Animal dispersal is often phenotype‐dependent and can exert evolutionary pressures on populations in which it occurs. The evolutionary pressure arising from phenotype‐dependent dispersal is called spatial sorting. We examined the evolutionary pressure arising from spatial sorting (sorting pressure) caused by downstream dispersal in juvenile white‐spotted char Salvelinus leucomaenis under ordinary flow conditions. We conducted outdoor experiments using an artificial channel with 10 steps to investigate the relationship between phenotypic characteristics and the occurrence or distance of downstream dispersal during five daytime hours. Six experiments were conducted using young‐of‐the‐year juveniles collected early in the morning of each experimental day. We focused on two phenotypes, fork length (body size) and station‐holding (SH) behavior, where juveniles remain sedentary on the substrate. Juveniles were assigned to the “SH group” if they exhibited SH behavior for more than 10 s during a 540‐s observation period, and to the “swimming group” if they exhibited SH behavior for less than 10 s. Juveniles in the swimming group had a higher occurrence of downstream dispersal than in the SH group. In addition, large juveniles in the SH group and small juveniles in the swimming group tended to show long dispersal distances. These results suggest an effective sorting pressure against juveniles with active swimming behavior. This sorting pressure may accumulate in isolated char populations located above a tall migration barrier and contribute to the creation and maintenance of the reported interpopulation variation in SH behavior.

中文翻译：

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幼年白斑炭在普通流动条件下表型依赖性下游扩散

动物扩散通常依赖于表型，并且会对发生扩散的种群产生进化压力。由表型依赖性扩散产生的进化压力称为空间排序。我们研究了幼年白点炭下游扩散引起的空间分选（分选压力）所产生的进化压力白斑红点鲑在普通流动条件下。我们使用具有 10 个台阶的人工通道进行了室外实验，以研究白天 5 小时内表型特征与下游扩散的发生或距离之间的关系。使用每个实验日清晨收集的当年幼鱼进行了六次实验。我们重点关注两种表型，即叉长（身体大小）和保持姿势（SH）行为，其中幼鱼在基质上保持静止不动。如果青少年在 540 秒的观察期内表现出 SH 行为超过 10 秒，则被分配到“SH 组”；如果他们表现出 SH 行为少于 10 秒，则被分配到“游泳组”。游泳组的幼鱼下游扩散的发生率高于SH组。此外，SH组中的大型幼鱼和游泳组中的小型幼鱼往往表现出较长的分散距离。这些结果表明对具有活跃游泳行为的幼体存在有效的分选压力。这种分选压力可能会在位于高迁移屏障上方的孤立的炭种群中积累，并有助于形成和维持所报告的种群间SH行为差异。