Genic population structure and gene flow in the Northern Flicker (Colaptes Auratus) hybrid zone

-The Yellow-shafted Flicker (Colaptes auratus auratus) and Red-shafted Flicker (C. a. cafer) form a stable, narrow hybrid zone on the western Great Plains of North America. Allozyme data were obtained from 31 structural gene loci for 33 samples representing 246 Northern Flickers from throughout the Great Plains. Flickers were approximately equivalent to other birds in terms of proportion of polymorphic loci (P = 0.207) and average heterozygosity (H = 0.056). There was no concordant variation between plumage characters and allelic frequencies. Gene-diversity analysis indicated that 92.5% of the genic variation occurred as within-deme heterozygosity (GD = 0.925), approximately 7% occurred among individual demes (GDT = 0.07), and only 0.9% occurred among major river drainages (GST = 0.009). Even less diversity was found among parental and hybrid groups (GST = 0.002). There is substantial allozymic structuring of the Northern Flicker species population, but the structuring is not associated with the hybrid zone, and there is, at most, very weak structuring into riparian zones of habitat. The electrophoretic data support the inference that gene flow among Northern Flicker populations is high (Nm = 1.9-4.4/generation). If the high gene-flow estimates are correct, then geographical selection gradients would be the most likely mechanism maintaining the narrow hybrid zone of plumage and morphometric traits. Received 21 July 1986, accepted 16 April 1987. THE Yellowand Red-shafted subspecies of the Northern Flicker (Colaptes auratus) form a narrow hybrid zone in woodland areas of the North American Great Plains (Fig. 1). Extensive hybridization between these two forms (Allen 1892, Short 1965) has resulted in the assignment of Yellow-shafted (C. a. auratus) and Red-shafted (C. a. cafer) flickers to subspecific status (A.O.U. 1983). Hybrid individuals do not have lower fitness than the parental types (Moore and Koenig 1986), and there is no evidence of preferential mating between flickers with similar plumages (Moore 1987). The position and width of the hybrid zone have been stable in historical times (Moore and Buchanan 1985). The central and northern portions of the hybrid zone probably formed as secondary contacts during the retreat of the Pinedale (Wisconsin) cordilleran glaciers in the Rocky Mountains (Short 1965, Hubbard 1969, Barrowclough 1980a). This occurred 10,000-13,000 years before the present (YBP) in the central Rockies 3 Present address: Department of Anatomy, Wayne State University, Detroit, Michigan 48202 USA. (Baker 1976, Madole 1976) and as recently as 7,500 YBP in the Canadian Rockies (Barrowclough 1980a). The hybrid zone south of Colorado may have a different history, and the possibility that divergence between the subspecies occurred primarily during interglacials, with the central prairies and deserts serving as a geographical barrier, cannot be excluded (Mengel 1970). Electrophoretic analysis of birds sampled throughout a hybrid zone, coupled with statistical techniques for estimating genetic population structure and gene flow, could help clarify the nature of the hybrid zone and provide insight into the processes of speciation. Despite the potential of such studies, genetic analyses of avian hybrid zones are rare. Genetic differentiation has been examined among several avian taxa known to hybridize occasionally (e.g. Martin and Selander 1975, Johnson and Zink 1983). At present, however, there are only five allozymic studies that specifically address the question of genetic variation in avian hybrid zones: Northern Orioles, Icterus galbula galbula x I. g. bullockii (Corbin et al. 1979); Yellowrumped Warblers, Dendroica coronata coronata x 654 The Auk 104: 654-664. October 1987 This content downloaded from 157.55.39.49 on Mon, 29 Aug 2016 05:05:43 UTC All use subject to http://about.jstor.org/terms October 1987] Northern Flicker Population Structure 655 NORTHERN FLICKER COLLECTING SITES