Why do isolated or nonmigratory waterfowl populations develop racial differences?

In isolated or nonmigratory waterfowl populations, racial differences develop primarily because the restriction of gene flow prevents the homogenization of the population.1

In migratory species, rapid gene flow typically maintains uniformity across the population. However, when populations are nonmigratory or geographically isolated, they are no longer subject to this mixing. In these smaller, isolated groups, two primary mechanisms drive the development of distinct traits:1

• Genetic Drift: In small populations, chance events can cause allele frequencies to change significantly over generations, leading to the loss or fixation of certain traits regardless of their survival value.23
• Natural Selection: Isolated groups often adapt to their specific local environments. As these populations face different ecological pressures, natural selection favors mutations that are locally valuable, leading to phenotypic differentiation.45

Additionally, some waterfowl maintain family groupings even away from breeding grounds, which fosters learned, traditional attachments to specific birth areas. This behavior further reinforces isolation and contributes to the development of distinct regional characteristics.1