Fig. 2: Coalescent model illustrating how a more recently established population of yellow-billed pintails at high altitude likely acquired beneficial β-globin allele(s) from an older, more established population of speckled teal at high altitude.

This example of adaptive introgression presumably involved a founder event by yellow-billed pintails followed by hybridization and backcrossing to the parental population and a selective sweep on newly-acquired beneficial β-globin alleles, which appear identical by descent (IBD) across the entire β-globin cluster. A selective sweep likely occurred earlier the original speckled teal β-globin prior to yellow-billed pintail becoming established at high altitude.