Body size predicts the rate of contemporary morphological change in birds

Variation in evolutionary rates among species is a defining characteristic of the tree of life and may be an important predictor of species’ capacities to adapt to rapid environmental change. It is broadly assumed that generation length is an important determinant of microevolutionary rates, and body size is often used as a proxy for generation length. However, body size has myriad biological correlates that could affect evolutionary rates independently from generation length. We leverage two large, independently collected datasets on recent morphological change in birds (52 migratory species breeding in North America and 77 South American resident species) to test how body size and generation length are related to rates of contemporary morphological change. Both datasets show that birds have declined in body size and increased in wing length over the past 40 years. We found, in both systems, a consistent pattern wherein smaller species declined proportionally faster in body size and increased proportionally faster in wing length. By contrast, generation length explained less variation in evolutionary rates than did body size. Although the mechanisms warrant further investigation, our study demonstrates that body size is an important predictor of contemporary variation in morphological rates of change. Given the correlations between body size and a breadth of morphological, physiological, and ecological traits predicted to mediate phenotypic responses to environmental change, the relationship between body size and rates of phenotypic change should be considered when testing hypotheses about variation in adaptive responses to climate change.

Methods
For information on data collection see https://doi.org/10.5061/dryad.8pk0p2nhw and https://doi.org/10.5061/dryad.fqz612jsp.

物种间演化速率的差异是生命之树的标志性特征，同时或可作为预测物种适应快速环境变化能力的重要指标。学界普遍认为，世代长度（generation length）是微演化速率（microevolutionary rates）的重要决定因素，而体型（body size）常被用作世代长度的替代指标。然而，体型存在诸多生物学关联性状，这些性状可独立于世代长度之外对演化速率产生影响。本研究依托两套独立采集的大型鸟类近期形态变化数据集（分别包含52种在北美繁殖的迁徙鸟类与77种南美留居鸟类），检验体型与世代长度如何与当代形态变化速率相关联。两套数据集均显示，在过去40年间，鸟类的体型呈下降趋势，而翅长则有所增加。在两类研究体系中，我们均发现了一致的规律：体型更小的物种，其体型的比例性下降速率更快，翅长的比例性增长速率也更快。相较之下，世代长度对演化速率变异的解释度低于体型。尽管其内在机制尚需进一步探究，但本研究证实，体型是当代形态变化速率变异的重要预测因子。鉴于体型与一系列可介导生物对环境变化做出表型响应（phenotypic responses）的形态、生理及生态性状存在关联，在检验气候变化适应性响应（adaptive responses）变异的相关假说时，应考虑体型与表型变化速率之间的关系。

研究方法
有关数据采集的详细信息，请参阅：https://doi.org/10.5061/dryad.8pk0p2nhw 与 https://doi.org/10.5061/dryad.fqz612jsp。