Data from: Reversal of response to artificial selection on body size in a wild passerine

A general assumption in quantitative genetics is the existence of an intermediate phenotype with higher mean individual fitness in the average environment than more extreme phenotypes. Here we investigate the evolvability and presence of such a phenotype in wild bird populations from an eleven-year experiment with four years of artificial selection for long and short tarsus length, a proxy for body size. The experiment resulted in strong selection in the imposed directions. However, artificial selection was counteracted by reduced production of recruits in offspring of artificially selected parents. This resulted in weak natural selection against extreme trait values. Significant responses to artificial selection were observed at both the phenotypic and genetic level, followed by a significant return towards pre-experimental means. During artificial selection, the annual observed phenotypic response closely followed the predicted response from quantitative genetic theory (ryear = 0.96, rcohorts = 0.56). The rapid return to pre-experimental means was induced by three interacting mechanisms: selection for an intermediate phenotype, immigration and recombination between selected and unselected individuals. The results of this study demonstrates the evolvability of phenotypes and that selection may favour an intermediate phenotype in wild populations.数量遗传学（quantitative genetics）中的一项通用假设认为，相较于极端表型（phenotype），存在一类中间表型在平均环境中具备更高的平均个体适合度（fitness）。本研究基于一项为期11年的野生鸟类种群野外实验，该实验针对作为体长替代指标的跗跖长度（tarsus length）开展了为期4年的长短方向人工选择（artificial selection），以此探究这类中间表型的可进化性（evolvability）及其在野生种群中的存在性。实验在所施加的选择方向上产生了强烈的选择效应，然而人工选择受到人工选择亲本的后代招募量（recruits）下降的抵消，这使得针对极端性状值的自然选择作用相对微弱。研究在表型与遗传水平（genetic level）上均观测到了对人工选择的显著响应，随后出现了向实验前均值的显著回归现象。在人工选择阶段，年度观测到的表型响应与数量遗传学理论的预测响应高度吻合（ryear=0.96，rcohorts=0.56）。快速回归至实验前均值的现象由三种相互作用的机制引发：中间表型的选择、种群迁入（immigration）以及选择个体与非选择个体间的重组（recombination）。本研究结果证实了表型的可进化性，同时表明在野生种群中，自然选择可能会青睐中间表型。