Figure 2.

Hybrid zones are defined as geographic regions of genetic mixing between divergent populations. Hybrid indices and admixture coefficients tell us about the proportion of an individual's genome inherited from one or the other parental species. In this example, the same individual ancestry proportions are represented in (a–d) ordered by the proportion of ancestry belonging to parental species A and in (e–h) as a frequency distribution. This distribution in a hybrid zone may reflect the strength and direction of reproductive barriers. In strongly bimodal hybrid zones (a, e) few hybrids are present and pure parental genotypes dominate, suggesting a strong barrier with very low levels of gene exchange. By contrast, for unimodal hybrid zones (b, f) the distribution spans a range of admixture and backcrosses toward both parents, which suggests weaker barriers and high levels of gene exchange. In trimodal hybrid zones (c, g) high frequencies of F₁s and limited backcrosses may suggest F₁ sterility and/or heterosis. Highly skewed distributions with backcrossing predominantly to one parental type (d, h) may reflect the direction of gene flow and imply asymmetrical strength in reproductive barriers.