What is introgression?
Introgression is the transfer of alleles across species or subspecies boundaries. In other words, it describes gene flow between populations that are partially isolated. For archaic humans, there is no test of the strength or permeability of boundaries between populations; it is common to use the term “introgression” to describe gene flow in such situations, even if such gene flow is fairly common. The paper by Evans and colleagues describes a scenario of adaptive introgression. In such cases, an allele with a selective advantage moves from one population to another.
