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Science and technology
科技版块
Evolution in action
行为影响的进化论
Spoils of war
“战利品”
How tuskless elephants evolved in Mozambique
莫桑比克的无象牙大象是如何进化的
EVOLUTION ENSURES that animals are well-adapted to their circumstances.
进化可以确保动物能很好地适应它们的环境。
Sometimes, as with predators and prey, those circumstances include the behaviour of other creatures.
有时，就像捕食者和猎物一样，这些情况也包括其他生物的行为。
And, as a paper just published in Science describes, that includes the behaviour of human beings, which can force drastic changes on a species in an evolutionary eyeblink.
正如刚刚发表在《科学》杂志上的一篇论文所描述的那样，人类的行为可以迫使一个物种在进化的瞬间发生巨大变化。
Shane Campbell-Staton, a biologist at Princeton University, studies how animals adapt to human creations like cities and pollution.
普林斯顿大学的生物学家坎贝尔-斯塔顿研究动物如何适应人类的创造，如城市和污染。
His interest was piqued by a film about the tuskless female elephants of Gorongosa National Park, in Mozambique.
引起他兴趣的是一部关于莫桑比克戈龙戈萨国家公园里无象牙母象的电影。
Their lack of tusks was thought to be a consequence of another human creation—the Mozambican civil war, which lasted from 1977 to 1992 and was partly paid for by the killing of elephants for their ivory.
他们缺乏象牙被认为是另一个人类创造的结果——莫桑比克内战，这场内战从1977年持续到1992年，部分费用来自于猎杀大象获得象牙。
Around 90% of the pachyderms living in Gorongosa are thought to have been killed.
大约90%生活在戈龙戈萨的厚皮动物被认为已经被杀死。
Biologists therefore wondered if rising tusklessness might be an adaptation to make elephants less attractive to human hunters.
因此，生物学家想知道，是不是越来越多的无牙是一种适应，因为可以降低大象对人类猎人的吸引力。
It was a plausible theory, says Dr Campbell-Staton, but no one had actually tested it.
坎贝尔-斯塔顿博士说，这是一个貌似合理的理论，但没有人真正验证过它。
Through a mix of old video footage and surveys, he and his colleagues concluded that around 18% of the female elephants in Gorongosa lacked tusks before the war.
通过老视频和相关调查，他和他的同事们得出结论，内战前大约18%的戈龙戈萨母象缺少象牙。
Three decades later, after it was over, that number had risen to 50%.
三十年后，内战结束，无象牙母象的数量上升到了50%。
Computer simulations suggested that the likelihood of such a rapid change happening by chance, even in a diminished population, was tiny.
计算机模拟表明，即使在人口减少的情况下，如此迅速的变化偶然发生的可能性很小。
Besides confirming the change, the researchers managed to unravel its genetic roots.
除了证实这一变化外，研究人员还设法揭开了它的基因根源。
Tusklessness is caused by a mutation in a gene on the elephantine X chromosome.
无牙是由大象X染色体上的一个基因突变引起的。
(As with humans, two X chromosomes make a female, while an X and a Y make a male.)
（和人类一样，两条X染色体构成雌性，而X和Y染色体构成雄性。）
Unfortunately for males, the mutation is a package deal, coming with changes to nearby genes that interfere with embryonic development.
不幸的是，对于雄性来说，这种突变是一揽子交易，伴随而来的是干扰胚胎发育的邻近基因的变化。
Males who inherit the mutant gene die before birth.
继承突变基因的雄性在出生前就会死亡。
Females can avoid the lethal side-effects if one of their two X chromosomes contains a non-mutated gene—but they will still grow up without tusks.
如果雌性的两条X染色体中有一条含有未突变的基因，则可以避免致命的副作用——但她们长大后仍然没有象牙。
Fortunately for the females, the specifics of how the mutant gene is inherited make it impossible for them to inherit two copies.
幸运的是，对于雌性来说，突变基因遗传的具体方式使她们不可能继承两个副本。
Since mutant males die before being born, those which survive to reproductive age carry only non-mutated versions of the X chromosome, ensuring that their daughters will have at least one copy too.
由于突变的雄性在出生前死亡，那些活到育龄的雄性只携带非突变版本的X染色体，这确保了他们的女儿也会至少有一个副本。
At the moment, the continual reintroduction of non-mutant X chromosomes from males puts a limit on how far tusklessness can spread through the female population.
目前，来自雄性的非突变X染色体的不断重新引入限制了无牙现象在雌性群体中传播的程度。
But, given time and genetic recombination, says Dr Campbell-Staton, evolution might disentangle the mutation for tusklessness from the maladaptive mutations in its neighbouring genes, opening the door for males to shed their tusks, too.
但是，坎贝尔-斯塔顿博士说，经过一定的时间和基因重组，进化可能会将无牙突变从邻近基因的不适应突变中解脱出来，也为雄性打开无牙的大门。
There are occasional rumours, he says, of tuskless male elephants in the wild, but—so far at least—no firm evidence.
他说，偶尔会有传言说野外有无牙公象，但至少到目前为止还没有确凿的证据。
Finding one now seems unlikely.
现在看来似乎不太可能找到。
With the war over, the evolutionary pressure from poaching has eased.
随着战争的结束，偷猎带来的进化压力有所缓解。
Tusks have gone back to being useful tools, helping their owners strip bark from trees and dig for water.
象牙已经重新成为有用的工具，帮助它们的主人剥树皮和挖洞取水。
In recent years the prevalence of tuskless females has fallen, to around 33%.
近年来，无牙母象的流行率已经下降到33%左右。
But the speed of the change is a reminder that wars can alter evolutionary history as well as the human sort.
但变化的速度提醒人们，战争不仅可以改变人类，也可以改变进化史。
