Genetic analyses show that Spot-billed Duck and Mallard diverged recently and hybridized in the process.
Telling male and female Mallards (Anas platyrhynchos) apart is not difficult. The male has an obvious fluorescent-green head, while the female is…well…brown (but nonetheless very beautiful). But did you know that most species in the Mallard complex are monochromatic (i.e. males and females have the same color)? Just have a look at the American Black Duck (A. rubripes) or the Hawaiian Duck (A. wyvilliana).
Most waterbird researchers argue that these monochromatic species originated from a dichromatic Mallard-like ancestor (see for example Omland 1997). A recent study in Current Zoology explored the evolutionary history of another monochromatic duck, the Spot-billed Duck (A. zonorhyncha).
Mallards and Spot-billed Ducks are difficult to separate with genetic data. When you construct a family tree for these duck species, you will arrive at clusters that contain both species. In technical terms, they do not form monophyletic groups. This pattern can be the outcome of two processes: recent divergence or hybridization.
Previous work in Russia suggested that hybridization was the main cause for this non-monophyly. Wenjuan Wang and colleagues expanded the sampling to China and reconstructed the evolutionary history of the Spot-billed Duck in more detail.
Too Little Time
The genetic analyses revealed that the non-monophyly between Mallard and Spot-billed Duck can be explained by both recent divergence and hybridization. Both species went their own evolutionary ways about 40,000 years ago. There has been too little time for the genetic variation to sort among the lineages.
In addition, hybridization between both species prevented faster differentiation. Isolation-with-migration analyses of mitochondrial DNA (mtDNA) indicated that gene flow was mainly from Spot-billed Ducks into Mallards. Given that mtDNA is maternally inherited, this suggests that female Spot-billed Ducks mostly mated with male Mallards.
This study nicely shows that ducks are a goldmine for those who want to understand the dynamics of hybridization.
Wang, W., Wang, Y., Lei, F., Wang, H. & Chen, J. (2018) Incomplete lineage sorting and introgression in the diversification of Chinese spot-billed ducks and mallards. Current Zoology
This paper has been added to the Anseriformes page.