Data from: Local adaptation despite high gene flow in the waterfall-climbing Hawaiian goby, Sicyopterus stimpsoni.

Environmental heterogeneity can promote the emergence of locally adapted phenotypes among subpopulations of a species, whereas gene flow can result in phenotypic and genotypic homogenization. For organisms like amphidromous fishes that change habitats during their life history, the balance between selection and migration can shift through ontogeny, making the likelihood of local adaptation difficult to predict. In Hawaiian waterfall-climbing gobies, it has been hypothesized that larval mixing during oceanic dispersal counters local adaptation to contrasting topographic features of streams, like slope gradient, that can select for predator avoidance or climbing ability in juvenile recruits. To test this hypothesis, we used morphological traits and neutral genetic markers to compare phenotypic and genotypic distributions in recruiting juveniles and adult subpopulations of the waterfall-climbing amphidromous goby, Sicyopterus stimpsoni, from the islands of Hawai'i and Kaua'i. We found that body shape is significantly different between adult subpopulations from streams with contrasting slopes, and that trait divergence in recruiting juveniles tracked stream topography more so than morphological measures of adult subpopulation differentiation. Though no evidence of population genetic differentiation was observed among adult subpopulations, we observed low but significant levels of spatially and temporally variable genetic differentiation among juvenile cohorts, which correlated with morphological divergence. Such a pattern of genetic differentiation is consistent with chaotic genetic patchiness arising from variable sources of recruits to different streams. Thus, at least in S. stimpsoni, the combination of variation in settlement cohorts in space and time coupled with strong post-settlement selection on juveniles as they migrate upstream to adult habitats provides the opportunity for morphological adaptation to local stream environments despite high gene flow.

环境异质性可推动物种种群亚群产生局部适应性表型，而基因流则会引发表型与基因型的同质化。对于生活史中会改变栖息环境的两游性鱼类（amphidromous fishes）这类生物而言，选择压力与迁移之间的平衡会随个体发育动态变化，这使得局部适应性的发生概率难以预测。针对夏威夷攀瀑虾虎鱼（waterfall-climbing gobies）类群，已有假说提出：海洋扩散过程中的幼体混合，会抵消种群对溪流不同地形特征（如坡度）的局部适应性演化——而这类地形特征会对幼体定居者的避敌能力或攀附能力产生定向选择。为验证该假说，本研究以夏威夷岛与考爱岛的史氏爬虾虎（Sicyopterus stimpsoni，一种攀瀑两游虾虎鱼）的定居幼体与成体亚群为研究对象，利用形态性状与中性遗传标记（neutral genetic markers），对比分析其表型与基因型的分布特征。研究结果显示：来自坡度差异显著的溪流的成体亚群，其体型存在显著差异；且相较于成体亚群的形态分化指标，定居幼体的性状分化与溪流地形的关联度更高。尽管未在成体亚群中检测到种群遗传分化的迹象，但我们在幼体群簇中发现了程度较低但显著的、随空间与时间变化的遗传分化，且该分化与性状分化存在相关性。这类遗传分化模式，与不同溪流的定居幼体来源各异所引发的混沌遗传斑块现象（chaotic genetic patchiness）相符。因此，至少在史氏爬虾虎（S. stimpsoni）中，定居群簇的时空变异，结合幼体洄游至成体栖息溪流过程中所经历的强烈定居后选择压力，即便存在高强度基因流，仍为其适应溪流局部环境的形态演化提供了可能。