[The hypothesis of immune testing of partners--coordinated adaptations and changes in mating preferences]

Reproductive isolation plays the key role in speciation. According to the prevailing ideas, the main speciation mechanism is gradual accumulation of genetic differences in isolated populations (allopatric phase of speciation) based on mutations, selection, and genetic drift. In this case, reproductive isolation emerges as an occasional byproduct of adaptation to different conditions (ecological speciation) or accumulation of random changes in the gene pool resulting from long-term isolation. Pure sympatric speciation assumes isolation as a direct product of selection (divergent or disruptive selection) that favors individuals selectively mating with their likes. A third possibility is substantiated below. We believe that isolation can be a regular and determined product rather than occasional byproduct of divergence. It can rely on the friend/foe discrimination mechanisms, some of which can be "immune-based" and compare the partner's and own properties (signaling molecules, pheromones, and other antigens in a broad sense). Antigens of the major histocompatibility complex (MHC) can play a substantial role in such testing of potential mates.