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Message: <blockquote data-quote="Kratter" data-source="post: 1360430" data-attributes="member: 50001">Even if carolinensis is more closely related to flavirostris (and not crecca), it does not mean that carolinensis and crecca cannot be lumped into one species, and flavirostris put into another species, if your follow the BSC (and a good majority of the AOU-CLC does).  Theoretically, two populations (say, A & C) that are very different genetically can be phenotypically identical (thus no reproductive isolation), while it may take only one nucleotide substitution (in population B that has recently split from A) to create a phenotypic difference that leads to reproductive isolation. Thus A & B are more closely related than A & C, but A & C are considered the same species because they are not reproductively isolated, while A/C & B are different species.  Many who follow the BSC do not think that monophyly is an absolute critereon at the species level. Andy Kratter</blockquote>