The parasite-driven-wedge model provides a mechanism of parapatric speciation (the evolution of adjacent species across the range of an ancestral species without allopatric separation). Regionally localized coevolutionary races between parasites and their hosts result in three locally adaptive antiparasite behaviors: mating and other social preference for local conspecifics, avoidance of nonlocal conspecifics and philopatry (limited dispersal). These three behaviors comprise behavioral immunity. They become linked within individuals through linkage disequilibrium. Genetic immunity to local parasites also links through the same genetic mechanism with the traits of behavioral immunity. These linked traits are mutually reinforcing in that as any one increases in frequency due to its adaptiveness, the others do as well. Also, preference for locals is self-reinforcing because both the locals preferred and those preferring them have the same preference. These events create a wedge and associated boundaries that effectively fractionate and diversify the original range of a species, leading to the genesis of contiguous multiple species from one. The higher the parasite stress in a region, the greater the frequency and intensity of the parasite-driven wedge in splitting species. We do not deny an important role for allopatric speciation, but argue that parasite-driven parapatric processes will be relatively predominant in regions of high parasite adversity (e.g. low latitudes), leading to the high diversity of species in the regions. The fractionation of host populations through the parasite-driven wedge also diversifies parasites, leading to even greater geographic localization of host–parasite races. Methods are discussed for empirically distinguishing parasite-driven parapatric speciation and allopatric speciation. We hypothesize that host coevolution with beneficial symbionts may also be an engine of parapatric speciation by way of a wedge-like process.
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