Figure 1.

Description of the study design and associated predictions.a) The design involves three steps. First, locus-specific genetic divergence is estimated between the allopatric populations LA and PRC. Second, locus-specific introgression is estimated in two admixed populations (HV and MR). Third, the results from the first two steps are compared. b) The two admixed populations examined are geographically closer to LA than PRC (Additional file 1: Figure S1). Thus, the environmental conditions in the admixed populations might mirror those in LA, resulting in selection against alleles from PRC (denoted by ‘X’s representing selection against black alleles). This process could result in excess LA ancestry for a given value of hybrid index, as depicted in the genomic cline figure (bottom right).