Coevolutionary negative frequency-dependent selection has been hypothesized to maintain genetic variation in host and parasites. Despite the extensive literature pertaining to host–parasite coevolution, the temporal dynamics of genetic variation have not been examined in a matching-alleles model (MAM) with a finite population size relative to the expectation under neutral genetic drift alone. The dynamics of the MA coevolution in an infinite population, in fact, suggests that genetic variation in these coevolving populations behaves neutrally. By comparing host heterozygosity to the expectation in a single-species model of neutral genetic drift we find that while this is also largely true in finite populations two additional phenomena arise. First, reciprocal natural selection acting on stochastic perturbations in host and pathogen allele frequencies results in a slight increase or decrease in genetic variation depending on the parameter conditions. Second, following the fixation of an allele in the parasite, selection in the MAM becomes directional, which then rapidly erodes genetic variation in the host. Hence, rather than maintain it, we find that, on average, matching-alleles coevolution depletes genetic variation.

假设协同进化的负频率依赖性选择可以维持宿主和寄生虫的遗传变异。尽管有大量关于宿主-寄生虫协同进化的文献，但是在具有有限种群规模的匹配等位基因模型（MAM）中，相对于仅在中性遗传漂移下的预期，尚未研究遗传变异的时间动态。实际上，无限数量群体中MA协同进化的动力学表明，这些共同进化群体中的遗传变异行为是中性的。通过将宿主杂合性与中性遗传漂移的单物种模型中的期望值进行比较，我们发现，尽管在有限种群中这也是正确的，但还会出现另外两种现象。第一，交互自然选择作用于宿主和病原体等位基因频率的随机扰动，导致遗传变异略有增加或减少，具体取决于参数条件。其次，在将等位基因固定在寄生虫中之后，MAM中的选择将变成定向的，然后迅速侵蚀宿主中的遗传变异。因此，我们发现而不是维持它，平均而言，匹配等位基因的共同进化会耗尽遗传变异。