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.

杂交在自然界中十分常见，即便在‘典型’物种之间亦是如此。这一现象引出了核心问题：为何强化选择并非总能成功推动完全生殖隔离的演化？为探究这一问题，我们提出了一种全新的准连锁不平衡（quasi-linkage disequilibrium, QLD）近似方法，首次获得了针对增强对杂交个体与异种个体配偶甄别的修饰基因演化的解析结果。当此类修饰基因的效应较小时，相较于双等位基因机制，单等位基因机制下其演化过程更为顺利（依照Felsenstein 1981年的定义）。修饰基因所受的选择强度会随杂交个体频率下降而降低，因此当杂交事件足够稀少时，进一步的强化选择便无法发生。然而，当修饰基因的效应较大时，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。在此情形下，双等位基因机制中的修饰基因所受的选择强度，可与单等位基因机制下的修饰基因相当，甚至更强。此外，它们可在相当广泛的条件下实现扩散。因此，通过强化选择实现基因渐渗的完全阻断是否能够发生，或许取决于突变对配偶选择所依赖的感官系统所产生的效应大小。