Direction and magnitude of natural selection on body size differs among age classes of seaward migrating Pacific salmon

Due to the mediating role of body size in determining fitness, the ‘bigger is better’ hypothesis still pervades evolutionary ecology despite evidence that natural selection on phenotypic traits varies in time and space. For Pacific salmon (genus Oncorhynchus), most individual studies quantify selection across a narrow range of sizes and ages; therefore, uncertainties remain concerning how selection on size may differ among diverse life-histories. Here, we quantify the direction and magnitude of natural selection on body size among age-classes of multiple marine cohorts of O. nerka (sockeye salmon). Across four cohorts of seaward migrants, we calculated standardized selection differentials by comparing observed size distributions of out-migrating juvenile salmon to back-calculated smolt length from the scales of surviving, returning adults. Results reveal the magnitude of selection on size was very strong (> 90th percentile compared to a database of 3,759 linear selection differentials) and consistent among years. However, the direction of selection on size consistently varied among age-classes. Selection was positive for fish migrating to sea after two years in freshwater (age 2) and in their first year of life (age 0), but negative for fish migrating after 1 year in freshwater (age 1). The absolute magnitude of selection was negatively correlated to mean ocean-entry timing, which may underpin negative selection favoring small age-1 fish, given associations between size and timing of seaward migration. Collectively, these results indicate that ‘bigger is not always better’ in terms of survival and emphasize trade-offs that may exist between fitness components for organisms with similarly diverse migratory life-histories.由于体型在决定适合度（fitness）中起到中介作用，尽管已有证据表明表型性状（phenotypic traits）的自然选择随时间和空间而异，但“体型越大越佳”假说仍在进化生态学（evolutionary ecology）中占据主导地位。对于太平洋鲑属（Oncorhynchus）鱼类而言，绝大多数现有研究仅在狭窄的体型和年龄范围内量化选择强度，因此针对不同生活史类型下体型选择模式的差异，目前仍存在诸多不确定性。本研究针对多组海洋种群的红大马哈鱼（Oncorhynchus nerka，俗称红鲑（sockeye salmon））不同年龄组，量化了体型相关自然选择的方向与强度。针对4组入海洄游种群，本研究通过对比出洄幼鲑的实测体型分布与从存活洄游成鱼鳞片反推所得的降海幼鲑（smolt）体长，计算了标准化选择差（standardized selection differentials）。研究结果显示，体型选择的强度极高（相较于包含3759个线性选择差（linear selection differentials）的数据库，其值超过第90百分位数），且在不同年份间保持一致。但体型选择的方向在不同年龄组间始终存在显著差异：对于在淡水环境中生活2年后入海的个体（年龄组2）以及生命第一年即入海的个体（年龄组0），选择呈正向；而对于在淡水生活1年后入海的个体（年龄组1），选择则呈负向。选择强度的绝对值与平均入海时间呈负相关，考虑到体型与入海时间存在关联，这一结果或可解释为何年龄组1的小型个体更受负向选择青睐。综合来看，本研究结果表明，就生存适配而言，“体型越大越佳”并非普适规律，同时也凸显了对于具有多样洄游生活史的生物而言，适合度组分间可能存在权衡关系。