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 ).