We provide the R code and morphological measurements used to compare body size divergence in different parts of the range of a monophyletic group of scincid lizards (Plestiodon skiltonianus species group) in western North America. Behavioral experiments have shown that successful mating among members of the group occurs more than 50% of the time if the body size difference of a male-female pair lies within 10 mm of an optimal threshold but quickly falls to ~5% once it reaches 15 mm. Beyond 15 mm, individuals tend to avoid any interaction at all. We used this information to test whether a greater proportion of heterotypic pairs drawn from inside two introgression zones fall within the ±10 mm optimum compared to pairs drawn from outside those areas. To test this hypothesis, we used rangewide specimen data from Richmond (2005) to calculate the body size difference (measured as snout-vent-length [SVL] in millimeters) of 500 randomly sampled pairs in each of four groups: (1) heterotypic pairs from the Diablo subrange of the inner Coast Ranges (ICR; Fig. 1) of central California, or introgression zone 1; (2) heterotypic pairs from the vicinity of the Kern River Valley (KRV), or introgression zone 2; (3) heterotypic pairs from areas without introgression; and (4) rangewide within the same morphotype. For heterotypic pairs, we sampled male P. gilberti and female P. skiltonianus to match the direction of mitochondrial introgression described in the paper. For group (4), the male was sampled first and then the female was randomly sampled from the same morphotype and mitochondrial clade to negate the effects of geographic variation in body size. We performed the permutation test in R (R_Core_Team 2021) using a custom script and plotted the distributions of the body size differences for each of the four groups in the paper.

We obtained the morphological data from whole skink specimens and organized it in an excel spreadsheet, which we then saved as a .cvs vile for analysis in R.