Department of Biology, Pacific Lutheran University, Tacoma, WA 98447, USA.
Proc Biol Sci. 2012 Oct 22;279(1745):4223-9. doi: 10.1098/rspb.2012.1500. Epub 2012 Aug 22.
How reproductive isolation is related to divergent natural selection is a central question in speciation. Here, we focus on several ecologically specialized 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 by-product 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 by-product. 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系是物种形成的核心问题。在这里，我们专注于几种生态特化的分类群或“鸣叫类型”红交嘴雀（Loxia curvirostra 复合体），这是少数为生态物种形成提供大量证据的鸟类群体之一。鸣叫类型在喙大小和取食能力上存在差异，同时在叫声上也存在差异，因此接触鸣叫提供了有关交嘴雀表型的信息。我们发现，两种红交嘴雀的鸣叫类型比异型的鸣叫类型更有可能接近自己的鸣叫类型的回放，而它们接近异型的倾向随着喙大小的差异增大而减小。尽管鸣叫的相似性也随着喙大小的差异增大而减小，但对熟悉和不熟悉的鸣叫类型的反应比较表明，随着喙大小的差异增大而接近异型的倾向的减小是一种学习反应，而不是随着喙大小的差异增大而趋异的鸣叫的副产品。由于交嘴雀在群体中选择配偶，因此趋同聚集可能会间接地导致作为副产品的趋同交配。因此，这些关联模式提供了一种机制，通过这种机制，趋异选择的增加可以导致生殖隔离的增加。
