Reinforcement, the increase of assortative mating driven by selection against unfit hybrids, is conditional on pre-existing divergence. Yet, for ecological divergence to precede the evolution of assortment, strict symmetries between fitnesses in niches must hold, and/or there must be low gene flow between the nascent species. It has thus been argued that conditions favouring sympatric speciation are rarely met in nature. Indeed, we show that under disruptive selection, violating symmetries in niche sizes and increasing strength of the trade-off in selection between the niches quickly leads to loss of genetic variation, instead of evolution of specialists. The region of the parameter space where polymorphism is maintained further narrows with increasing number of loci encoding the diverging trait and the rate of recombination between them. Yet, evolvable assortment and pre-existing assortment both substantially broaden the parameter space within which polymorphism is maintained. Notably, pre-existing niche preference speeds up further increase of assortment, thus facilitating reinforcement in the later phases of speciation. We conclude that in order for sympatric ecological divergence to occur, niche preference must coevolve throughout the divergence process. Even if populations come into secondary contact, having diverged in isolation, niche preference substantially broadens the conditions for coexistence in sympatry and completion of the speciation process.

This article is part of the theme issue ‘Towards the completion of speciation: the evolution of reproductive isolation beyond the first barriers'.

补强是由针对不适合的杂种的选择推动的交配交配的增加，其条件是预先存在分歧。然而，要使生态学差异先于物种进化，就必须在生态位之间保持严格的对称性，和/或新生物种之间的基因流必须低。因此，有人认为，自然界中很少有满足同胞形态形成的条件。的确，我们表明，在破坏性选择下，违反生态位大小的对称性和生态位之间选择权衡的强度增加会迅速导致遗传变异的丧失，而不是专家的进化。保留多态性的参数空间区域随着编码变异性状的基因座数目的增加以及它们之间的重组率而进一步缩小。然而，可进化分类和既有分类都大大拓宽了保持多态性的参数空间。值得注意的是，先前存在的利基市场偏好会加速品种的进一步增加，从而有助于在物种形成的后期阶段进行强化。我们得出结论，为了使同伴生态发散发生，生态位偏好必须在整个发散过程中共同发展。即使种群处于孤立的状态而进行了二次接触，利基的偏好也大大拓宽了共生和物种形成过程共存的条件。因此有助于在物种形成的后期阶段进行强化。我们得出结论，为了使同伴生态发散发生，生态位偏好必须在整个发散过程中共同发展。即使种群处于孤立的状态而进行了二次接触，利基的偏好也大大拓宽了共生和物种形成过程共存的条件。因此有助于在物种形成的后期阶段进行强化。我们得出结论，为了使同伴生态发散发生，生态位偏好必须在整个发散过程中共同发展。即使种群处于孤立的状态而进行了二次接触，利基的偏好也大大拓宽了共生和物种形成过程共存的条件。

本文是主题“走向物种形成：生殖隔离超越第一个障碍的演变”的一部分。