Introgression

Introgression is the transfer of genetic material from one species into the gene pool of another through repeated backcrossing of hybrid offspring with one of the parental species. Unlike occasional hybridization, which may produce sterile or unfit offspring, introgression results in the stable incorporation of alleles from one species into another — a one-way gene flow across the species boundary.

The classic view held species as reproductively isolated units with impermeable boundaries. Genomic studies have demolished this view: introgression is common, particularly in rapidly radiating groups such as Heliconius butterflies, Darwin's finches, and even between Neanderthals and anatomically modern humans. The transfer of immune-related genes from Neanderthals into human populations is among the clearest documented cases of adaptive introgression.

Introgression is not random. Genomic regions involved in reproductive isolation resist introgression because hybrids carrying them are selected against; regions conferring local advantage introgress readily because selection favors their spread. The genome is therefore a mosaic of introgression histories — some regions species-specific, others shared across species boundaries.

The biological species concept cannot survive the genomic era without becoming either trivial or false. If introgression is common, species are not isolated gene pools but partially connected nodes in a reticulate network. The question is no longer whether species exchange genes but which regions of the genome participate in the exchange — and what determines their participation.