Whatever one's definition of species, it is generally expected that individuals of the same species should be genetically more similar to each other than they are to individuals of another species. Here we show that in the presence of cross-species gene flow, this expectation may be incorrect. We use the multispecies coalescent model with continuous-time migration or episodic introgression to study the impact of gene flow on genetic differences within and between species and highlight a surprising but plausible scenario in which different population sizes and asymmetrical migration rates cause a genetic sequence to be on average more closely related to a sequence from another species than to a sequence from the same species. Our results highlight the extraordinary impact that even a small amount of gene flow may have on the genetic history of the species. We suggest that contrasting long-term migration rate and short-term hybridization rate, both of which can be estimated using genetic data, may be a powerful approach to detecting the presence of reproductive barriers and to define species boundaries.

中文翻译：

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当有基因流动时定义物种

无论对物种的定义如何，通常都期望同一物种的个体在遗传上应该比其他物种的个体在遗传上更相似。在这里我们表明，在存在跨物种基因流的情况下，这种期望可能是不正确的。我们使用具有连续时间迁移或间歇性渗入的多物种聚结模型来研究基因流动对物种内部和物种之间遗传差异的影响，并突出了一个令人惊讶但合理的场景，其中不同的种群规模和不对称的迁移率导致基因序列为平均而言，与来自另一个物种的序列的关系比与来自同一物种的序列的关系更密切。我们的结果强调了即使是少量的基因流动也可能对物种的遗传历史产生非凡的影响。我们建议对比长期迁移率和短期杂交率，这两者都可以使用遗传数据估计，可能是检测生殖障碍存在和定义物种边界的有力方法。