Every park has at least one weird duck. It’s the wrong colors—all white, or patchy white; its bill bright storybook orange or its face weirdly red and lumpy. Next to the other ducks it looks oversized and bulky, like a linebacker in a crowd of quarterbacks.
How new species form, and what determines whether they last, is one of the major topics in evolutionary biology; and much of this topic is embodied by that one weird duck.
It’s not hard to see why people are interested in speciation. Look at any group of animals, and the obviously question is: how did they become so many shapes/colors/sizes? Why are they so diverse?
The more you think about these questions, the more they seem to hatch new questions: why do we see all of this variation everywhere? How do not-ducks become ducks? Why are there so many ducks? How do theropod dinosaurs become birds?
Fundamentally, the diversity that we see around us—duck or otherwise—is a function of how quickly new species form and how quickly those species then disappear.
The formation of new species
New species form when a population is subdivided and those subgroups experience different environmental pressures. It may be strange to think of duck populations as being separated: ducks are champion fliers, and no mountain or river is a true barrier to a determined duck.
But as long as the populations do not breed together, they can speciate. Consider these geese:
Canada Geese winter across North America. This Emperor Goose has gotten a bit lost, and was wintering in Pacifica, CA. But even though these two geese were hanging out together in February, they will remain separate species, because the Canada Goose will breed somewhere in the North American mainland while the Emperor Goose will breed in coastal Alaska or Siberia. The Emperor Goose will continue to experience, and adapt to, the pressures of life in the extreme north; the Canada Goose will continue to exploit the lawns-and-golf courses niche. These environmental differences drive the physical differences between the two species.
Not all selection pressures are natural. Breeds of domestic duck also arise through the isolation of groups and their subsequent evolution; the difference is that humans deliberately determine the traits these breeds acquire. Generally, humans want big, tame ducks that lay lots of eggs. Often we like our fowl to be white, because white birds have pale pink skin that we find more appetizing than the darker skin of dark-plumaged birds. (If you ate a standard grocery-store turkey for Thanksgiving, that turkey was almost certainly white, despite our collective mental image of Thanksgiving turkeys as brown-feathered birds.)
Wild species adapt to fly far, to fend off predators, and to find food in barren places.
As they become more specialized, they may develop distinct plumage to show off their health and quality to prospective mates.
Meanwhile, domestic breeds are shaped into distinctive forms of their own by selective breeding. The mechanism is the same—restricted and unequal breeding—but the resultant creatures are quite different.
The loss of species
You can lose species in two ways. One is extinction: if all of the individuals in a species die, that species is gone.
The second is hybridization. Species remain distinct entities by having little or no gene exchange with other species. A male Mallard mates with a female Mallard, and they have little Mallard ducklings.
For this to work, however, there needs to be something preventing the species from breeding with other species. Just as during species formation, for species to persist, there needs to be a mechanism for reproductive isolation. Without reproductive isolation, species could hybridize to the point of uniformity, turning the original two species into a single hybrid species.
Sometimes the reproductive isolation between species is geographical: two species live on other sides of the world, so they never meet, and therefore never mate. Sometimes it is physical: the two species are different enough in their biology that if they do mate, the hybrid babies don’t fare well. This is the case with mules, the hybrid offspring of a female horse and male donkey: mules are almost always infertile.
Often some part of reproductive isolation is behavioral: the species prefer to mate with individuals of their own species. This reproductive barrier can be prone to break down. In ducks—as in most birds—the babies learn very early what they should look for in a mate by imprinting on what their own parents look like. This usually works. However, if a duckling happens to be adopted into the family of another species (which can happen, either naturally or due to researcher experiments), it may imprint on that species instead.
Among birds, ducks seem to be especially poor at maintaining reproductive isolation. Hybrids between wild duck species are, if not exactly common, more numerous than hybrids of other types of birds.
Now consider those artificially-selected domestic breeds. While breeding for large size, tame demeanor, large numbers of eggs, and so on, is it likely that humans also selected for ducks that were discerning about their choice in mates? No. So when those calm, oversized domestic ducks escape or are released into the wild, they aren’t particularly opposed to being courted by ducks of other species. The wild ducks may be a bit less eager to mate with these giant weirdos, but really, no male duck is going to pass up an opportunity to sire some ducklings.
Their hybrid offspring—part domestic, part wild—couldn’t choose to mate exclusively with their own species even if they wanted to, not being a member of a single species themselves.
The hybrids may mate with wild ducks, domestic ducks, and other hybrids.
The hybrids are generally not picky. The Mallard-colored male in the above photos, after mating with the dark hybrid and the other Mallard-looking hybrid, was so pleased with his own attractiveness that he went after a third partner.
The American Coot was having none of this, and took off immediately. All of the other coots began keeping their distance and eyeing the duck mistrustfully.
Just to be clear, a female duck looks like this:
And an American Coot looks like this:
Not similar, mister hybrid duck.
This is the story of those weird ducks at the park: species being forged by the challenges of the wild and being sculpted by humans in domestication; then the barriers between those species breaking down when the two groups meet in the half-wild, half-human settings of urban parks. Natural speciation, artificial speciation, and species loss all play a role in producing that one hybrid duck.
P.S. This post is, necessarily, an oversimplification of these processes. Speciation is complicated!