Ellner Stephen P, Hairston Nelson G, Kearns Colleen M, Babaï Dariouche
Section of Ecology and Systematics, Corson Hall, Cornell University, Ithaca, New York, 14853.
Biomathematics Graduate Program, Department of Statistics, North Carolina State University, Raleigh, North Carolina, 27695-8203.
Evolution. 1999 Feb;53(1):111-122. doi: 10.1111/j.1558-5646.1999.tb05337.x.
Direct observations of selection response in natural, unmanipulated populations in the wild are rare. Those that exist have resulted from major changes in environment during an ongoing study. Selection response should be more common and more readily observable in short-lived organisms where the direction of selection changes from year to year. We examined how the interaction of fluctuating selection, and emergence from long-term diapause, caused ongoing microevolutionary change over eight years in an important life-history trait (diapause timing) in the freshwater calanoid copepod Diaptomus sanguineus. Emergence from long-term diapause releases into the population lineages that did not experience the most recent bout of selection, thereby promoting the maintenance of the heritable trait variation that allows continual selection response. A mechanistic selection model was created on the basis of field and laboratory studies to predict how interannual variations in predation intensity generate year-to-year changes in mean diapause timing and in net reproductive success for alternate trait values. The predicted selection response and the estimated effect of emergence from diapause were both significantly correlated with observed changes in trait mean. A linear model combining selection response and emergence from diapause explained 59% of the variance in year-to-year changes in trait mean. According to this model, strong selection occurred in about half of the years studied, and the average annual contributions to changes in trait mean from selection and emergence were roughly equal. Thus, both fluctuating natural selection and emergence from prolonged diapause affect the expression of diapause timing by D. sanguineus. Fluctuating selection is ubiquitous in nature and may provide opportunities in other populations to witness ongoing natural selection without directional trends in mean phenotype.
在自然状态下、未受人工干预的野生种群中，对选择响应进行直接观测的情况十分罕见。现有的此类观测结果均来自于在一项正在进行的研究期间环境发生的重大变化。在短期生物中，选择响应应该更为常见且更容易被观察到，因为这些生物的选择方向每年都会发生变化。我们研究了波动选择与长期滞育后的羽化之间的相互作用，是如何在长达八年的时间里，导致淡水哲水蚤血红哲水蚤（Diaptomus sanguineus）一个重要生活史性状（滞育时间）发生持续的微进化变化的。长期滞育后的羽化会使种群谱系中那些未经历最近一轮选择的个体得以补充进来，从而促进可遗传性状变异的维持，进而实现持续的选择响应。基于野外和实验室研究，构建了一个机制性选择模型，以预测捕食强度的年际变化如何导致平均滞育时间以及不同性状值的净繁殖成功率逐年发生变化。预测的选择响应以及滞育羽化的估计效应均与性状均值的观测变化显著相关。一个结合了选择响应和滞育羽化的线性模型解释了性状均值逐年变化中59%的方差。根据该模型，在所研究的年份中约有一半发生了强烈选择，而且选择和羽化对性状均值变化的年均贡献大致相等。因此，波动的自然选择和长期滞育后的羽化都会影响血红哲水蚤滞育时间的表达。波动选择在自然界中无处不在，可能会为其他种群提供机会，见证持续的自然选择，而无需考虑平均表型的定向趋势。
