Assortative flocking in crossbills and implications for ecological speciation

How reproductive isolation is related to divergent natural selection is a central question in speciation. Here we focus on several ecologically specialised taxa or “call types” of red crossbills (Loxia curvirostra complex), one of the few groups of birds providing much evidence for ecological speciation. Call types differ in bill sizes and feeding capabilities, and also differ in vocalizations, such that contact calls provide information on crossbill phenotype. We found that two call types of red crossbills were more likely to approach playbacks of their own call type than those of heterotypics, and that their propensity to approach heterotypics decreased with increasing divergence in bill size. Although call similarity also decreased with increasing divergence in bill size, comparisons of responses to familiar versus unfamiliar call types indicate that the decrease in the propensity to approach heterotypics with increasing divergence in bill size was a learned response, and not a byproduct of calls diverging pleiotropically as bill size diverged. Because crossbills choose mates while in flocks, assortative flocking could lead indirectly to assortative mating as a byproduct. These patterns of association therefore provide a mechanism by which increasing divergent selection can lead to increasing reproductive isolation.

生殖隔离与趋异自然选择的关联机制，是物种形成研究中的核心科学问题。本研究聚焦红交嘴雀（Loxia curvirostra complex）复合种的数个生态特化类群，即“鸣唱型”——该类群是少数获得充分生态物种形成证据的鸟类类群之一。不同鸣唱型的红交嘴雀在喙部大小与取食能力上存在差异，鸣唱特征亦有所不同，其联络鸣唱可传递交嘴雀的表型信息。研究发现，两种红交嘴雀鸣唱型更倾向于趋近同种鸣唱型的回放鸣唱，而非异型鸣唱；且其趋近异型鸣唱的倾向，随喙部大小趋异程度的增加而降低。尽管鸣唱相似性同样随喙部大小趋异程度的增加而降低，但通过对比对熟悉与陌生鸣唱型的反应可知，趋近异型鸣唱的倾向随喙部大小趋异程度增加而下降这一现象，属于后天习得的响应，而非喙部大小趋异时鸣唱特征因多效性发生分化的附带产物。由于交嘴雀会在集群中选择配偶，选择性集群可间接作为附带结果促成选择性交配。因此，这些关联模式为趋异选择的增强如何导致生殖隔离加强提供了一套作用机制。