Evolutionary genetics of flycatchers. III. Variation in Empidonax hammondii (Aves: Tyrannidae)

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Abstract
We assessed variability at 36 structural gene loci in 149 individuals from nine breeding populations of the Hammond's Flycatcher (Empidonax hammondii), a species showing striking morphologic homogeneity over a nesting distribution spanning 30 degrees of latitude. Although moderately polymorphic (44.4%) at these loci, the species demonstrated low levels of mean observed heterozygosity (0.026), mean percentage of polymorphic loci (9.56), and mean number of alleles per locus (1.19), perhaps as a result of past population bottlenecks. Geographic genetic structuring was minimal (FSt = 0.043) and paralleled the lack of morphologic variation. Six of the eight unique alleles recorded occurred in Oregon, near the center of present abundance. Gene flow, as estimated by the rare allele method, was moderately high to high at 8–31 individuals per generation. Because most or all of the modern distribution of the species in western Canada and Alaska was either covered by ice or otherwise unsuitable for occupancy until the post-Wisconsin – Holocene interval, recolonization of the high latitudes in recent millenia is indicated. Reduced genetic variability and extreme morphologic uniformity of present populations is in agreement with this hypothesis.

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