Preference for one's own kind?

Eels from rivers in Europe, North Africa and America converge on the Sargasso Sea in the Atlantic to reproduce. Once there it was thought that they mate in a free-for-all, a theory that has been accepted since the 1920s. But research published in the 22 February Nature shows that eels breed only with others from their home region.

Thierry Wirth and Louis Bernatchez of the Université Laval in Quebec, Canada, analysed highly polymorphic genetic markers from 13 samples of eel taken from the North Atlantic, and the Baltic and Mediterranean seas (Nature 2001, 409:1037-1040). They found that eels from the different regions could be differentiated by means of seven microsatellite loci. Furthermore, the size of the genetic distance between two eels correlated with the geographical distance between their home rivers.

According to Wirth and Bernatchez, the most likely explanation is that eels from different latitudes arrive at different...