Mallet James, Mullen Sean P
Department of Organismic and Evolutionary Biology, Harvard University, Cambridge, Massachusetts, USA.
Department of Biology, Boston University, Boston, Massachusetts, USA.
J Evol Biol. 2022 Sep;35(9):1175-1182. doi: 10.1111/jeb.14052.
Reproductive isolation is the heuristic basis of the biological species concept, but what is it? Westram et al. (this issue) propose that it is a measurable quantity, "barrier strength," that prevents gene flow among populations. However, their attempt to make the concept of reproductive isolation more scientific is unlikely to satisfy the diverse opinions of all evolutionary biologists. There are many different opinions about the nature of species, even under the biological species concept. Complete reproductive isolation, where gene flow is effectively zero, is regarded by some biologists as an important end point of speciation. Others, including Westram et al., argue for a more nuanced approach, and they also suggest that reproductive isolation may differ in different parts of the genome due to variation in genetic linkage to divergently selected loci. In contrast to both these approaches, we favour as a key criterion of speciation the stable coexistence of divergent populations in sympatry. Obviously, such populations must be reproductively isolated in some sense, but neither the fraction of the genome that is exchanged, nor measures of overall barrier strength acting on neutral variation will yield very precise predictions as to species status. Although an overall measure of reproductive isolation is virtually unattainable for these reasons, its early generation components, such as assortative mating, divergent selection, or hybrid inviability and sterility are readily measurable and remain informative. For example, we can make the prediction that to remain divergent in sympatry, almost all sexual species will require strong assortative mating, as well as some sort of ecological or intrinsic selection against hybrids and introgressed variants.
生殖隔离是生物物种概念的启发式基础，但它究竟是什么呢？韦斯特拉姆等人（本期）提出，它是一个可测量的量，即“隔离强度”，可阻止种群间的基因流动。然而，他们使生殖隔离概念更具科学性的尝试不太可能满足所有进化生物学家的不同观点。即使在生物物种概念下，对于物种的本质也存在许多不同的观点。完全生殖隔离，即基因流动实际上为零，被一些生物学家视为物种形成的一个重要终点。包括韦斯特拉姆等人在内的其他人则主张采用更细致入微的方法，他们还指出，由于与分歧选择位点的遗传连锁变化，生殖隔离在基因组的不同部分可能会有所不同。与这两种方法不同，我们支持将同域分布的分歧种群的稳定共存作为物种形成的关键标准。显然，这样的种群在某种意义上必须是生殖隔离的，但无论是交换的基因组部分，还是作用于中性变异的整体隔离强度衡量指标，都无法对物种状态做出非常精确的预测。尽管由于这些原因几乎无法获得生殖隔离的整体衡量指标，但其早期阶段的组成部分，如选型交配、分歧选择或杂种不存活和不育等，很容易测量且仍然具有参考价值。例如，我们可以预测，为了在同域中保持分歧，几乎所有有性物种都需要强烈的选型交配，以及某种针对杂种和渗入变体的生态或内在选择。
