Coexisting (sympatric) pairs of closely related species are often characterized by exaggerated trait differences. This widespread pattern is consistent with adaptation for reduced similarity due to costly interactions (i.e., “character displacement”)—a classic hypothesis in evolutionary theory. But it is equally consistent with a community assembly bias in which lineages with greater trait differences are more likely to establish overlapping ranges in the first place (i.e., “species sorting”), as well as with null expectations of trait divergence through time. Few comparative analyses have explicitly modeled these alternatives, and it remains unclear whether trait divergence is a general prerequisite for sympatry or a consequence of interactions between sympatric species. Here, we develop statistical models that allow us to distinguish the signature of these processes based on patterns of trait divergence in closely related lineage pairs. We compare support for each model using a dataset of bill shape differences in 207 pairs of New World terrestrial birds representing 30 avian families. We find that character displacement models are overwhelmingly supported over species sorting and null expectations, indicating that exaggerated bill shape differences in sympatric pairs result from enhanced divergent selection in sympatry. We additionally detect a latitudinal gradient in character displacement, which appears strongest in the tropics. Our analysis implicates costly species interactions as powerful drivers of trait divergence in a major vertebrate fauna. These results help substantiate a long-standing but equivocally supported linchpin of evolutionary theory.

密切相关物种的共存（同域）对通常以夸大的性状差异为特征。这种普遍的模式与由于代价高昂的交互（即“字符置换”）而导致的相似性降低的适应是一致的——进化理论中的一个经典假设。但它同样与群落组装偏差一致，其中具有更大特征差异的谱系更可能首先建立重叠范围（即“物种分类”），并且随着时间的推移特征差异的预期为零。很少有比较分析明确地对这些替代品进行建模，目前尚不清楚性状差异是同情的一般先决条件还是同域物种之间相互作用的结果。这里，我们开发了统计模型，使我们能够根据密切相关的谱系对中的特征差异模式来区分这些过程的特征。我们使用代表 30 个鸟类家族的 207 对新世界陆生鸟类的喙形状差异数据集来比较对每个模型的支持。我们发现字符置换模型在物种排序和空期望上得到压倒性的支持，表明同域对中夸大的喙形状差异是由于同域中增强的发散选择导致的。我们还检测到字符位移的纬度梯度，这在热带地区显得最强。我们的分析表明，代价高昂的物种相互作用是主要脊椎动物群中性状分化的强大驱动因素。