Gene flow and local adaptation in a sunfish-salamander system

Abstract There is increasing evidence that populations may not be well adapted to their local environments, and as a result, recent interest has focused on understanding factors that constrain adaptive evolution. This study presents data suggesting gene flow may constrain the ability of larvae of the streamside salamander Ambystoma barbouri to avoid predation by fish via escape behavior and life history tactics. Streamside salamander larvae face conflicting selection pressures in different streams. Some streams are ephemeral, where larvae should be active to feed, grow, and reach metamorphosis before stream drying. Other streams contain predatory fish, where larvae should be generally inactive to avoid predation. Previous work has shown that streamside salamander larvae exhibit ineffective antipredator behavior by having inappropriately high activity levels with fish, resulting in high predation in laboratory and field experiments. This study investigated the possibility that gene flow from larvae in ephemeral habitats may reduce the escape performance of larvae from populations with fish and alter their life history characteristics to increase their susceptibility to fish predation. I assayed escape behavior (speed, acceleration, and duration of escape) and life history characteristics (hatching date, size, stage) associated with predator avoidance among laboratory-reared larvae from four populations. As predicted, two populations (one with fish and the other fishless and ephemeral) connected by gene flow were not significantly different in almost all assays. In contrast, larvae from an isolated population with fish had significantly stronger escape behaviors and delayed hatching than both an isolated population that lacked a history of fish co-occurrence and the population with fish but gene flow from a fishless population. These results support theory suggesting that gene flow can constrain adaptive evolution.

摘要 越来越多的证据表明，种群可能无法很好地适应当地环境，因此，近期的研究兴趣集中在理解限制适应性进化的因素上。本研究提供的数据表明，基因流可能会限制溪边蝾螈（Ambystoma barbouri）幼体通过逃避行为和生活史策略来躲避鱼类捕食的能力。溪边蝾螈幼体在不同的溪流中面临相互冲突的选择压力。一些溪流是季节性的，在溪流干涸之前，幼体应该积极觅食、生长并完成变态。其他溪流中有捕食性鱼类，在这些溪流中，幼体通常应该不活跃以避免被捕食。先前的研究表明，溪边蝾螈幼体在有鱼的情况下表现出不适当的高活动水平，从而表现出无效的反捕食行为，导致在实验室和野外实验中被捕食率较高。本研究调查了来自季节性栖息地的幼体的基因流是否可能降低有鱼种群幼体的逃避能力，并改变它们的生活史特征，从而增加它们对鱼类捕食的易感性。我测定了来自四个种群的实验室饲养幼体的逃避行为（速度、加速度和逃避持续时间）以及与躲避捕食者相关的生活史特征（孵化日期、大小、发育阶段）。正如预测的那样，通过基因流相连的两个种群（一个有鱼，另一个无鱼且是季节性的）在几乎所有的测定中都没有显著差异。相比之下，来自一个有鱼的孤立种群的幼体比一个没有与鱼共存历史的孤立种群以及有鱼但有来自无鱼种群基因流的种群的幼体具有明显更强的逃避行为和延迟的孵化时间。这些结果支持了基因流能够限制适应性进化的理论。