Data from: On the Coyne and Orr-igin of species: effects of intrinsic postzygotic isolation, ecological differentiation, X-chromosome size, and sympatry on Drosophila speciation

Coyne and Orr found that mating discrimination (premating isolation) evolves much faster between sympatric than allopatric Drosophila species pairs. Their meta-analyses established that this pattern, expected under reinforcement, is common and that Haldane's rule is ubiquitous in Drosophila species divergence. We examine three possible contributors to the reinforcement pattern: intrinsic postzygotic isolation, dichotomized as to whether hybrid males show complete inviability/sterility; host-plant divergence, as a surrogate for extrinsic postzygotic isolation; and X chromosome size, whether roughly 20% or 40% of the genome is X-linked. We focus on “young” species pairs with overlapping ranges, contrasted with allopatric pairs. Using alternative criteria for “sympatry” and tests that compare either level of prezygotic isolation in sympatry or frequency of sympatry, we find no statistically significant effects associated with X chromosome size or our coarse quantifications of intrinsic postzygotic isolation or ecological differentiation. Although sympatric speciation seems very rare in animals, the pervasiveness of the reinforcement pattern and the commonness of range overlap for close relatives indicate that speciation in Drosophila is often not purely allopatric. It remains to determine whether increased premating isolation with sympatry results from secondary contact versus parapatric speciation and what drives this pattern.

科因（Coyne）与奥尔（Orr）研究发现，同域分布（sympatric）的果蝇属（Drosophila）物种种对之间的交配识别（交配前生殖隔离，premating isolation）演化速率远快于异域分布（allopatric）的物种种对。他们的元分析（meta-analyses）证实，这种符合生殖强化（reinforcement）理论预期的模式普遍存在，且霍尔丹法则（Haldane's rule）在果蝇物种分化中无处不在。本研究探讨了可能影响生殖强化模式的三类因素：一是内在合子后生殖隔离（intrinsic postzygotic isolation），以杂交雄性是否完全致死或不育作为二分划分标准；二是宿主植物分化，作为外在合子后生殖隔离（extrinsic postzygotic isolation）的替代指标；三是X染色体大小，即基因组中约20%或40%的序列为X连锁（X-linked）。我们聚焦于分布区重叠的“年轻”物种种对，并与异域分布的物种种对进行对比。通过采用“同域分布”的不同判定标准，以及分别比较同域种群间交配前生殖隔离水平或同域分布频率的检验方法，我们未发现与X染色体大小、我们对内在合子后生殖隔离或生态分化的粗略量化结果相关的统计学显著效应。尽管同域物种形成（sympatric speciation）在动物界中似乎极为罕见，但生殖强化模式的普遍性以及近缘类群间分布区重叠的常见性，表明果蝇的物种形成往往并非完全异域式的。仍有待阐明的是，同域分布下交配前生殖隔离的增强究竟源于次生接触（secondary contact）还是邻域物种形成（parapatric speciation），以及究竟是什么因素驱动了这一模式。