Selection of Intermediate Rates of Increase in Parasite-Host Systems

Abstract

On theoretical grounds, we argue that parasite-host systems are ideal candidates for interdemic or "group" selection because of the potential for selection for avirulence based upon reduced host (hence group) survival. Such selection appears to have been an important ingredient in the stabilization of the myxomatosis-rabbit system in Australia, although clearly the evolution of resistance in the rabbit population also played a part. We present a simple mathematical model to demonstrate how easily group selection can (in theory) stabilize a parasite-host system. This model is not meant to be a literal translation of the myxomatosis-rabbit interaction, and in fact intentionally disregards host evolution; its purpose is to isolate the role of group selection in the parasite population. Fenner has established that both interdemic selection and host evolution were important in the stabilization of that system, although our model demonstrates that in theory interdemic selection alone could stabilize the interaction. Our central point is that selection for avirulence in the parasite population is important in the evolution of stability in parasite-host systems (Fenner and Myers 1978). The obvious next step, both from the general theoretical viewpoint and specifically with regard to the myxoma-rabbit system, will be to consider models in which both host and parasite are permitted to coevolve. We feel, as do many others, that modeling of coevolution of parasite-host systems has not received the attention deserved in theoretical ecology, and that because of the interdependence of the elements these associations represent much better candidates for such theory than the more popular predator-prey or competition systems. Moreover, as Donald R. Strong, Jr., has pointed out to us, a broad range of organisms are functionally parasitic and the ideas that we have developed herein apply to these as well as to the more restricted class of species which are the conventional organisms treated in parasitology.