<p>Comparison of introgression and strong contemporary barriers between the control and heat-selection treatments.</p>

<p><b>(A)</b> Mean hybrid index (HI), representing the amount of introgression from the more heat-tolerant <i>D. montana</i>, between the control and heat-selection treatments. Differences between the respective heat-selection and control replicate lines are shown in <a href="plosbiology.org/article/info:doi/10.1371/journal.pbio.3004028#pbio.3004028.s008" target="_blank">S2B Fig</a>. <b>(B)</b> Variability in mean HI, calculated as the absolute deviation from the mean, between the control and heat-selection treatment. <b>(C)</b> Proportion of SNPs with no introgression from <i>D. montana</i> (HI = 0) in any of the six control or six heat-selection lines, indicating strong contemporary barriers.

Each metric was calculated separately for colinear (COL) and inverted (INV) autosomal (Auto) and X chromosomal regions. Black stars indicate statistically significant differences between the control and heat-selection lines (GLMM: ***<i>P</i> < 0.001). The data underlying this figure can be found at <a href="doi.org/10.5281/zenodo.22277897" target="_blank">doi.org/10.5281/zenodo.22277897</a>.</p>