Biological species and taxonomic species: Will a new null hypothesis help? (A comment on Gill 2014)

In his article “Species taxonomy of birds: Which null hypothesis?”, Gill (2014) recommended that we might apply our growing knowledge of avian speciation more effectively, particularly to avian taxonomy and the definition of species. Specifically, Gill (2014) suggested that committees on avian nomenclature should operate under a new null hypothesis for species designation: Genetically and phenotypically distinct taxa would be considered full species, a priori, in the absence of any natural tests of reproductive isolation (i.e. sympatric populations) or additional evidence of possible isolating barriers. There are several useful aspects to this suggestion. However, in this Commentary, I present a number of issues that suggest that such a proposal may be premature. More generally, I recommend that unless a more compelling argument is made for altering the status quo, it seems prudent for nomenclature committees to continue to use the best available evidence to make informed decisions about the extent of reproductive isolation between putative avian species.