Fig. 1.

Premating, conspecific sperm precedence, noncompetitive gametic isolation, and postzygotic isolation increase with pairwise genetic distance between species. Proxies of premating isolation (red), conspecific sperm precedence (CSP, yellow), noncompetitive gametic isolation (NCGI, green), and postzygotic isolation (blue) were regressed against phylogenetic distance (K_s between species and π_s within species). The four types of isolation increase with genetic distance, and premating isolation evolves faster than hybrid inviability. The thick red, yellow, green, and blue lines represent fitted logistic regressions for the premating, and postzygotic data, respectively. The thinner lines of each of the four colors are the regressions for each of 10,000 bootstrap resamplings of the data.