Background Information
The white-throated sparrow (Zonotrichia albicollis) is a socially monogamous passerine that breeds throughout the northeastern United States and Canada. This species is polymorphic, and both sexes can be separated into tan and white morphs based on the color of the median crown stripe (Lowther 1961; Vardy 1971; Atkinson & Ralph 1980; Watt 1986; Piper & Wiley 1989b). The polymorphism does not correlate with geographic factors, and morphic proportions have remained stable for over 100 years in thecentral region of Canada. It is therefore reasonable to assume that the polymorphism has attained equilibrium.
The plumage polymorphism correlates with a chromosomal polymorphism resulting from a pericentric inversion of the second chromosome. White birds are heterozygous for the inversion (i.e., 2m/2, where 2m represents the inverted chromosome and 2 represents the non-inverted form) whereas, tan birds are homozygous non-carriers (i.e. 2/2) (Thorneycroft 1966, 1975). In a survey of 400 white-throated sparrows Thorneycroft (1975) found only 1 female homozygous for the inversion and no homozygous males. The autosomal inversion segregates in a Mendelian fashion, so that birds possessing a single 2m pass this chromosome to 1/2 of their progeny (Thorneycroft 1975).
The polymorphism is also correlated with behavioral differences. White birds are more aggressive than tan birds (Hailman 1975; Ficken et al. 1978; Watt et al. 1984; Kopachena & Falls 1993a; Tuttle, unpublished data), and they initiate more aggressive attacks (Ficken et al. 1978; Kopachena & Falls 1993a); yet both morphs are equi-probable recipients of aggression (Ficken et al. 1978). It is unclear if aggression translates into dominance, since white birds are not always dominant to tan (Watt et al. 1984; Piper & Wiley 1989b). White males sing more than tan males (Lowther & Falls 1968; Falls 1988; Tuttle 1993, 1997 ms.a) and are more likely to mate polygynously (Knapton & Falls 1983; Tuttle 1993, 1997 ms.a). Tan birds expend more effort in parental care than white birds (Knapton & Falls 1983; Whillans & Falls 1990; Kopachena & Falls 1993b).
White-throated sparrows are unusual in that they mate disassortatively with respect to the polymorphism, so that white almost always mates with tan (Lowther 1961; Thorneycroft 1975; Tuttle 1993). Disassortative mating ensures that both morphs are maintained in relatively equal proportions in the population, since, when white (2m/2) mates with tan (2/2), Mendelian genetics ensures that 1/2 of the brood will be 2m/2 and the other half will be 2/2. Disassortative mating is sufficient within itself to maintain balanced polymorphism in this species (Thorneycroft 1975). Therefore, the key to understanding the adaptive significance of the white-throated sparrow polymorphism lies in determining the evolutionary causes and consequences of disassortative mating.
