Relationships
mtDNA data-wise, this species is more closely related to the Speckled Teal than to the Common Teal. This would require that sexual dimorphism either was lost in the Speckled Teal or that it evolved in near-identical forms in the Green-winged and Common Teal after the divergence of the Green-winged and Speckled Teal lineages.
Both hypotheses seem rather spurious initially, with the GWT and Common Teal's male nuptial plumage being unique and very complex, and the tendency to gain, not lose, strong sexual dimorphism overwhelming in the dabbling ducks. An alternative explanation given the high frequency of hybridisation in ducks would be that the mtDNA of the present-day GWT is originally derived from Spotted Teal females by introgression and thus the molecular data gives a misleading picture of the species' true relationships. This is supported by the observation that in Mallard × American Black Duck hybrids, females of both taxa prefer the sexually dimorphic mallard drakes over the dull-plumaged Black Duck drakes; that the GWT is in some aspects—such as the less contrasting nuptial plumage—intermediate between the Common and Speckled Teal is also interesting to note.
Alternatively, the Common Teal might be derived from the GWT, with the molecular difference being due to genetic drift or a founder effect in the latter and possibly Speckled Teal introgression in the former. The three teals certainly belong to a superspecies in the teals ("Nettion"); the ancestors of this group were most likely sexually monomorphic Southern Hemisphere forms (Johnson & Sorenson, 1999; note that their "African" distribution includes Bernier's Teal taxa which like many Madagascar taxa is of Indo-Australian origin).
Another possibility is that the American lineage is derived from stray Common Teals, with the founder effect/genetic drift and/or hybrid introgression phenomena applying as above, only in the reverse direction for the former two. Still, this would require loss of sexual dimorphism in the ancestors of the Speckled Teal, but while extremely rare in dabbling ducks, it is not per se impossible.
The close Speckled-GWT relationship suggested by mtDNA data could of course still apply to the taxa in general, not just to sequences in 2 maternally inherited genes in a few individual ducks (for which it without doubt does apply), but the overall failure of Johnson & Sorenson to seriously take hybridization into account and their small sample sizes and obsolete conceptions of Indian Ocean biogeography do not help at all to resolve the issue, but in 1999, the methodology and interpretation were reasonable enough and in fact, the study was pioneering in many respects due to dense taxon-level sampling and still represents one of the default references for interpreting the pyhlogeny of the genus.
It is worthy of note that the post-copulatory displays of the Common and GWT are identical, but those of the Speckled Teal have some additional elements.
All in all, a firm conclusion cannot be reached at present beyond a tentative rejection of the phylogeny suggested by the mtDNA data. Nuclear DNA sequence information is required, but may not be sufficient, to resolve the puzzling relationships in the crecca-carolinensis-flavirostris complex of teals.
Read more about this topic: Green-winged Teal