When we think of evolution, we tend to picture slow changes occurring over very long periods of time, typically millions of years. However, evolution can actually happen much faster, over only a few generations: think COVID-19 strains, for example. And this fast evolution isn’t just restricted to viruses and microbes.
當我們思考演化時，傾向於想像，在通常是數百萬年之很長一段時間內，發生的緩慢變化。然而，演化實際上能發生得更快，僅在幾代內：譬如，思索2019冠狀病毒症(COVID-19：Coronavirus Disease-19)的病毒株。而這種快速演化，不只限於病毒及微生物。
Our study, recently published in Evolutionary Biology, documents rapid evolution in tiger snakes (Notechis scutatus). In less than a century, these snakes evolved the ability to swallow whole seagull chicks, allowing them to survive on a tiny island.
我們最近發表於，歐洲演化生物學學會發行之《演化生物學》期刊的研究，記實性描述了，於虎蛇(Notechis Scutus)中的快速演化。在不到一個世紀中，此些蛇演化出，吞嚥整隻海鷗雛鳥的能力，使其得以在一處小島上存活下來。
Most Australians are familiar with the tiger snake, one of our most iconic animals. The snakes in our study were from Carnac Island, a tiny islet off the coast of Perth in Western Australia. They were introduced here less than a century ago, possibly dumped by a travelling snake performer to avoid trouble with the law.
大多數澳大利亞人熟悉虎蛇，這是我們最標誌性的動物之一。這些在我們研究中的蛇，來自西澳大利亞珀斯海岸外的一個小島─卡納克島。在不到一個世紀前，它們可能是一位巡迴表演蛇的人，為了避免觸犯法律而拋棄，才被引進到這裡。
These marooned tiger snakes are now adept at swallowing the large chicks of the seagulls that nest there. Adult snakes have little else to eat on this island in order to survive and reproduce.
目前，這些被隔絕於孤島上的虎蛇，擅長吞嚥築巢於那裡的海鷗大雛鳥。在這個島上，為了存活及繁殖，成年的蛇幾乎沒有其他食物可吃。
But how did they adapt so well, and so quickly? Tiger snakes on the mainland (the ancestral stock of the snakes on Carnac Island) typically feed on much smaller creatures, especially frogs.
不過，它們如何適應得這麼好，且這麼快？在此大陸上的虎蛇(此些於卡納克島上之蛇的原種蛇群)，通常以小得多的生物為食，特別是青蛙。
圖1. 虎蛇的頭部能快速演化，來對以大型獵物為食作出反應。
The heads of tiger snakes can evolve rapidly in response to feeding on large prey.
The secret is a process called “phenotypic plasticity”, which is a phenomenon where an organism physically changes its body within its own lifetime to match the environment it is experiencing.
秘密是一種被稱為“表現型可塑性”的過程。這是一種，生物在其一生中，徹底改變其身體，來適應正經歷之環境的現象。
Myriad examples exist: from water fleas (Daphnia) that can develop a spiky “helmet” in the presence of predators, to the tadpoles of some spadefoot toads (Spea), which morph from omnivore to carnivore bodies when exposed to environments packed with small prey (including other tadpoles!).
存在著無數例子：從面臨捕食者，能發展出一種尖頭“頭盔”的水蚤(Daphnia)，到當曝露於充滿小獵物(包括其他蝌蚪！)的環境時，能從雜食動物變成食肉動物身驅的鋤足蟾(Spea)蝌蚪。
Our research team took a detailed look at how the snakes from Carnac Island have adapted to survive there. We raised some baby snakes taken from the island and some from the nearby mainland, and divided them further into two groups – one group fed on small mice, and the other on large mice.
我們的研究團隊詳細探究了，此些來自卡納克島的蛇，如何已經適應，在那裡存活下來。我們飼養了，一些取自該島及附近大陸的小蛇，並進一步將它們分成兩群。一群以小的小老鼠為食，另一群以大的小鼠為食。
After the snakes matured, we looked into their heads using a high-resolution version of a medical CT scanner and took measurements of the different skull bones. This showed us how the island and mainland groups responded to different prey sizes.
在此些蛇成熟後，我們使用一種，高解析度版的醫用電腦斷層(CT：Computed Tomography)掃描儀，來檢視它們的頭部，並進行不同頭骨的測量。這向我們展現了，此島嶼及大陸群體，如何應對不同大小的獵物。
Mainland snakes always had the same head shape, regardless of whether they were fed small or large prey. But Carnac Island snakes showed an intriguing reponse: those fed large prey developed a much bigger bite, with longer jaw bones (especially in the lower jaw and palate).
大陸蛇，無論被餵食之獵物的大小，始終具有相同頭部形狀。不過，卡納克島的蛇展現了，引人好奇的反應：那些被餵食大獵物的蛇，發展出具有較長顎骨(特別是在下顎及上顎)之較大很多的咬合。
Previous studies had only looked at external head dimensions. We confirmed their conclusions and further revealed exactly which skull bones were responding to the drastic dietary change.
