女性到了一定的年龄都会绝经（menopause），这种现象一直以来是个进化谜题。英国科学家近日研究认为，绝经是为了最小化同一家庭单位内不同代女性之间的生殖竞争。相关研究论文3月31日在线发表于美国《国家科学院院刊》（PNAS）上。
50年前，美国进化生物学家George Williams针对女性停经的现象提出了“祖母假设”（grandmother hypothesis），即如果老年的不育女性能够帮助她们的子女存活和生殖，那么自然选择会提高她们生殖后的存活率。
然而，这一理论存在说不通的地方。自然生育（natural fertility，不借助现代医学或技术的生育）的数据显示，祖母哺育（grandmothering）带来的好处太小，女性不该因此在50岁时关闭生殖能力。所以，虽然祖母假设能够解释为什么女性在绝经后仍然能够存活，却不能解释女性为什么绝经。
在最新的研究中，英国剑桥大学的Rufus Johnstone和埃克塞特大学的Michael Cant分析认为，人类的生殖活动非常特殊，生殖代之间几乎没有重叠。在自然生育的群体中，女性平均19岁时生第一个孩子，平均38岁时生最后一个孩子。换句话说，在下一代开始生育时，上一代正好停止了生育。
此外，研究人员还分析了女性分散系统（female-dispersal system）中的生殖竞争。在这种系统中，生殖竞争包括“婆婆”与“儿媳”之间的竞争，而年轻女性具有决定性的优势。研究人员针对这种竞争建立了一个数学模型指出，当同一社会单位里的年轻女性开始生育时，年长的女性应该停止生育。
这一模型解释了人类的绝经现象，也有助于更好理解绝经的进化。不论医学水平或其它条件如何，各种社会的女性都会经历绝经。这表明作为进化的后果，人类的生育时间表同遗传结构是“硬”连接的。
Johnstone表示，“此次结果为研究人类绝经和生育开启了新的途径，并为理解其它物种（如虎鲸和巨头鲸）发生在自然条件下的绝经现象提供了新的视角。”（科学网 梅进/编译）
生物谷推荐原始出处：
（PNAS），doi：10.1073/pnas.0711911105，Michael A. Cant，Rufus A. Johnstone
Reproductive conflict and the separation of reproductive generations in humans
Michael A. Cant*, and Rufus A. Johnstone
*Centre for Ecology and Conservation, University of Exeter, Cornwall Campus, Penryn, Cornwall TR10 9EZ, United Kingdom; and Department of Zoology, University of Cambridge, Cambridge CB2 3EJ, United Kingdom
Edited by Gordon H. Orians, University of Washington, Seattle, WA, and approved February 14, 2008 (received for review December 18, 2007)
Abstract
An enduring puzzle of human life history is why women cease reproduction midway through life. Selection can favor postreproductive survival because older females can help their offspring to reproduce. But the kin-selected fitness gains of helping appear insufficient to outweigh the potential benefits of continued reproduction. Why then do women cease reproduction in the first place? Here, we suggest that early reproductive cessation in humans is the outcome of reproductive competition between generations, and we present a simple candidate model of how this competition will be resolved. We show that among primates exhibiting a postreproductive life span, humans exhibit an extraordinarily low degree of reproductive overlap between generations. The rapid senescence of the human female reproductive system coincides with the age at which, in natural fertility populations, women are expected to encounter reproductive competition from breeding females of the next generation. Several lines of evidence suggest that in ancestral hominids, this younger generation typically comprised immigrant females. In these circumstances, relatedness asymmetries within families are predicted to give younger females a decisive advantage in reproductive conflict with older females. A model incorporating both the costs of reproductive competition and the benefits of grandmothering can account for the timing of reproductive cessation in humans and so offers an improved understanding of the evolution of menopause.
