The effect of the environment on fitness in natural populations is a fundamental question in evolutionary biology. However, experimental manipulations of both environment and phenotype at the same time are rare. Thus, the relative importance of the competitive environment versus intrinsic organismal performance in shaping the location, height, and fluidity of fitness peaks and valleys remains largely unknown. Here, we experimentally tested the effect of competitor frequency on the complex fitness landscape driving adaptive radiation of a generalist and two trophic specialist pupfishes, a scale‐eater and molluscivore, endemic to hypersaline lakes on San Salvador Island (SSI), Bahamas. We manipulated phenotypes, by generating 3407 F4/F5 lab‐reared hybrids, and competitive environment, by altering the frequency of rare transgressive hybrids between field enclosures in two independent lake populations. We then tracked hybrid survival and growth rates across these four field enclosures for 3–11 months. In contrast to competitive speciation theory, we found no evidence that the frequency of hybrid phenotypes affected their survival. Instead, we observed a strikingly similar fitness landscape to a previous independent field experiment, each supporting multiple fitness peaks for generalist and molluscivore phenotypes and a large fitness valley isolating the divergent scale‐eater phenotype. These features of the fitness landscape were stable across manipulated competitive environments, multivariate trait axes, and spatiotemporal heterogeneity. We suggest that absolute performance constraints and divergent gene regulatory networks shape macroevolutionary (interspecific) fitness landscapes in addition to microevolutionary (intraspecific) competitive dynamics. This interplay between organism and environment underlies static and dynamic features of the adaptive landscape.

中文翻译：

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测量混合适应度的独立现场实验揭示了多峰适应度景观令人惊讶的时空稳定性

环境对自然种群适应性的影响是进化生物学的一个基本问题。然而，同时对环境和表型进行实验操作的情况很少见。因此，竞争环境与内在有机体表现在塑造适应度峰值和谷值的位置、高度和流动性方面的相对重要性仍然很大程度上未知。在这里，我们通过实验测试了竞争者频率对驱动适应性辐射的复杂健身景观的影响，该适应性辐射是一种多面性和两种营养专科鳉鱼（一种食鳞鱼和软体动物），是巴哈马圣萨尔瓦多岛（SSI）高盐湖的特有种。我们通过生成 3407 个 F4/F5 实验室饲养的杂交体来操纵表型，并通过改变两个独立湖泊种群的围场之间罕见的海侵杂交体的频率来操纵竞争环境。然后，我们在 3-11 个月内跟踪了这四个野外围栏中杂种的存活率和生长率。与竞争性物种形成理论相反，我们没有发现任何证据表明杂交表型的频率影响它们的生存。相反，我们观察到了与之前的独立现场实验惊人相似的适应度景观，每个实验都支持通才和食软体动物表型的多个适应度峰值，以及隔离不同鳞食动物表型的大适应度谷。适应度景观的这些特征在受操纵的竞争环境、多变量特征轴和时空异质性中保持稳定。我们认为，除了微观进化（种内）竞争动态之外，绝对性能限制和不同的基因调控网络还塑造了宏观进化（种间）适应性景观。有机体与环境之间的相互作用是适应性景观的静态和动态特征的基础。