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Science and technology
科技版塊
Evolution in action
行為影響的進化論
Spoils of war
“戰利品”
How tuskless elephants evolved in Mozambique
莫桑比克的無象牙大象是如何進化的
EVOLUTION ENSURES that animals are well-adapted to their circumstances.
進化可以確保動物能很好地適應它們的環境。
Sometimes, as with predators and prey, those circumstances include the behaviour of other creatures.
有時，就像捕食者和獵物一樣，這些情況也包括其他生物的行為。
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.
正如剛剛發表在《科學》雜志上的一篇論文所描述的那樣，人類的行為可以迫使一個物種在進化的瞬間發生巨大變化。
Shane Campbell-Staton, a biologist at Princeton University, studies how animals adapt to human creations like cities and pollution.
普林斯頓大學的生物學家坎貝爾-斯塔頓研究動物如何適應人類的創造，如城市和污染。
His interest was piqued by a film about the tuskless female elephants of Gorongosa National Park, in Mozambique.
引起他興趣的是一部關于莫桑比克戈龍戈薩國家公園里無象牙母象的電影。
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.
他們缺乏象牙被認為是另一個人類創造的結果——莫桑比克內戰，這場內戰從1977年持續到1992年，部分費用來自于獵殺大象獲得象牙。
Around 90% of the pachyderms living in Gorongosa are thought to have been killed.
大約90%生活在戈龍戈薩的厚皮動物被認為已經被殺死。
Biologists therefore wondered if rising tusklessness might be an adaptation to make elephants less attractive to human hunters.
因此，生物學家想知道，是不是越來越多的無牙是一種適應，因為可以降低大象對人類獵人的吸引力。
It was a plausible theory, says Dr Campbell-Staton, but no one had actually tested it.
坎貝爾-斯塔頓博士說，這是一個貌似合理的理論，但沒有人真正驗證過它。
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.
通過老視頻和相關調查，他和他的同事們得出結論，內戰前大約18%的戈龍戈薩母象缺少象牙。
Three decades later, after it was over, that number had risen to 50%.
三十年后，內戰結束，無象牙母象的數量上升到了50%。
Computer simulations suggested that the likelihood of such a rapid change happening by chance, even in a diminished population, was tiny.
計算機模擬表明，即使在人口減少的情況下，如此迅速的變化偶然發生的可能性很小。
Besides confirming the change, the researchers managed to unravel its genetic roots.
除了證實這一變化外，研究人員還設法揭開了它的基因根源。
Tusklessness is caused by a mutation in a gene on the elephantine X chromosome.
無牙是由大象X染色體上的一個基因突變引起的。
(As with humans, two X chromosomes make a female, while an X and a Y make a male.)
（和人類一樣，兩條X染色體構成雌性，而X和Y染色體構成雄性。）
Unfortunately for males, the mutation is a package deal, coming with changes to nearby genes that interfere with embryonic development.
不幸的是，對于雄性來說，這種突變是一攬子交易，伴隨而來的是干擾胚胎發育的鄰近基因的變化。
Males who inherit the mutant gene die before birth.
繼承突變基因的雄性在出生前就會死亡。
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.
如果雌性的兩條X染色體中有一條含有未突變的基因，則可以避免致命的副作用——但她們長大后仍然沒有象牙。
Fortunately for the females, the specifics of how the mutant gene is inherited make it impossible for them to inherit two copies.
幸運的是，對于雌性來說，突變基因遺傳的具體方式使她們不可能繼承兩個副本。
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.
由于突變的雄性在出生前死亡，那些活到育齡的雄性只攜帶非突變版本的X染色體，這確保了他們的女兒也會至少有一個副本。
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，來自雄性的非突變X染色體的不斷重新引入限制了無牙現象在雌性群體中傳播的程度。
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，坎貝爾-斯塔頓博士說，經過一定的時間和基因重組，進化可能會將無牙突變從鄰近基因的不適應突變中解脫出來，也為雄性打開無牙的大門。
There are occasional rumours, he says, of tuskless male elephants in the wild, but—so far at least—no firm evidence.
他說，偶爾會有傳言說野外有無牙公象，但至少到目前為止還沒有確鑿的證據。
Finding one now seems unlikely.
現在看來似乎不太可能找到。
With the war over, the evolutionary pressure from poaching has eased.
隨著戰爭的結束，偷獵帶來的進化壓力有所緩解。
Tusks have gone back to being useful tools, helping their owners strip bark from trees and dig for water.
象牙已經重新成為有用的工具，幫助它們的主人剝樹皮和挖洞取水。
In recent years the prevalence of tuskless females has fallen, to around 33%.
近年來，無牙母象的流行率已經下降到33%左右。
But the speed of the change is a reminder that wars can alter evolutionary history as well as the human sort.
但變化的速度提醒人們，戰爭不僅可以改變人類，也可以改變進化史。
