如果说自然选择既是普遍的又是无情的，那么群体中的差异又是如何维持的呢？
演化生物学中这一迫切需要回答的问题，由Kimberly Hughes及同事在对虹鳉(孔雀鱼)所做的一项研究中得到了回答。
虹鳉是一个特别有用的模型系统，因为雄性虹鳉的颜色是已知遗传变异性最大的生物性状之一。以前的研究表明，在一个由颜色鲜艳的虹鳉组成的种群中，出乎意料的是，具有罕见颜色图案的雄性往往比较兴旺。
Hughes等人利用精心控制的野生种群发现，不仅雌性虹鳉喜欢与这些罕见雄性交配，而且这些雄性也比那些身上条纹比较普通的雄性能生出更多后代。
这一现象（被称为“负频率依赖性选择”）表明与众不同是有价可循的，同时也为种群中差异性的维持提供了一个机制。（生物谷Bioon.com）
生物谷推荐的英文摘要
Nature doi:10.1038/nature12717
Mating advantage for rare males in wild guppy populations
Kimberly A. Hughes，Anne E. Houde，Anna C. Price& F. Helen Rodd
To understand the processes that maintain genetic diversity is a long-standing challenge in evolutionary biology， with implications for predicting disease resistance， response to environmental change， and population persistence1， 2， 3. Simple population genetic models are not sufficient to explain the high levels of genetic diversity sometimes observed in ecologically important traits2. In guppies (Poecilia reticulata)， male colour pattern is both diverse and heritable， and is arguably one of the most extreme examples of morphological polymorphism known4， 5. Negative frequency-dependent selection (NFDS)， a form of selection in which genotypes are favoured when they are rare6， can potentially maintain such extensive polymorphism， but few experimental studies have confirmed its operation in nature7， 8. Here we use highly replicated experimental manipulations of natural populations to show that males with rare colour patterns have higher reproductive fitness， demonstrating NFDS mediated by sexual selection. Rare males acquired more mates and sired more offspring compared to common males and， as previously reported， had higher rates of survival8. Orange colour， implicated in other studies of sexual selection in guppies， did predict male reproductive success， but only in one of three populations. These data support the hypothesis that NFDS maintains diversity in the colour patterns of male guppies through two selective agents， mates and predators. Similar field-based manipulations of genotype frequencies could provide a powerful approach to reveal the underlying ecological and behavioural mechanisms that maintain genetic and phenotypic diversity.
