EVOLUTION OF MUTUALISTIC SYMBIOSIS - A DIFFERENTIAL-EQUATION MODEL

In geological history, rapid speciation, called adaptive radiation, ha s occurred repeatedly. The origins of such newly developing taxa often evolved from the symbiosis of different species. Mutualistic symbiose s are generally considered to evolve from parasitic relationships. As well as the previous model of host population with discrete generation s, a differential equation model of host population with overlapping g enerations shows that vertical transmission, defined as the direct tra nsfer of infection from a parent host to its progeny, is an important factor which can stimulate reduction of parasite virulence. Evolution of the vertical transmission rate from both points of view, the parasi te and the host, is analyzed. There is a critical level of the rate, b elow which an evolutionary conflict arises (the parasite would want an increase in the rate while the host would not), and above which both species would correspond to increase the rate. Therefore, once the par asite dominates the evolutionary race so as to overcome this critical level, one-way evolution begins toward a highly mutualistic relationsh ip with a high vertical transmission rate, possibly creating a new org anism through symbiosis with perfect vertical transmission. Changes in other parameters may decrease the critical level, initiating one-way evolution. However, changes in traits, probably developed through a lo ng interrelationship in parasitism, do not necessarily induce the evol ution of mutualism. Establishment of the ability to make use of metabo lic and digestive wastes from the partner certainly facilitates the ev olution of mutualism, while improvements in reproductive efficiency of parasites and reduction of negative effects from exploitation in host s on the contrary disturb mutualism.