SYMBIOSIS THROUGH EXPLOITATION AND THE MERGER OF LINEAGES IN EVOLUTION

A model for the coevolution of two species in facultative symbiosis is used to investigate conditions under which species merge to form a si ngle reproductive unit. Two traits evolve in each species, the first a ffecting loss of resources from an individual to its partner, and the second affecting vertical transmission of the symbiosis from one gener ation to the next. Initial conditions are set so that the symbiosis in volves exploitation of one partner by the other and vertical transmiss ion is very rare. It is shown that, even in the face of continuing exp loitation, a stable symbiotic unit can evolve with maximum vertical tr ansmission of the partners. Such evolution requires that eventually de aths 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 defences, builds up sufficiently large costs in the free-living s tate. This result expands the set of initial conditions from which sep arate lineages can be expected to merge into symbiotic units.