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.

中文翻译：

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不同年龄段的向海迁移的太平洋鲑鱼的自然选择方向和大小的大小不同。

由于身体大小在确定适应性方面的中介作用，尽管有证据表明表型性状的自然选择会随时间和空间变化，但“更大更好”的假设仍然贯穿进化生态学。对于太平洋鲑鱼（Oncorhynchus属），大多数个体研究都在狭窄的大小和年龄范围内对选择进行量化。因此，关于大小的选择在不同的生活史中可能会如何变化仍存在不确定性。在这里，我们量化了O. nerka多个海洋队列年龄组中自然选择的方向和大小的规模。（鲑鱼）。在四组向海移徙者中，我们通过比较观察到的迁出幼鲑的大小分布与从成活后成年成年后的重新计算的鲑鱼长度来计算标准化选择差异。结果表明，选择的规模非常强（与3,759个线性选择差异的数据库相比，排名高出第90个百分位数），并且年间一致。但是，尺寸选择的方向在各个年龄段之间始终不同。在淡水两年（2岁）和在生命的第一年（0岁）之后，迁移到海中的鱼的选择为阳性，而在淡水1年后（1岁）迁移的鱼，则选择为阴性。选择的绝对幅度与平均海洋进入时间负相关，考虑到规模和迁徙时间之间的联系，这可能会支持偏向于1岁小鱼的负面选择。总的来说，这些结果表明，就生存而言，“更大并不总是更好”，并强调了在迁徙生活史相似的生物体的适应性成分之间可能存在的取舍。