neutral or selective means (akin to finding a fixed diagnostic phenotypic trait), may be a reasonable criterion for delimiting species.
It is certain that molecular approaches will continue to play an important role in species delimitation. However, the battle over species delimitation between the nuclear and mitochondrial genomes, and between the BSC and PSC, will have no victor. Nuclear gene trees will not provide enough phylogenetic resolution to satisfy avian systematists. Furthermore, the high levels of recombination detected in the first surveys of avian SNP variation appear prohibitive for standard phylogenetic analysis (Edwards and Dillon, 2004; International Chicken Polymorphism Map Consortium, 2004). On the other hand, in our view, maternally inherited mtDNA can never capture enough of a species history to delimit species on its own. Although mtDNA will frequently deliver clean phylogeographic breaks within avian species, these breaks need not have their origin in reproductive isolation (Irwin, 2002) and may in some cases be driven by natural selection (Hudson and Turelli, 2003). These same species showing clean mitochondrial breaks will frequently look very messy with regard to nuclear gene splits, as decades of allozyme analyses have already confirmed. We suggest that, despite its disproportionate contribution to revealing phylogeographic patterns and its ability to reflect cessation of female gene flow more rapidly than nuclear genes, mtDNA should not have priority over nuclear genes in avian species delimitation. Ours is a generation of avian systematists raised primarily on single locus analyses of avian phylogeny and divergence. But nuclear genes can and should be interrogated in questions of avian taxonomy, even if the interpretation of nuclear histories and the contrast with mtDNA histories will be challenging.
As suggested by Mayr (1942), divergence of populations in allopatry appears to be the dominant mode of speciation in birds. Molecular phylogenies have provided new opportunities for testing alternative geographic models, and avian sister taxa generally meet the expectation of having allopatric distributions, whether because of vicariance or dispersal (Drovetski et al., 2004; Pereira and Baker, 2004; Voelker, 1999; Zink et al., 2000). Current distributions, however, do not necessarily reflect the geo-