How genetic polymorphisms are maintained in a population is a key question in evolutionary ecology. Previous work on a plumage colour polymorphism in the common buzzard Buteo buteo suggested heterozygote advantage as the mechanism maintaining the co‐existence of three morphs (light, intermediate and dark). We took advantage of 20 years of life‐history data collected in a Dutch population to replicate earlier studies on the relationship between colour morph and fitness in this species. We examined differences between morphs in adult apparent survival, breeding success, annual number of fledglings produced and cumulative reproductive success. We found that cumulative reproductive success differed among morphs, with the intermediate morph having highest fitness. We also found assortative mating for colour morph, whereby assortative pairs were more likely to produce offspring and had longer‐lasting pair bonds than disassortative pairs. Over the 20‐year study period, the proportion of individuals with an intermediate morph increased. This apparent evolutionary change did not just arise from selection on individual phenotypes, but also from fitness benefits of assortative mating. The increased frequency of intermediates might also be due to immigration or drift. We hypothesize that genetic variation is maintained through spatial variation in selection pressures. Further studies should investigate morph‐dependent dispersal behaviour and habitat choice.

中文翻译：

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荷兰常见秃鹰 Buteo buteo 种群的变形依赖性适应度和变形频率随时间的方向变化

如何在种群中维持遗传多态性是进化生态学中的一个关键问题。先前对普通秃鹰 Buteo buteo 羽毛颜色多态性的研究表明，杂合子优势是维持三种形态（浅色、中色和深色）共存的机制。我们利用在荷兰人群中收集的 20 年生活史数据复制了早期关于该物种颜色变形与适应性之间关系的研究。我们检查了成虫表观存活率、繁殖成功率、年产雏鸟数量和累积繁殖成功率方面的变体之间的差异。我们发现不同变种的累积繁殖成功率不同，中间变种的适应度最高。我们还发现了颜色变形的分类交配，因此，与非分类对相比，组合配对更有可能产生后代并且具有更持久的配对键。在 20 年的研究期间，具有中间变形的个体比例增加。这种明显的进化变化不仅源于对个体表型的选择，还源于组合交配的适应性益处。中间体频率增加也可能是由于移民或漂移。我们假设遗传变异是通过选择压力的空间变异来维持的。进一步的研究应该调查依赖于形态的扩散行为和栖息地选择。这种明显的进化变化不仅源于对个体表型的选择，还源于组合交配的适应性益处。中间体频率增加也可能是由于移民或漂移。我们假设遗传变异是通过选择压力的空间变异来维持的。进一步的研究应该调查依赖于形态的扩散行为和栖息地选择。这种明显的进化变化不仅源于对个体表型的选择，还源于组合交配的适应性益处。中间体频率增加也可能是由于移民或漂移。我们假设遗传变异是通过选择压力的空间变异来维持的。进一步的研究应该调查依赖于形态的扩散行为和栖息地选择。