Brown Charles R, Brown Mary Bomberger, Roche Erin A, O'Brien Valerie A, Page Catherine E
Department of Biological Sciences, University of Tulsa, Tulsa, OK 74104
Department of Biological Sciences, University of Tulsa, Tulsa, OK 74104.
Proc Natl Acad Sci U S A. 2016 May 3;113(18):5113-8. doi: 10.1073/pnas.1600218113. Epub 2016 Apr 18.
Most animal groups vary extensively in size. Because individuals in certain sizes of groups often have higher apparent fitness than those in other groups, why wide group size variation persists in most populations remains unexplained. We used a 30-y mark-recapture study of colonially breeding cliff swallows (Petrochelidon pyrrhonota) to show that the survival advantages of different colony sizes fluctuated among years. Colony size was under both stabilizing and directional selection in different years, and reversals in the sign of directional selection regularly occurred. Directional selection was predicted in part by drought conditions: birds in larger colonies tended to be favored in cooler and wetter years, and birds in smaller colonies in hotter and drier years. Oscillating selection on colony size likely reflected annual differences in food availability and the consequent importance of information transfer, and/or the level of ectoparasitism, with the net benefit of sociality varying under these different conditions. Averaged across years, there was no net directional change in selection on colony size. The wide range in cliff swallow group size is probably maintained by fluctuating survival selection and represents the first case, to our knowledge, in which fitness advantages of different group sizes regularly oscillate over time in a natural vertebrate population.
大多数动物群体在大小上差异很大。由于特定规模群体中的个体往往比其他群体中的个体具有更高的表观适合度，所以大多数种群中群体大小为何存在广泛差异仍无法解释。我们对群居繁殖的崖燕（白额燕）进行了一项为期30年的标记重捕研究，以表明不同群体规模的生存优势在不同年份间波动。群体规模在不同年份既受到稳定选择也受到定向选择，并且定向选择的方向经常发生逆转。定向选择部分是由干旱条件预测的：在较凉爽湿润的年份，较大群体中的鸟类往往更具优势，而在较炎热干燥的年份，较小群体中的鸟类更具优势。对群体规模的振荡选择可能反映了食物可获得性的年度差异以及信息传递的重要性，和/或体外寄生虫感染水平，在这些不同条件下社会性的净收益有所不同。多年平均来看，对群体规模的选择没有净定向变化。据我们所知，崖燕群体规模的广泛范围可能是由波动的生存选择维持的，这代表了首例在自然脊椎动物种群中不同群体规模的适合度优势随时间定期振荡的情况。
