Evolutionary Ecology of Variation Group, Max Planck Institute for Ornithology, 82319 Seewiesen, Germany.
J Evol Biol. 2013 Sep;26(9):2031-43. doi: 10.1111/jeb.12210. Epub 2013 Aug 13.
Negative density dependence of clutch size is a ubiquitous characteristic of avian populations and is partly due to within-individual phenotypic plasticity. Yet, very little is known about the extent to which individuals differ in their degree of phenotypic plasticity, whether such variation has a genetic basis and whether level of plasticity can thus evolve in response to selection. Using 18 years of data of a Dutch great tit population (Parus major), we show that females reduced clutch size with increasing population density (slopes of the reaction norms), differed strongly in their average clutch size (elevations of the reaction norms) at the population-mean density and that the latter variation was partly heritable. In contrast, we could not detect individual variation in phenotypic plasticity ('I × E'). Level of plasticity is thus not likely to evolve in response to selection in this population. Observed clutch sizes deviated more from the estimated individual reaction norms in certain years and densities, implying that the within-individual between-year variance (so-called residual variance) of clutch size was heterogeneous with respect to these factors. Given the observational nature of this study, experimental manipulation of density is now warranted to confirm the causality of the observed density effects. Our analyses demonstrate that failure to acknowledge this heterogeneity would have inflated the estimate of 'I × E' and led to misinterpretation of the data. This paper thereby emphasizes the fact that heterogeneity in residuals can provide biologically insightful information about the ecological processes underlying the data.
巢大小的负密度依赖性是鸟类种群的普遍特征，部分原因是个体内表型可塑性。然而，人们对个体在表型可塑性程度上的差异程度知之甚少，这种变异是否具有遗传基础，以及可塑性水平是否可以因此对选择做出反应而进化。利用荷兰大山雀种群 18 年的数据（Parus major），我们表明，随着种群密度的增加，雌性会减少巢的大小（反应规范的斜率），在种群平均密度下，它们的平均巢大小（反应规范的高程）存在强烈差异，而且这种差异具有部分遗传性。相比之下，我们无法检测到个体在表型可塑性上的差异（I×E）。因此，在这个种群中，可塑性水平不太可能对选择做出反应而进化。在某些年份和密度下，观察到的巢大小与估计的个体反应规范的偏离更大，这意味着巢大小的个体内年际方差（所谓的残差方差）在这些因素上存在异质性。鉴于本研究的观察性质，现在需要进行密度的实验处理，以确认观察到的密度效应的因果关系。我们的分析表明，如果不承认这种异质性，就会夸大“I×E”的估计，并导致对数据的错误解释。本文因此强调了这样一个事实，即残差的异质性可以为数据背后的生态过程提供有生物学意义的信息。
