Introgression

Introgression is the incorporation of genetic material from one species or divergent population into another through hybridization followed by backcrossing. It can introduce donor-derived genomic segments into the recipient genome, which may be widespread or localized and can persist across generations if they are neutral, advantageous, or not strongly deleterious.

The process requires initial hybrid offspring between the donor and recipient, followed by repeated backcrossing to the recipient lineage. Over time, most of the donor genome may be purged, but segments that confer a fitness advantage or are linked to advantageous traits can be retained. Introgression is common among closely related species and populations and can occur across geographic ranges or in localized contact zones.

Detection and study rely on population-genomic data and methods that distinguish introgression from other processes such as incomplete lineage sorting. Tools include local ancestry inference and tests like the ABBA-BABA (D-statistic), as well as genome-wide association and phylogenetic approaches. Notable examples include ancient human introgression from Neanderthals and Denisovans, introgression of crop traits from wild relatives (for example, maize from teosinte, sunflower, and other crops), and adaptive exchanges in various animal groups such as Heliconius butterflies and some cichlid fishes.

Significance and applications vary. Adaptive introgression can introduce beneficial alleles for climate tolerance, disease resistance, or other traits, influencing evolution and adaptation. Conversely, introgressed DNA can be maladaptive in certain environments. In conservation and breeding, introgression is considered for preserving diversity, introducing desirable traits, or managing hybridization risks.