INTERPLAY BETWEEN HUMANS AND INFECTIVE AGENTS - A POPULATION GENETIC-STUDY

The genetic composition of present day human populations is determined largely by the interaction between the human host and infective agent s. Therefore, theoretical analysis of the host-infective-agent system is required in order for us to be able to understand human evolution. Classical population genetics has been confined largely to analysing t he interplay of various mechanisms, such as selection, mutation and dr ift, in one species at a time. Unfortunately, there have been few stud ies of such interactive systems. In the present investigation, these s tudies have been enlarged, with problems of human genetics in mind, by mathematical examination of a model in which a diploid host with thre e alleles interacts with a haploid infective agent with two alleles. T he results are compared with those obtained from simpler models analys ed in the past. The assumptions inherent in such ''gene for gene'' mod els and our results are discussed. An increase in the number of allele s appears to enhance the chances for the establishment of permanent ge netic polymorphisms, improving genetic ''elasticity'' of a population for coping with changing challenges by various infective agents. Inter action between two haploid species leads to a loss of polymorphism in both of them and, hence, to a severe loss of evolutionary elasticity. The hypothesis that the evolution of diploidy might have been favoured by a selective advantage of diploid organisms interacting with enviro nmental challenges, such as infective agents, is supported.