Figure 2: Schematic presentation of models of parasite-mediated speciation. (a) Reproductive isolation due to reduced viability or fecundity of immigrants and hybrids. (1) Immigrants from host populations (a) (dark grey) and (b) (light grey) suffer higher infection levels in the habitat of the other population resulting in reduced survival or fecundity. (2) Hybrids (middle grey) between divergently adapted parent populations (a) and (b) have higher infection levels and reduced survival or fecundity in either of the parental habitats as their intermediate defence profiles cannot match with the parasite pressure of the parental habitats. (b) Reproductive isolation due to pleiotropic effects of MHC on mate choice. Divergence of parasite infections between host populations (indicated by darker and lighter grey background) with initially similar MHC profiles leads to divergent adaptation in MHC profiles to the particular infection conditions (dark grey and light grey). Reproductive isolation between the populations increases in the course of the process through the pleiotropic effects of MHC on mate choice. (c) Ecologically based sexual selection. Two host populations that differ in parasite infections because of habitat or diet, diverge in their use of mating cues because different cues better signal heritable resistance to the different infections (here red and blue). Initially they are weakly reproductively isolated with frequent occurrence of hybrid individuals (purple). Sexual selection for individuals that better resist parasites in a given environment (bright blue and red) over more heavily infected individuals (pale blue and red) facilitates divergent adaptation and results in reproductive isolation between the populations.