Data from: Can reinforcement complete speciation?

Hybridization is common in nature, even between "good" species. This observation poses the question of why reinforcement is not always successful in leading to the evolution of complete reproductive isolation. To study this question, we developed a new "quasi-linkage disequilibrium" (QLD) approximation to obtain the first analytic results for the evolution of modifiers that increase mate discrimination against hybrids and heterospecifics. When such modifiers have small effects, they evolve more readily under a one-allele than a two-allele mechanism (sensu Felsenstein 1981). The strength of selection on the modifier decreases as hybrids decrease in frequency, and so further reinforcement may not occur once hybridization is sufficiently rare. The outcome is qualitatively different when modifiers have large effects, however, for example when a single mutation can cause complete reproductive isolation. In this case modifiers in a two-allele mechanism can be selected as or more strongly than those in a one-allele mechanism. Further, they can spread under quite general conditions. Thus whether complete closure of genetic introgression by reinforcement occurs may depend on the size of effects that mutations have on the sensory systems used in mate choice.杂交在自然界中极为普遍，即便在“优良物种”（good species）之间也会发生。这一现象引出了核心问题：为何强化作用（reinforcement）并非总能成功推动完全生殖隔离（reproductive isolation）的演化？为解答该问题，我们提出了一种全新的“拟连锁不平衡（quasi-linkage disequilibrium, QLD）”近似方法，首次获得了针对增强对杂种与异种个体交配识别能力的修饰因子演化的解析结果。当此类修饰因子的效应较小时，其演化在单等位基因机制下比双等位基因机制下更为容易（参照Felsenstein 1981年的定义）。修饰因子所受选择的强度会随杂种频率的降低而减弱，因此当杂交事件足够稀少时，进一步的强化作用可能无法发生。然而，当修饰因子的效应较大时，演化结果会出现质的差异——例如当单个突变即可引发完全生殖隔离时。在此类情形下，双等位基因机制中的修饰因子所受选择强度可与单等位基因机制中的修饰因子相当，甚至更强。此外，此类修饰因子可在相当广泛的条件下完成扩散。由此可见，通过强化作用实现遗传渐渗（genetic introgression）的完全阻断是否可行，或许取决于突变对交配选择所用感官系统的效应大小。