Speciation in Sapsuckers (Sphyrapicus): I. Genetic Differentiation

--We report the results of an electrophoretic analysis at 39 presumptive genetic loci of 88 specimens in the picid genus Sphyrapicus, here treated as comprising the following species: Williamson's Sapsucker (S. thyroideus), Yellow-bellied Sapsucker (S. varius), Red-naped Sapsucker (S. nuchalis), and Red-breasted Sapsucker (S. ruber). Seventeen loci (43.5%) were polymorphic. Values for observed heterozygosity in S. tuber tuber, S. tuber daggetti, and in three populations of S. nuchalis were uniform and averaged 0.043, the mean value reported for other birds. However, S. varius, at 0.022, and S. thyroideus, at 0.016, have low values of H. Values for percentage of polymorphic loci ranged from 12.8 to 20.5 in S. tuber and S. nuchalis, 12.8 in S. varius, and 7.7 in S. thyroideus. In contrast, all four species exhibited a similar mean number of alleles per locus (1.12-1.21). Genetic distances between populations of the same species are either very low or zero. Genetic distances are also very low between the phenotypically dissimilar tuber and nuchalis (I• = 0.004), two forms we consider to be biological species based on their assortative mating in sympatry. This is the lowest avian interspecific D-value ever reported and is similar to that found between avian subspecies. Phenotypically, varius and nuchalis are very similar, yet the average genetic distance between samples of these forms, 0.029, is comparable to interspecific values reported for other avian congeners and is our chief basis for treating the two forms as separate species. S. thyroideus is strongly differentiated genetically from varius (O = 0.142), from nuchalis (O = 0.197), and from tuber (I• = 0.186). Fsr statistics point to little or no population subdivision within tuber and nuchalis but substantial subdivision within the superspecies varius (S. varius + S. nuchalis + S. tuber). The genetic information suggests that the Williamson's Sapsucker epresents the oldest lineage in the genus. S. thyroideus plit from its sister clade (superspecies S. varius) between 3 and 4 MYBP. S. tuber and S. nuchalis are sister species that evolved very recently, either during or since the Pleistocene and sometime after their common ancestor diverged from S. varius. Received 3 January 1983, accepted 4 May 1983. THE recent upswing in the number of avian electrophoretic studies has included few titles by authors (e.g. Martin and Selander 1975, Barrowclough 1980) concerned specifically with taxa known to hybridize, despite the potential of this approach for clarifying the genetic aspects of hybridization and the speciation process. In the present paper, we offer the results of an electrophoretic analysis of the major taxa of North American sapsuckers (genus Sphyrapicus), specialized woodpeckers that illustrate prominent patterns of geographic variation and hybridization. Many past publications (Howell 1952, A.O.U. 1957) have treated S. varius, nuchalis, and ruber, three basically allopatric forms, as geographic representatives of a single species, the Yellow-bellied Sapsucker (S. varius). We regard these forms as specifically distinct, however, and consider the genus to comprise the following four species: Williamson's Sapsucker (S. thyroideus), Yellow-bellied Sapsucker (S. vat871 ius), Red-naped Sapsucker (S. nuchalis), and Redbreasted Sapsucker (S. ruber). This treatment follows Mayr and Short (1970), Short and Morony (1970) and Short (1982). Our view that nuchaIis is specifically distinct from ruber is based on a detailed field study (Johnson and Johnson in prep.) in south-central Oregon and northeastern California, where these two forms nest in local sympatry and mate assortatively, with infrequent hybridization. The opinion that nuchaIis is a distinct species is supported here by analysis of allozymic variation. The four species of sapsuckers are similar in size but fall into three categories of coloration (Fig. 1). S. thyroideus, in which the male is strongly melanic and the female barred, is the most divergent in plumage. S. ruber is basically sexually monomorphic, with the striking red of the head and upper breast superimposed over subtle whitish lateral head stripes and a faint dusky breast band resulting from melanic The Auk 100: 871-884. October 1983 872 JOHNSON AND /INK [Auk, Vol. 100 Fig. 1. Lateral aspect of head and breast plumage of males and females of four species of Sphyrapicus. feather bases under the red tips. S. varius and S. nuchalis are extremely similar in ventral and lateral plumage, the only significant difference being in the females. In the former species the female has a white throat whereas in nuchalis the throat patch of the female is basally red with a varying amount of white near the base of the bill. Dorsally, the head patterning of the Yellow-bellied Sapsucker and Red-naped Sapsucker is different in both sexes. In S. varius, a well-defined white mark begins behind the eyes and encircles the nape. In S. nuchaIis, a subtle white mark remains on the feather bases of the nape, but the tips of the feather barbs are red, resulting in a crimson nuchal patch. The dorsum of S. varius is also much whiter than that of S. nuchalis. In addition to delimiting species' boundaries, the electrophoretic data also throw light on the phylogenetic relationships of sapsuckers, a topic discussed by both Howell (1952) and Short and Morony (1970), who had no access to quantitative genetic information bearing on phylogenetic estimation. Finally, values here reported for heterozygosity, percentage polymorphism, number of alleles per locus, and population structure (genetic distances and F-statistics) add to the growing body of genetic information on populations of birds. MATERIALS AND METHODS This analysis is based on 88 specimens of Sphyrapicus (Table 1). A list of collecting localities can be October 1983] Speciation i Sapsuckers 873 obtained from the authors. Data from two S. thyroideus nataliae from Montana were combined with those from S. thyroideus thyroideus (n = 16) from Oregon and California after initial analysis showed the results to be indistinguishable. Samples of heart, liver, kidney, and pectoral muscle were removed from specimens 1-4 h after collection in the field and placed in liquid nitrogen before permanent storage at -76oC in Berkeley. Tissue extracts were prepared for electrophoresis as described by Selander et al. (1971) and Zink (1982). Thirty-nine presumptive genetic loci were scored in this study. Electrophoretic procedures (i.e. gel and buffer combinations) essentially follow Yang and Patton (1981). We have basically used the "one-pass" approach (Aquadro and Avise 1982), although many of the loci have been scored on more than one gel type. Precise conditions are available from the authors. The most common allele at a locus was designated M, and alleles with more anodal or cathodal migrations were coded as F or S, respectively; pluses and minuses were used to code further different allelic states. The distribution of observed and expected numbers of heterozygotes, over all loci in a sample, was tested for departure from Hardy-Weinberg expectation (I-Iartl 1981) with a x2-test (see Barrowclough 1980). Individual heterozygosity was calculated by dividing the number of loci at which an individual was heterozygous by the total number of loci scored (39 for all birds). Two measures of average individual heterozygosity per population sample were calculated: first, the mean (nSD) of the observed individual heterozygosities (Hob•) and, second, the heterozygosity expected under conditions of Hardy-Weinberg equilibrium (i.e. Hex, = 1• x, 2, where x, is the frequency of the ith allele summed over k alleles at a locus). Individual genotypes were converted to allelic frequencies (Appendix) for each sample. Genetic distances between samples were calculated using the methods of Nei (1978) and Rogers (1972). Hierarchical F-statistics, which measure population subdivision in the absence of selection, were calculated according to Wright (1978). Branching diagrams, summarizing the pattern of genic similarity among samples, were constructed using the methods of Sneath and Sokal (1973; UPGMA and WPGMA phenograms), Farris [1972; distance Wagner networks, optimized according to Swofford (1981)], and Fitch and Margoliash (1967; "F-M" trees).