先前諸多研究，一直僅檢視頭外部尺寸。我們證實了他們的結論，並進一步確切揭露了，哪些顱骨對這種迅猛之飲食改變作出的反應。
圖2. 在來自西澳大利亞卡納克島的蛇，被餵食大型獵物一段長時間後，顯示骨骼長度增加之虎蛇頭骨的3D模型圖像：下顎骨(紅色)及上顎骨(黃色)。
A 3D model image of a tiger snake skull showing the bones that increase in length in the snakes from Carnac Island after being fed large prey for a prolonged period of time: lower jaws (red) and palate bones (yellow).
The explanation behind our results can be more easily understood if we compare our snakes to people at a gym – a human example of phenotypic plasticity. As all gym-goers know, some people can quickly build up muscle from weight training, the so-called easy-gainers (who doesn’t envy those!), while others struggle more – hard-gainers.
倘若將我們的蛇比喻作健身房裡的人(表現型可塑性的人類例子)，我們研究結果背後的解釋，能更容易被理解。因為所有去健身房的人皆知曉，有些人能從重量訓練快速增強肌肉。這是所謂的容易獲得者(誰不羨慕他們！)，而另一些人好不容易才達到更多，也就是難獲得者。
The snakes from Carnac Island are easy-gainers: they have a stronger intrinsic (genetic) capability to change their shape in response to a stimulus, in this case large prey items. This means they have a higher level of phenotypic plasticity.
來自卡納克島的蛇是容易獲得者：在對是大型獵物這種情況刺激下作出反應上，它們具有一種更強之本來具有的(遺傳)能力，來改變其形狀。這意味著，它們具有較高表現型可塑性的水平。
In any population, some individuals are more plastic than others. Of the tiger snakes marooned on Carnac Island and forced to exist on seagull chicks, the most “plastic” snakes would develop the biggest bites – and be more likely to survive and pass down their easy-gainer genes to their descendants.
在任何種群中，有些個體是比其他個體更可塑性的。在被隔絕於卡納克島，而被迫靠海鷗雛鳥生存的虎蛇中，最“可塑性”的蛇，能發展出最大的咬合。因此，更有可能存活下來，且將其容易獲得者的基因，傳遞給後代。
Over the course of a few generations (and less than 100 years), the entire population of tiger snakes would become easy-gainers.
在幾代(不到100年)的過程中，整個虎蛇種群會變成容易獲得者。
But evolution doesn’t stop there. Eventually, genetic mutations occur so that snakes are born with longer jaws; these would have a survival advantage even over the easy-gainers (who only develop longer jaws after “training”), and eventually replace them.
不過，演化並不止於此。最終，發生基因突變，因此出生具有較長顎的蛇；可能具有一種，甚至超越容易獲得者(在“受訓練”後，僅發展出較長的顎)的生存優勢，而最終取代它們。
Thus, a wide gape becomes fixed at birth. This phenomenon is called “trait canalization” or “assimilation”.
因此，在出生時，一張大的口成為定型的。這種現象被稱為“特徵渠道化”或“同化”。
In our study we observed examples of this as well: some skull bones were relatively longer in the Carnac Island snakes from birth, regardless of their diet.
在我們的研究中，也觀察到諸多此事的例子：在卡納克島的蛇中，不論其飲食，有些頭骨生而相對較長。
圖3. 來自南澳大利亞亞歷山德里納湖的虎蛇。不像來自卡納克島的蛇頭骨，生長出較長的下顎骨及上顎骨。於該大陸上的虎蛇頭骨，在長時間被迫以大型獵物為食時，沒有改變形狀。
Tiger snake from Lake Alexandrina in South Australia. The skulls of tiger snakes on the mainland don’t change shape when forced to feed on large prey for a prolonged period of time, unlike the skulls of snakes from Carnac Island, which grow longer jaws and palates.
So how much change has occurred in the Carnac Island snakes? We found that certain jaw bones can lengthen by about 5% simply as a result of feeding on large prey for a year.
那麼，在卡納克島的蛇曾發生多大的變化？我們發現，某些顎骨能僅因為以大型獵物為食一年，增長大約5%。
On top of this, the bones of the snakes from Carnac Island were also about 5% longer at birth than those in the mainland snakes (because of “trait canalisation” mentioned above). The island snakes are indeed on a rapid path to evolve very big jaws to effortlessly swallow baby seabirds.
除此之外，(因為，上面提及的“特徵渠道化”)來自卡納克島的蛇骨頭，在出生時，也比那些於大陸蛇中的骨頭長大約5%。該島上的蛇，確實以快速的途徑演化出，不用費力吞嚥幼海鳥的大顎。
Our study delves deeply into how rapid evolution occurs in complex organisms, and shows how diet has profound effects, even greatly affecting individual skull bones. It also reveals the resilience of tiger snakes, and how their “easy-gainer” bodies help them adapt almost instantaneously to extreme new environments.
我們的研究深入研究了，於複雜生物體中，快速演化如何發生，且證實了飲食如何具有深遠影響，甚至大大地影響個體頭骨。這也揭露了，虎蛇的適應力，及其“容易獲得者”的身體如何協助它們，幾乎瞬間適應極端的新環境。
This is one of the likely reasons tiger snakes have been so successful at colonising islands.
在移生島嶼中，這是虎蛇已經是如此成功的可能原因之一。
網址：https://theconversation.com/on-a-tiny-australian-island-snakes-feasting-on-seabirds-evolved-huge-jaws-in-a-surprisingly-short-time-197791
翻譯：許東榮
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