变动环境中达尔文雀终身适应性形成的原因

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Causes of lifetime fitness of Darwin's finches in a fluctuating environment

变动环境中达尔文雀终身适应性形成的原因

最新一期的PNAS（2011年1月11日）上报道了格兰特夫妇最新的研究成果，他们在加拉帕戈斯群岛上进行的经典进化论实验已经持续了37年,他们仍然在寻找达尔文雀身上进化的证据。

进化是由适应性这个特征来衡量的，就个体来说，适应性就是指他存活和产生后代的能力，一般是以个体对下一代的贡献---产生后代的个数---来衡量的。这个定义给格兰特夫妇带了很大的困扰。因为按照这个定义，只要能产生更多的后代，完全相反的特征如快-慢、大-小都可以表示适应。甚至能产生大量后代（比如昆虫）或者仅仅哺育非常少后代（如大熊猫）都可以称为具有“适应性”。

在这篇文章中,研究者摒弃了形态特征作为适应性的测度标准,而选择用“终生适应性”---一个个体在其一生中产生了多少后代---来定义个体的适应性。

为了调查终身适应性的决定性因素，格兰特夫妇以加拉帕戈斯群岛上两个长寿的一夫一妻制达尔文雀---勇地雀(Geospiza fortis) 和仙人掌地雀(G. scandens)---的四个种群为对象，研究了他们的寿命、后代的数量、终生的配偶数。回归分析表明终身产生的子代数量可以由终生窝卵数预测，而终生适应性可以由终身产生的子代数量和寿命预测。对雌性择偶这一假说有很少的证据支持。偶外交配产生的后代没有比正常配偶产生的后代具有更高的适应性。

在热带栖息地的季节性、海拔、结构和气候都变动极大的情况下，生物学上的成功由两个部分构成。首先是鸟类在旱季非繁殖季，当食物匮乏时能寻找到食物---种子。第二个部分是在繁殖期寻找食物---昆虫和蜘蛛，以及避免入侵者对鸟巢的干扰。这两个部分对鸟类行为和生理特征方面的要求是两套不同的系统，所以需要进一步的研究来明确适应性是如何最大化的。

这些研究可能会就繁殖投入与成鸟存活率之间的权衡提出一些新的理解。

链接: http://www.pnas.org/content/108/2/674.abstract

摘要：

The genetic basis of variation in fitness of many organisms has been studied in the laboratory, but relatively little is known of fitness variation in natural environments or its causes. Lifetime fitness (recruitment) may be determined solely by producing many offspring, modified by stochastic effects on their subsequent survival up to the point of breeding, or by an additional contribution made by the high quality of the offspring owing to nonrandom mate choice. To investigate the determinants of lifetime fitness, we measured offspring production, longevity, and lifetime number of mates in four cohorts of two long-lived species of socially monogamous Darwin's finch species, Geospiza fortis and G. scandens, on the equatorial Galápagos Island of Daphne Major. Regression analysis showed that the lifetime production of fledglings was predicted by lifetime number of clutches and that recruitment was predicted by lifetime number of fledglings and longevity. There was little support for a hypothesis of selective mating by females. The offspring sired by extrapair mates were no more fit in terms of recruitment than were half-sibs sired by social mates. These findings provide insight into the evolution of life history strategies of tropical birds. Darwin's finches deviate from the standard tropical pattern of a slow pace of life by combining tropical (long lifespan) and temperate (large clutch size) characteristics. Our study of fitness shows why this is so in terms of selective pressures (fledgling production and adult longevity) and ecological opportunities (pulsed food supply and relatively low predation).