Data from: Heteropatric speciation in a duck, Anas crecca

Heteropatric differentiation is a mode of speciation with gene flow in
which divergence occurs between lineages that are in sympatry and
allopatry at different times during cyclic spatial movements. Empirical
evidence suggests that heteropatric differentiation may prove to be common
among seasonally migratory organisms. We examined genetic differentiation
between the sedentary Aleutian Islands population of green-winged teal
(Anas crecca nimia) and its close migratory relative, the Eurasian, or Old
World (OW), Anas c. crecca population, a portion of which passes through
the range of nimia during its seasonal migrations. We also examined its
relationship with the parapatric North American, New World (NW), A. c.
carolinensis population. Sequence data from eight nuclear introns and the
mtDNA control region showed that the nimia-crecca divergence occurred much
more recently than the deeper crecca-carolinensis split (~83,000 y vs.
~1.1 My). Despite considerable spatial overlap between crecca and nimia
during seasonal migration, three key predictions of heteropatric
differentiation are supported: significant genetic divergence (overall
mean Φst = 0.07), low gene flow (2Nem ~1.8), and an effective population
size in nimia that is not especially low (Ne ~ 80,000 individuals).
Similar levels of gene flow have come into nimia from carolinensis, but no
detectable nuclear gene flow has gone out of nimia into either OW (crecca)
or NW (carolinensis) populations. We infer that adaptations of these
populations to local optima in different places (e.g., each matching their
reproductive effort to different resource blooms) promote genetic
isolation and divergence despite periods of sympatry between them, as the
heteropatric model predicts.