Minor genes can delay evolution of major resistance

A model was used to study natural selection acting upon a population possessing both a common, slightly adaptive allele (i.e. minor gene) and a much rarer, yet highly adaptive allele at a second locus (i.e. major gene). As expected, the population evolves resistance to the toxin imposing high selection intensity. The interesting aspect is that a single resistance gene contributing less than 10% of the fitness provided by the major resistance gene and as little as 1-5% additional survival relative to susceptible homozygotes can significantly delay the evolution of resistance measured either as an increase in mean population fitness or the allele frequency of the major resistance allele. This occurs when the minor allele is dominant to partially dominant and the initial, pre-selection, minor allele frequency is greater than that of the major allele. Thus, the minor allele will evolve faster and actually delay evolution of the major allele.