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.