Constraints on Rapid Adaptation in Sticklebacks: availability & maintenance of genetic variation used in adaptation to freshwater environments.

了解影响快速适应新的或变化的环境的机制和制约因素是进化生物学中的一个重要而重大的挑战。一个主要的焦点是作为快速适应变化的底物的遗传变异的类型和来源。这一点特别令人感兴趣，因为进化史和人口统计过程会影响适应性遗传变异的可用性，促进或限制适应的速度，并影响种群未来的进化轨迹。适应的理论种群遗传模型传统上侧重于从头突变作为适应性遗传变异的来源。然而，在暴露于新环境或受到环境变化影响的种群中，与从头突变相反，先前存在的长期遗传变异可能提供更容易获得的底物，可能通过历史选择事件预先筛选，从而促进快速适应。与此相一致的是，定量遗传实验和对自然种群的研究表明，对时间尺度上的选择的快速表型反应被认为是太短而不是由从头开始的突变引起的。而最近的适应群体遗传模型表明，当底物是恒定的遗传变异而不是从头开始的突变时，适应的速度显着提高。然而，尽管有这些进展和理论模型，但关于自然种群快速适应背后的遗传变异的类型、来源、流行率和特性的经验数据相对较少。在这里，我们将来自最先进的三刺鱼基因组测序的经验数据与维持来源海洋种群的长期遗传变异的种群遗传模型以及淡水种群的选择性扫描相结合，以识别和量化形成、促进和限制自然种群快速适应的关键因素。除了估计影响适应的因素的合理参数范围外，我们的研究还将为遗传变异的起源、传播和维持提供有价值的见解，这对于保持自然种群未来的适应潜力至关重要。

Understanding the mechanisms and constraints influencing rapid adaptation to new or changing environments is an important and significant challenge in evolutionary biology. One major focus centres on the type and source of genetic variation that acts as a substrate for rapid adaptive change. This is of particular interest because evolutionary history and demographic processes can affect the availability of adaptive genetic variation, promoting or constraining the speed of adaptation, and influencing the future evolutionary trajectory a population. Theoretical population genetic models of adaptation have traditionally focused on de novo mutations as the source of adaptive genetic variation. However, in populations exposed to a new environment or subject to environmental change, pre-existing standing genetic variation, as opposed to de novo mutation, may provide a more readily available substrate, potentially pre-screened by historical selection events, that facilitates rapid adaptation. Consistent with this, quantitative genetic experiments and studies of natural populations have revealed rapid phenotypic responses to selection on time scales considered to be too short to be caused by de novo mutations. And more recent population genetic models of adaptation have shown that the speed of adaptation increases significantly when the substrate is standing genetic variation rather than de novo mutation. However, despite these advances and theoretical models, relatively little empirical data is available on the type, source, prevalence and properties of genetic variation underlying rapid adaptation in natural populations. Here, we combine empirical data from state-of-the-art genomic sequencing of threespine stickleback fish with population genetic models of maintenance of standing genetic variation in source marine populations, and selective sweeps in freshwater populations, in order to identify and quantify key factors that shape, promote and constrain rapid adaptation in the natural populations. In addition to estimating the plausible parameter range of factors influencing adaptation, our study will provide valuable insight into the origin, spread and maintenance of genetic variation that is essential for preserving the future adaptive potential of natural populations.