Populations of generalist grazers often contain genotypes with "powerful" and "efficient" strategies. Powerful genotypes grow rapidly on rich-quality resources, but slowly on poorer-quality ones, while efficient genotypes grow relatively better on poorer resources but cannot exploit richer resources as well. Via a "power-efficiency" trade-off, variation in resource quality could maintain genetic diversity. To evaluate this mechanism, we sampled six populations of the freshwater cladoceran Daphnia pulicaria. In persisting (year-round) populations, Daphnia consume resources that vary in quality, whereas in non-persisting (spring-only) populations, Daphnia primarily encounter rich-quality resources. We hypothesized that non-persisting populations harbor no efficient clones (hence should show lower growth on poor-quality resources). Although individuals from non-persisting populations remained smaller than individuals from persisting populations, no evidence arose for a trade-off between powerful and efficient strategies. In fact, growth rates on the two diets were positively correlated (instead of negatively, as predicted). Furthermore, in the persisting populations, we predicted that clonal selection from spring to summer should shift the distribution of genotypes from powerful (specialists on richer spring resources) to efficient (poorer, summer resources). Genetic composition of populations shifted from spring to summer, but not toward more efficient genotypes. Therefore, in these lakes, maintenance of variation among genotypes must stem from more complicated factors than population persistence patterns or seasonal shifts in resource quality alone.

中文翻译：

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资源使用的种内差异无法通过人口持续性或季节性来解释。

通才放牧者的种群通常包含具有“强大”和“有效”策略的基因型。强大的基因型在优质资源上快速增长，而在劣质资源上缓慢增长，而有效基因型在较贫困资源上增长相对较好，但也无法利用较丰富的资源。通过“功率效率”的权衡，资源质量的变化可以维持遗传多样性。为了评估这种机制，我们对淡水枝角类水蚤Daphnia pulicaria的六个种群进行了采样。在持续存在的种群（全年）中，水蚤消耗质量不同的资源，而在非持续生存的种群（仅春季）中，水蚤主要遇到质量较高的资源。我们假设非持续性种群没有有效的克隆（因此，在劣质资源上应表现出较低的增长）。尽管非持久人口的个体比持久人口的个体小，但没有证据表明有力的策略与有效的策略之间需要权衡取舍。实际上，两种饮食的增长率呈正相关（而不是如预期的负相关）。此外，在持续存在的人群中，我们预测春季至夏季的克隆选择应将基因型的分布从强大的（春季资源丰富的专家）转移到高效的（夏季资源较差）。种群的遗传组成从春季转移到夏季，但没有向更有效的基因型转移。因此，在这些湖泊中，