HYBRID PHENOTYPES AND GENOTYPES

Following Parkes (1951), we consider two hybrid phenotypes, the Brewster’s Warbler (V. leucobronchialis , Brewster (1874)) and Lawrence’s Warbler (V. lawrencii, Herrick (1874)). Parkes described the color patterns as if they were due to two genes each having a dominant and a recessive allele. This two gene model provides a fairly accurate predictor of the pattern of phenotype inheritance (Toews et al. 2016), although it is insufficient to explain occasional intermediate phenotypes. Brewster’s Warblers are the F1 product of primary hybridization between genetically pure Golden-winged and Blue-winged Warblers, but can also result from matings of other genotypes within the Golden-winged and Blue-winged warbler complex. This phenotype is characterized by the contour plumage of a Golden-winged Warbler with a gray back and white underside coupled with the facial pattern of a Blue-winged Warbler (Fig. 1). The Lawrence’s Warbler has the body color of a Blue-winged Warbler and the facial pattern of a Golden-winged Warbler (Fig. 1). In Parkes’ model the Lawrence’s phenotype is homozygous recessive for both genes and can be produced by an F1 × F1 cross.
We created a plumage index to quantify the degree of difference among phenotypes in this complex. We compare the frequency of pairing among the phenotypes to this degree of difference, testing if more similar phenotypes pair more frequently. Our plumage index is qualitatively similar to existing plumage indices (Gill 1980, Toews et al. 2016), for which we scored 11 plumage patches on males and females of each phenotype (Appendix 1). Information on plumage in Pyle (1997) and Bent (1953) were used to generate plumage scores for our plumage index.
While there is a strong correlation between phenotype and genotype of individuals in this system (Toews et al. 2016), it is important to note that some phenotypically “pure” individuals show signs of introgression in their genetic background (Debrosky et al. 2005, Vallender et al. 2009, Wood et al. 2016). The presence of these “cryptic hybrids” will inflate our estimates of reproductive isolation (see below), and overestimate the reduction in gene flow due to a given barrier. Nonetheless, assortative mating by plumage phenotype and/or sexual selection against males with intermediate phenotypes would still act to reduce gene flow between lineages, thus promoting speciation. The main goal of this study was to determine whether there is non-random mating based on these phenotypic differences, and thus whether patterns of mating in the field are consistent with patterns of genomic divergence primarily in regions related to plumage development (Toews et al. 2016). We note that an imperfect relationship between phenotype and genotype is precisely what is expected in systems that are in the early stages of speciation (Dobzhansky 1958, Roux et al. 2016), and thus not unique to Golden-winged and Blue-winged Warblers.