女性到了一定的年齡都會(huì)絕經(jīng)（menopause）,，這種現(xiàn)象一直以來是個(gè)進(jìn)化謎題。英國科學(xué)家近日研究認(rèn)為,，絕經(jīng)是為了最小化同一家庭單位內(nèi)不同代女性之間的生殖競爭,。相關(guān)研究論文3月31日在線發(fā)表于美國《國家科學(xué)院院刊》（PNAS）上。
50年前,，美國進(jìn)化生物學(xué)家George Williams針對(duì)女性停經(jīng)的現(xiàn)象提出了“祖母假設(shè)”（grandmother hypothesis）,，即如果老年的不育女性能夠幫助她們的子女存活和生殖，那么自然選擇會(huì)提高她們生殖后的存活率,。
然而,，這一理論存在說不通的地方。自然生育（natural fertility,，不借助現(xiàn)代醫(yī)學(xué)或技術(shù)的生育）的數(shù)據(jù)顯示,，祖母哺育（grandmothering）帶來的好處太小，女性不該因此在50歲時(shí)關(guān)閉生殖能力,。所以,，雖然祖母假設(shè)能夠解釋為什么女性在絕經(jīng)后仍然能夠存活，卻不能解釋女性為什么絕經(jīng),。
在最新的研究中,，英國劍橋大學(xué)的Rufus Johnstone和埃克塞特大學(xué)的Michael Cant分析認(rèn)為,，人類的生殖活動(dòng)非常特殊,，生殖代之間幾乎沒有重疊。在自然生育的群體中,，女性平均19歲時(shí)生第一個(gè)孩子,，平均38歲時(shí)生最后一個(gè)孩子。換句話說,，在下一代開始生育時(shí),，上一代正好停止了生育。
此外，研究人員還分析了女性分散系統(tǒng)（female-dispersal system）中的生殖競爭,。在這種系統(tǒng)中,，生殖競爭包括“婆婆”與“兒媳”之間的競爭，而年輕女性具有決定性的優(yōu)勢,。研究人員針對(duì)這種競爭建立了一個(gè)數(shù)學(xué)模型指出,，當(dāng)同一社會(huì)單位里的年輕女性開始生育時(shí)，年長的女性應(yīng)該停止生育,。
這一模型解釋了人類的絕經(jīng)現(xiàn)象,，也有助于更好理解絕經(jīng)的進(jìn)化。不論醫(yī)學(xué)水平或其它條件如何,，各種社會(huì)的女性都會(huì)經(jīng)歷絕經(jīng),。這表明作為進(jìn)化的后果，人類的生育時(shí)間表同遺傳結(jié)構(gòu)是“硬”連接的,。
Johnstone表示,，“此次結(jié)果為研究人類絕經(jīng)和生育開啟了新的途徑，并為理解其它物種（如虎鯨和巨頭鯨）發(fā)生在自然條件下的絕經(jīng)現(xiàn)象提供了新的視角,。”（科學(xué)網(wǎng) 梅進(jìn)/編譯）
生物谷推薦原始出處：
（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.
