The genetic basis of adaptation in gradually changing environments.

生物适应环境的观察结果是显而易见的，但我们只能解释这种情况如何在极端情况下发生，例如抗生素和农药耐药性的进化，重金属耐受性和饥饿。典型的研究旨在了解生物如何适应环境变化，突然将种群置于新的压力环境中。例如，一个细菌群可能从一个营养丰富的环境转移到一个几乎没有某种营养的环境。然后，种群通过新突变的连续固定来适应，这些突变增加了其在新环境中的生长和繁殖。使用这一框架的理论和实验使我们能够描述一个种群随着时间的推移适应的速度有多快，在典型的一轮适应中涉及多少突变，以及如果同一种群多次适应相同的压力环境，我们预计会有多少不同的结果。然而，在实验室和自然灾害之外，很少有环境变化涉及从一个相对稳定的环境突然过渡到另一个完全不同的稳定环境。相反，随着时间的推移，环境往往会逐渐改变，因此大多数种群所处的环境与最近的祖先只有轻微的不同，尽管它可能与更遥远的祖先有很大的不同。全球变化就是一个例子，植物种群目前暴露在二氧化碳水平下的程度是一万年前最后一次冰期的两倍多，但只比十年前高几个百分点。因此，在任何给定的时间，人口都在适应环境的微妙变化，但在他们适应的过程中，环境并没有保持不变。这表明，对适应的研究应该同时考虑环境变化的幅度和速度。我的研究使用实验室实验、计算机模拟、数学模型和对自然种群的研究来研究大量的单细胞藻类是如何对不同的环境变化速度做出反应的，无论是单独的还是在群体中。我已经表明，当环境变化缓慢时，大型微生物种群能够变得更加适应，并且适应的结果随着环境变化的速度而不同。这里提出的工作进化短的寡核苷酸（DNA）在不同的环境变化速率下进行数百轮复制。这使我能够跟踪自然选择对新的有益突变的固定作用。在此过程中，我将提供一个一般的机制（遗传学）解释，说明较慢的环境变化速度如何影响适应。这项研究将之前从适应性变化的角度描述适应性变化的工作联系在一起，并为生物学中最基本的过程之一——适应提供了见解。这项研究的结果将帮助我们更好地了解大型微生物种群，如海洋浮游植物，如何对全球变化做出反应，也将有助于将实验室模型系统中获得的结果与自然种群中发生的反应联系起来。更一般地说，了解不同的环境变化速率如何影响适应将有助于我们解释浮游植物种群中发生的遗传变化的历史数据，以响应先前的冰期-间冰期旋回和其他环境变化，并为我们提供适应如何发生的更现实的一般描述。

The observation that organisms are adapted to their environment is obvious, yet we can only explain how this occurs in extreme scenarios such as the evolution of antibiotic and pesticide resistance, heavy metal tolerance, and starvation. Typical studies that aim to understand how organisms adapt following an environmental change suddenly place populations in a new stressful environment. For example, a bacterial population may be transferred from a nutrient-rich environment to one where a particular nutrient is nearly absent. The population then adapts by the sequential fixation of novel mutations that increase its growth and reproduction in the new environment. Theory and experiments that use this framework have allowed us to describe how fast a population adapts over time, how many mutations are involved in a typical round of adaptation, and how many different outcomes we expect if the same population adapts to the same stressful environment many times. However, very few environmental changes outside of laboratories and natural disasters involve the sudden transition from one relatively stable environment to a second, drastically different, stable environment. Instead, environments tend to change gradually over time, such that most populations exist in an environment that is only slightly different from that of a recent ancestor, even though it may differ substantially from a more distant ancestor. Global change is an example of this, where plant populations are currently exposed to levels of carbon dioxide more than twice as high as those of the last glaciation 10,000 years ago, but only a few percent higher than those of a decade ago. Thus, at any given time, populations are adapting to a subtle shift in environment, but the environment does not hold still while they do it. This suggests that studies of adaptation should incorporate both the magnitude and rate of environmental change. My research uses laboratory experiments, computer simulations, mathematical models and studies of natural populations to examine how large populations of single-celled algae respond to different rates of environmental change, either alone or in communities. I have already shown that large microbial populations are able to become more adapted when the environment changes slowly, and that the outcomes of adaptation differ with the rate of environmental change. The work proposed here evolves short oligonucleotides (DNA) for hundreds of rounds of replication at different rates of environmental change. This allows me to follow the fixation of novel beneficial mutations by natural selection. In doing so, I will provide a general mechanistic (genetic) explanation of how slower rates of environmental change affect adaptation. This work ties together previous work that described adaptive change in terms of changes in fitness and provides insight into one of the most fundamental processes in biology, that of adaptation. The results of this research will help us to understand better how large microbial populations, such as marine phytoplankton, may respond to global change, and will also help link results obtained in laboratory model systems to responses that occur in natural populations. More generally, understanding how different rates of environmental change affect adaptation will help us to interpret historical data on genetic changes that occurred in phytoplankton populations in response to previous glacial-interglacial cycles and other environmental shifts, as well as give us a more realistic general description of how adaptation occurs.