Speciation in Stickleback Facilitated by Admixture – Where is the Evidence?

Where genetic variation promoting speciation originates is a crucial question in evolutionary genomics. In a recent article, Marques et al. (2019) seek to address this question in lake and stream threespine stickleback fish from the Lake Constance (hereafter LC) basin in Central Europe. Based on population genetic methods, they conclude that incipient speciation between lake and stream stickleback was facilitated by the mixing of genetic variation from old lineages evolved in isolation (i.e., admixture following secondary contact). In this comment, I discuss conceptual and methodological problems and unrecognized conflicts with existing evidence that cast doubt on Marques et al.’s conclusion.