经济学人下载:性选择和物种形成(1)

Sexual selection and speciation 

性选择和物种形成

Choice and determinism 

选择和决定论

Just four genes separate two species of South American bird 

南美两种鸟类之间只有四种基因的区别

Natural selection, as propounded in Charles Darwin's master work, "On the Origin of Species", explains how organisms evolve and adapt to their circumstances. Paradoxically, though, it is a bit hazy on the actual subject of its title, namely how parent species spin off new, daughter species. Darwin recognised that diverse ecological niches encourage speciation (though he did not use those as-yet-uninvented terms). But he did not ask how incipient daughter species were prevented from remixing before they had properly separated. 

“自然选择”由查尔斯·达尔文在其著作《物种起源》中提出，解释了生物如何进化并适应环境。然而矛盾的是，它对标题的实际主题——即母物种是如何衍生出新子代物种——有点含糊不清。达尔文认识到不同的生态位促进了物种形成(尽管他没有使用那些尚未发明的术语)，但他并没有对“如何阻止早期的子物种在它们完全分离之前重新混合”提出疑问。

Somehow, barriers need to be erected against miscegenation. They might be geographical—fish in different lakes, for example. Or they might be ecological, such as a shift in food preferences causing insects formerly of the same species to feed in different types of tree. But sometimes species form in circumstances where no such barriers are apparent. That is puzzling. 

无论如何，我们需要建立障碍来反对种族通婚。它们可能是地理上的——例如，不同的湖泊里的鱼。或者它们可能是生态的，比如食物偏好的改变导致原来同一物种的昆虫在不同类型的树上觅食。但有时物种是在没有明显障碍的情况下形成的。这是令人费解的。

One established exception to the rule that speciation requires separation is hybridisation. This is a common source of new plant species, but is rarer for animals. It is, though, thought to explain several brightly coloured animal groups, including the cichlid fish of Africa's Great Lakes, the heliconid butterflies of South and Central America, and the southern capuchino seedeaters, a family of songbirds that also live in the New World tropics. Now, Sheela Turbek of the University of Colorado, in Boulder, has taken matters a step further, by showing that a process called sexual selection plays a role in what is going on. 

杂交是“物种形成需要分离”这一规则的例外，是新植物物种的常见来源，但在动物中相对罕见。然而它被认为可以解释几种颜色鲜艳的动物群体，包括非洲五大湖的丽鲷科鱼类，南美洲和中美洲的螺旋形蝴蝶，以及南部卡普契诺食种子鸟(一种同样生活在新世界热带地区的鸣禽)。目前，来自科罗拉多大学(位于博尔德)的席拉·图尔贝克通过展示一种被称为性选择的过程在正在发生的事情中起著作用使得这一说法更进一步。

Picky, picky 

吹毛求疵

Sexual selection, a phenomenon first described scientifically by Darwin, occurs when one sex (usually the female) chooses another (usually the male) on the basis of a distinctive genetically derived characteristic. The classic example is a peacock's tail. But sexual selection could also serve to separate species in what is more-or-less a one-step process, by the sudden appearance of such a characteristic. And that, as she describes in a paper in Science, is what Ms Turbek thinks has happened in the case of the Ibera and tawny-bellied seedeaters. 

“性选择”是达尔文首次在科学上描述的一种现象，在一个性别(通常是女性)基于一种独特的遗传特征来选择另一个性别(通常是男性)之时出现。孔雀的尾巴就是比较经典的例子。但是性选择也可以通过这种特征的突然出现，在一个或多或少是一步的过程中分离物种。正如图贝克女士在《科学》杂志上的一篇论文中所描述的那样，这就是她认为发生在伊比利亚和茶色腹食种子动物身上的情况。

These two birds live, among other places, in the San Nicolas Reserve, in Argentina. They forage on the same grasses, breed at the same time, and have breeding territories which may be as close as 50 metres from one occupied by the other species. They are also similar enough to interbreed successfully in captivity. But, as far as is known, they never do so in the wild. 

这两种鸟生活在阿根廷的圣尼古拉斯保护区。它们在相同的草地上觅食，在同一时间繁殖，它们的繁殖领地大概离其他物种的领地有近50米。它们的相似程度也足以在圈养环境中成功杂交。但据我们所知，野外的它们从不进行杂交。