of the 20th century, started the process of synonymizing this glut of names. As the first half of the century progressed, evolutionary biologists revised their concept of just what they thought a species is. This discussion led to a change from species being simply things that are different from other things (typology) to evolutionary entities. By mid-century, the prevailing concept of a species was the Biological Species Concept.

The Biological Species Concept (BSC) was championed by Ernst Mayr and derived from the New Synthesis. When Darwin proposed natural selection as the mechanism of evolution, the actual process was still a little fuzzing. This was especially due to a lack of understanding of inheritance. But as genetic research blossomed during the first half of the 20th century, along with paleontology and developmental biology, the process began to come into focus. Species changed from being things that we different from each other to being organisms that shared genealogical connections.

According to Mayr & Ashlock (1991: 26), a species is “a group of interbreeding natural populations that is reproductively isolated from other such groups.”

A result of this change in persective was a drastic reduction in the number of species recognized, especially among hyper-variable taxa like freshwater mussels. Whereas before the BSC when every minor difference between two individual mussels was considered grounds to distinguish them as species, the BSC treated those differences as simply variation among geographically isolated populations that potentially retained the capacity to interbreed. Sympatry — being found together — became the test for the species: if two otherwise similar forms are found in isolated basins, the null hypothesis became that the differences were merely different geographical races or subspecies. Thus, instead of many species with narrow ranges, the Palearctic fauna was reduced to a few species with wide ranges and numerous subspecies (though fewer than the Nouvelle École would have admitted). This remains largely the prevailing system for Palearctic freshwater mussels today.

However, more recently a rival system has emerged: the "Comparatory" (or "Comparatorial") Method of the emeninent Russian malacologist, Ya.I. Starobogatov. The Comparatory is typological rather than evolutionary in basis, and it's practicioners recognize a much higher diversity in the eastern Palearctic than those who apply the BSC do.

References

• Mayr, E & P.D. Ashlock. 1991. Principles of Systematic Zoology. 2nd edition. McGraw-Hill, New York.