Biodiversity and the Parasite-Driven Wedge

Abstract

The parasite-stress theory of sociality includes a theory of biodiversity: the parasite-driven-wedge model. Regionally localized coevolutionary races between parasites and their hosts result in three anti-parasite behaviors: preference for in-group affiliation and interaction, out-group avoidance (xenophobia), and philopatry. These three behaviors of behavioral immunity become linked within individuals through genetic linkage disequilibrium. In the case of human cultural behavioral immunity, within-individual linkage of behavioral immunity traits results in what we refer to as cultural linkage disequilibrium. Linkage by either process also includes linkage with genetic immunity to local parasites. 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 in-group members with behavioral-immunity values and behavior is self-reinforcing, because the in-group members preferred and favored have the same preference. These events create a wedge that gives rise to intergroup boundaries that effectively fractionate, locally isolate, and diversify the original range of a culture or a species, leading to the genesis of two or more discrete groups from one. The higher the parasite stress in a region, the greater the frequency and intensity of these processes of biodiversity genesis. The parasite-driven-wedge model, then, provides a parapatric (side-by-side) diversification mechanism that we propose accounts for the high diversity of species and cultures in geographical regions of high parasite adversity. Parasite-driven divergence may lead to sympatric speciation, especially at low latitudes, and account for distinct sympatric classes of caste social systems.