Symbiosis Without Mutualism and the Merger of Lineages in Evolution [Revised 3 June 1998]

A model for the coevolution of two species in facultative symbiosis is used to investigate conditions under which species merge to form a single reproductive unit. Two traits evolve in each species, the first affecting loss of resources from an individual to its partner, and the second affecting vertical transmission of the symbiosis from one generation to the next. Initial conditions are set so that the symbiosis is not mutualistic and vertical transmission is very rare. It is shown that a stable symbiotic unit with maximum vertical transmission of the partners can evolve in the face of continued exploitation of one partner by the other. Such evolution requires that eventually deaths should exceed births for both species in the free-living state, a condition which can be met if the victim, in the course of developing its defenses, builds up sufficiently large costs in the free-living state. This result expands the set of initial conditions from which the separate lineages can be expected to merge into symbiotic units, and argues against any automatic assumption of mutualism between organism with a long history of symbiosis.

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