ESTIMATING THE STRENGTH OF REPRODUCTIVE ISOLATING BARRIERS

We estimated the strength of one prezygotic reproductive isolating barrier (BI or behavioral isolation) and one postzygotic reproductive isolating barrier (SH or sexual selection against hybrids) based on the social pairing data. To estimate the strength of each barrier, we used the RI index of Sobel and Chen (2014). Specifically, behavioral isolation was estimated as
\(BI=1-2\left[\frac{\text{Het}}{Con+Het}\right]\)[Eq. 1]
where Het denotes the number of heterospecific social pairs andCon denotes the number of conspecific social pairs. Behavioral isolation was only estimated relative to phenotypically pure Golden-winged and Blue-winged Warblers to evaluate the effectiveness of the plumage differences between these lineages as a prezygotic reproductive isolating barrier. Sexual selection against hybrids, which we refer to as hybrid fitness, was estimated as
\(SH=1-2\left[\frac{\text{Hyb}}{Pur+Hyb}\right]\)[Eq. 2]
where Hyb denotes the proportion of phenotypically hybrid males that formed a social pair with a female and Pur denotes the proportion of phenotypically pure males that formed a social pair.
These equations produce symmetrical values that represent the proportional reduction in gene flow relative to expectations under random mating (Sobel and Chen 2014). A slope of 2 ensures that values ofRI range from -1 to 1, with 1 denoting complete reproductive isolation, The strength of both reproductive isolating barriers was estimated for each population and 95% confidence intervals for individual RI indices were estimated using bootstrap resampling with 1000 replicates using the boot package (Canty and Ripley 2015) in R version 3.3.3.
We used the methods outlined in Coyne and Orr (1989) and Ramsey et al. (2003) to estimate the absolute contribution of each sequentially and independently acting reproductive isolating barrier (AC) to total reproductive isolation resulting from BI and SH. Because behavioral isolation acts first, ACBI = BI. The absolute contribution of sexual selection against hybrids (ACSH ) equals SH(1-ACBI ). Total reproductive isolation is the sum of the absolute contributions of behavioral isolation and sexual selection against hybrids (ACBI + ACSH ).