So if you thought the gray wolf species complex is controversial, it mirrored in another charismatic, widespread species.
I used to be quite into domestic ducks. My preferred variants were all domesticated Eurasian mallards, and I regularly encountered wild mallards at the various parks I would frequent in Morgantown, West Virginia.
I have no problem considering Pekins, Cayugas, Khaki Campbell’s, and Rouens just domestic variants of mallard. Most domestic mallard varieties cannot fly, and many lack proper brooding instincts and could never really exist in the wild. However, domestic duck genes do get into the wild mallard population every so often. So in this way, domestic ducks are to mallards what domestic dogs are to gray wolves.
But the analogy gets even more interesting. There are endemic mallards in North America that are often regarded as distinct species, and most experts would regard them as distinct species. However, they could easily be thought of as regional variants of what is really a mallard complex.
The most common endemic North American mallard is the black duck. The black duck is a large mallard that behaves almost exactly like the more common form. However, the drakes never develop the green heads or chestnut breasts. They never get that ornate gray penciling on their plumage. They are heavily mottled, rather dark large mallards.
Most authorities regard this creature as a distinct species, but a good case can be made that American black ducks are a regional form of mallard. They live over Northeastern and Midwestern states. They breed extensively in Eastern Canada and the Northern Great Lakes.
One hypothesis is that these ducks are descended from an early radiation of the ancestral mallard that adapted to living in bodies of water surrounded by extensive forests. Because there was so much predation in those areas, the ducks had a strong selection pressure never to evolve the ornate plumage of the more typical mallard.
These ducks evolved into a distinct population from Northeastern Mexico to Florida, which is called the mottled duck. The mottled duck is usually considered a distinct species as well.
And in deeper into Mexico, there is the Mexican duck, which is probably derived from black duck population.
One weird thing, though, is that mallards and black ducks really do not recognize much of a species barrier. Indeed, the genetic difference between black ducks and mallards appears to be decreasing.
Maybe a better reading of black duck taxonomy is that black ducks just represent a form of mallard that adapted to living in high predator density forests, and now that the forests have been opened, the more open country forms of mallard in which the drakes have ostentatious plumage have invaded their range.
And they have started to hybridize significantly.
Most waterfowl experts would disagree with me on this question, but the truth is the molecular work on mallards and their relatives is far behind canids. And yes, we do know that lots of Anas ducks hybridize. Hybridization itself is not a very good species indicator within these species, but if it is as significant as it is between black ducks and mallards, then we have to reconsider our classification.
I would love to see full genome comparisons of the various ducks in the genus Anas. We need to get a better idea of when all these various ducks diverged from a common ancestor and get a full handle on how much hybridization has happened.
Gadwalls are also somewhere in this mess. They might be an early offshoot of mallards that adapted to truly open environments, but I would not be surprised if their hybridization was at a level comparable to that of black ducks and mallards.
So we need more molecular work on these ducks. Their evolutionary history has quite strong parallels in the gray wolf complex, including their wide distribution, lots of hybridization between populations, and domestic form that casts a few genes into the wild population every once in a while.
The only way to resolve these issues is to have comparisons of full genomes. My guess is that we will someday, but until now, we’re still basing species on mtDNA samples and very limited genetic markers.