Sympatric speciation without borders?

The biogeography of speciation remains a controversial issue and the process of allopatric speciation reigns. Sympatric speciation differs from allopatric speciation in terms of geographic setting and the role of selection in bringing about reproductive isolating mechanisms, making it a particularly fascinating and controversial subject for evolutionary biologists. Mayr (1947) explained the difference eloquently: for allopatric speciation, populations spatially diverge and then become reproductively isolated; for sympatric speciation, populations first become reproductively isolated and then diverge. Because of this, sympatric speciation is difficult to show empirically and most evolutionary biologists agree that strict ecological, evolutionary, and geographic criteria must be met (Coyne & Orr 2004). In this issue, Crow et al. (2010) challenge us to expand the definition of sympatric speciation by studying species of marine fishes that they propose have arisen by sympatric speciation in a setting that does not appear to conform to the usual geographical criteria.