The genus Anas has a high propensity to hybridize, sometimes with bizarre results (Mallard X Muscovy, Mallard X Green-winged Teal, Mallard X Northern Pintail). This causes serious debate about classification of species within the genus. The Green-winged Teal is a perfect example of this. However, another complex group includes the Mallard, "Mexican" Duck (currently considered a subspecies of the Mallard by the AOU), Mottled Duck, and American Black Duck. The following website provides a very convincing argument in favour of splitting the "Mexican" Duck and giving it full species status (or, as an alternative, lump it with the more closely related Mottled Duck). It is worth a read:
One passage regarding recent studies of the genetic similarities between the Common Teal and Green-winged Teal that stood out in the article is the following:
"All of this contributes to the increasing realization that in waterfowl some plumage similarities represent tremendous plumage conservatism over time while outstanding plumage differences may be more recent changes."
Or, in other words, the Green-winged Teal and Common Teal may look incredibly similar, but are actually less closely related genetically than the Mallard is to the Northern Pintail. This is quite astonishing actually and the author's comment on "plumage conservatism" is very interesting.
This is exactly what is happening with the Mallard complex. The Mallard is more closely related, genetically, to the American Black Duck than it is the "Mexican" Duck and yet the "Mexican" Duck is still considered a subspecies of the Mallard. This would seem to be going against the AOU rulebook on species status and obviously needs further research. Although the Mexican Duck and Mallard hybridize, so do Mallards and Black Ducks.
The article also discusses a range of morphological and behavioural discrepancies between the two 'subspecies' as well as a detailed description of their ranges and how they differ.
In the compelling article, the author even goes as far as saying, "A reluctance on the part of some to advocate splitting Mexican Duck from Mallard stems from a concern about how a split could create a de facto threatened species in the U.S., especially a concern about how that could affect the popular sport of duck hunting. This is a brave statement surrounding the ethics of the AOU questioning whether their decisions are scientifically based or if they are influenced by bias and 'the easy way out'.
If nothing else, the article hits on a point I made in my last post: the importance of studying subspecies as though they were a separate species. Oftentimes, subspecies are given less notoriety, threatened or otherwise, and perhaps if the Mexican Duck was given full species status, it would receive the attention it deserves.