Data from: Color patterns of closely-related bird species are more divergent at intermediate levels of breeding range sympatry

Closely related species of birds often differ markedly in their color patterns. Here we examine the influence of breeding-range overlap (breeding sympatry) on the evolution of color pattern differences in a sample of closely related bird species. We used a sister-lineage method to analyze 73 phylogenetically independent comparisons among 246 species and 39 families of birds worldwide. We found that divergence of color patterns among closely related species was greater between sympatric than between allopatric lineages, but only at intermediate levels of sympatry (50%–80% breeding-range overlap). This pattern suggests that closely related species incur costs at intermediate levels of sympatry if they exhibit similar color patterns—costs that could include hybridization, interspecific aggression, competition for signaling space, or ecological interactions that secondarily influence color patterns. The decline in color pattern divergence with further increase in sympatry suggests either the relaxation of divergent selection, increased impediment of gene flow, or an increased role for counteracting selection at higher levels of sympatry. We also found that the differences in color pattern between sympatric and allopatric sister species were greatest at lower latitudes. The global scale and broad taxonomic coverage in our study suggest that the divergence of color patterns between sympatrically breeding closely related species is widespread in birds.

亲缘关系相近的鸟类类群，其羽色图案往往存在显著差异。本研究探讨繁殖范围重叠（breeding sympatry，繁殖同域分布）对亲缘鸟类羽色图案差异演化的影响。我们采用姊妹支系方法（sister-lineage method），对全球范围内246个鸟类物种、39个科的73组系统发育独立对比（phylogenetically independent comparisons）进行分析。结果显示，亲缘物种间的羽色图案分化程度，在同域支系间高于异域支系，但该现象仅在中等同域程度（繁殖范围重叠率50%~80%）下存在。
这一模式表明，若亲缘物种在中等同域程度下展现相似羽色图案，将承担相应演化成本——此类成本可能包括杂交、种间攻击、信号空间竞争，或间接影响羽色图案的生态相互作用。随着同域程度进一步升高，羽色图案分化程度反而下降，该现象提示三种潜在解释：一是歧化选择的松弛，二是基因流阻碍作用的增强，或是在更高同域程度下拮抗选择的作用愈发显著。
此外，我们发现同域与异域姊妹物种间的羽色图案差异，在低纬度地区最为显著。本研究具备全球尺度与宽泛的类群覆盖范围，表明同域繁殖的亲缘鸟类类群间的羽色图案分化现象，在鸟类中广泛存在。