Assortative mating in hybrid zones is remarkably ineffective in promoting speciation

<b>Abstract</b><br/><p>Partial prezygotic isolation is often viewed as more important than partial postzygotic isolation (low fitness of hybrids) early in the process of speciation. I simulate secondary contact between two populations (‘species’) to examine effects of assortative mating and low hybrid fitness in preventing blending. A small reduction in hybrid fitness (e.g., by 10%) produces a narrower hybrid zone than a strong but imperfect mating preference (e.g., 10x stronger preference for conspecific over heterospecific mates). In the latter case, rare F1 hybrids find each other attractive (due to assortative mating), leading to the buildup of a continuum of intermediates. The weakness of assortative mating compared to reduced fitness of hybrids in preventing blending is robust to varying genetic bases of these traits. Assortative mating is most powerful in limiting blending when it is encoded by a single locus, is essentially complete, or when there is a large mate search cost. In these cases assortative mating is likely to cause hybrids to have low fitness, due to frequency-dependent mating disadvantage of individuals of rare mating types. These results prompt a questioning of the concept of partial prezygotic isolation, since it is not very isolating unless there is also postzygotic isolation.</p>

<b>摘要</b><br/><p>在物种形成的早期阶段，部分合子前生殖隔离（prezygotic isolation）通常被认为比部分合子后生殖隔离（postzygotic isolation，表现为杂种适合度低下）更为重要。本研究通过模拟两个种群（或称“物种”）的二次接触场景，探究选型交配（assortative mating）与杂种适合度降低在阻止种群基因库融合过程中的作用效果。研究结果表明，杂种适合度小幅下降（例如降低10%）所形成的杂种带，相较于强烈但并不完美的交配偏好（例如对同种配偶的选择强度为异种配偶的10倍）更窄。在后一种情形中，罕见的F1杂种会因选型交配而相互吸引，最终导致中间型类群的连续谱系得以形成。相较于杂种适合度降低，选型交配在阻止种群融合方面的效力强弱，不受这些性状的遗传基础差异的影响。当选型交配由单个位点编码、选择程度近乎完全，或是存在较高的配偶搜寻成本时，选型交配在限制种群融合上的效果最为显著。在此类情况下，由于稀有交配类型个体存在频率依赖性交配劣势，选型交配可能会间接导致杂种适合度降低。本研究结果对部分合子前生殖隔离的概念提出了质疑，因为除非同时存在合子后生殖隔离，否则其隔离效果往往十分有限。</p>