Fig. 1. The Alba phenotypes of representative Colias species, the evolutionary relationships among major Colias lineages in light of their Alba phenotypes and regional distribution, and evidence for historical introgression.

(A) Representative Colias species from both sides of the Atlantic, illustrating the female-limited Alba phenotype along with a table of life history differences between the female morphs. (B) A densitree plot of chromosome-level trees (one tree per chromosome), generated using gene trees based on a single exon per single-copy gene (on average 129 genes per chromosome; n = 4011 BUSCO genes). (C) Each specimen’s wing color is indicated with colored boxes on branch tips (blue = Alba, orange = colored, gray = NA). (D) ASTRAL species tree, generated using the longest exon per BUSCO gene, with branches color-coded by their sample’s regional distribution (blue = North America, orange = Holarctic, green = Eurasia and Africa, purple = South America). (E) Time-calibrated SNAPP tree generated using a subset of taxa and 1314 SNPs, with millions of years on the x axis. Blue bars at nodes represent 95% highest posterior distribution of node ages, with nodes having posterior support of <0.9 indicated with a dot and their value. (F) Distribution of minimal D-statistic of all species trios that showed significant levels of introgression (Bonferroni-Holm–corrected P < 0.05). Trios are grouped by Eurasian or North American regions and their respective combinations with the Holarctic taxa.