生物学中最古老的问题之一是，为什么存在不同性别？通过异体受精繁殖的动物要多于通过自体受精繁殖的动物，尽管生出雄性后代及寻找交配对象都是有代价的。人们通常认为，异体受精或异型杂交具有可能超过其代价的两个优势：快速演化的能力和近亲繁殖的避免，但对此很难进行实验验证。
现在，使线虫的自体繁殖变种和异体繁殖变种置于选择压力之下的一个实验，为以上观点提供了一个实用的验证，而且该实验还证明，以上两个潜在的标准解释（即两个优势）在促进异型杂交中似乎都扮演一个角色。（生物谷Bioon.com）
生物谷推荐原始出处：
Nature 462, 350-352 (19 November 2009) | doi:10.1038/nature08496
Mutation load and rapid adaptation favour outcrossing over self-fertilization
Levi T. Morran1, Michelle D. Parmenter1 & Patrick C. Phillips1
Center for Ecology & Evolutionary Biology, 5289 University of Oregon, Eugene, Oregon 97403-5289, USA
Correspondence to: Patrick C. Phillips1 Correspondence and requests for materials should be addressed to P.C.P.
The tendency of organisms to reproduce by cross-fertilization despite numerous disadvantages relative to self-fertilization is one of the oldest puzzles in evolutionary biology. For many species, the primary obstacle to the evolution of outcrossing is the cost of production of males1, individuals that do not directly contribute offspring and thus diminish the long-term reproductive output of a lineage. Self-fertilizing ('selfing') organisms do not incur the cost of males and therefore should possess at least a twofold numerical advantage over most outcrossing organisms2. Two competing explanations for the widespread prevalence of outcrossing in nature despite this inherent disadvantage are the avoidance of inbreeding depression generated by selfing3, 4, 5 and the ability of outcrossing populations to adapt more rapidly to environmental change1, 6, 7. Here we show that outcrossing is favoured in populations of Caenorhabditis elegans subject to experimental evolution both under conditions of increased mutation rate and during adaptation to a novel environment. In general, fitness increased with increasing rates of outcrossing. Thus, each of the standard explanations for the maintenance of outcrossing are correct, and it is likely that outcrossing is the predominant mode of reproduction in most species because it is favoured under ecological conditions that are ubiquitous in natural environments.
