Commensal pathogens as a source of a coexistence mechanism

Most known organisms carry commensal viruses or bacteria. These parasites are often treated as an inevitable nuisance. We here show that they may be essential for the survival of the host species, and may actually be the force driving speciation. Viruses that do not hurt their natural host but are deadly for other species have been argued to facilitate invasion. We show using a generic SIR model that the opposite may be the general case. Such viruses may be the force sustaining multiple distinct populations through spatial segregation, in the absence of physical barriers. This segregation protects the hosts against invasion by neighboring, possibly more fit, populations. The virus induced segregation can eventually lead to allopatric speciation, with no animal dispersal, geographical changes or human activities. We further propose a speculative mechanism, where the introduction of a new virus to a population with a heterogeneous response (based for example on the MHC polymorphism) may lead to the segregation of distinct sub-populations reacting to different strains of the virus. The existence of such a mechanism will require further experimental validation.