Data from: Evolution and plasticity: divergence of song discrimination is faster in birds with innate song than in song learners in Neotropical passerine birds

Plasticity is often thought to accelerate trait evolution and speciation. For example, plasticity in birdsong may partially explain why clades of song learners are more diverse than related clades with innate song. This “song learning” hypothesis predicts that 1) differences in song traits evolve faster in song learners, and 2) behavioral discrimination against allopatric song (a proxy for premating reproductive isolation) evolves faster in song learners. We tested these predictions by analyzing acoustic traits and conducting playback experiments in allopatric Central American sister pairs of song learning oscines (N = 42) and non-learning suboscines (N = 27). We found that non-learners evolved mean acoustic differences slightly faster than did leaners, and that the mean evolutionary rate of song discrimination was 4.3 times faster in non-learners than in learners. This unexpected result may be a consequence of significantly greater variability in song traits in song learners (by 54–79%) that requires song-learning oscines to evolve greater absolute differences in song before achieving the same level of behavioral song discrimination as non-learning suboscines. This points to “a downside of learning” for the evolution of species discrimination, and represents an important example of plasticity reducing the rate of evolution and diversification by increasing variability.表型可塑性（Plasticity）通常被认为可加速性状演化与物种形成过程。例如，鸟鸣的表型可塑性或可部分解释为何鸣唱学习（song learning）类群的物种多样性高于具有先天鸣唱能力的近缘类群。这一“鸣唱学习假说”可推导出两项预测：其一，鸣唱学习类群的鸣唱性状差异演化速率更快；其二，鸣唱学习类群对异域鸣唱（allopatric song）——交配前生殖隔离（premating reproductive isolation）的替代指标——的行为识别演化速率更快。我们通过分析声学性状，并针对中美洲异域分布的姊妹类群对（sister pairs）开展鸣唱回放（playback）实验，对上述预测进行检验：其中鸣唱学习鸣禽（oscines）的样本量为42对，非学习亚鸣禽（suboscines）的样本量为27对。研究得到了出人意料的结果：非学习类群的平均声学差异演化速率略快于学习类群，且鸣唱识别的平均演化速率在非学习类群中是学习类群的4.3倍。这一意外结果或源于鸣唱学习类群的鸣唱性状变异程度显著更高（较非学习类群高出54%~79%）——这意味着鸣唱学习鸣禽需要演化出更大的鸣唱绝对差异，才能达到与非学习亚鸣禽相同水平的行为识别能力。该研究揭示了“学习行为的演化代价”，并为表型可塑性通过增加性状变异减缓演化与物种多样化速率提供了一则重要实证案例。