不會(huì)飛的鳥兒何以遍布全球？

不同的物種——甚至在進(jìn)化上相距甚遠(yuǎn)的物種，如果生活在條件相同的環(huán)境中，有可能產(chǎn)生功能相同或十分相似的形態(tài)結(jié)構(gòu)以適應(yīng)環(huán)境。例如哺乳綱的鯨和海豚、爬行類的魚龍等由于長(zhǎng)期生活在水中，因而進(jìn)化出與魚類相似的體型;澳大利亞的袋食蟻獸、非洲的土豚、亞洲的穿山甲和南美洲的食蟻獸都具有相似的生活方式和適于捕食白蟻的相似生理結(jié)構(gòu)。此種現(xiàn)象被稱為趨同進(jìn)化（convergent evolution）。

Here’s a riddle： how did an order of flightless birds manage to spread the places they would have had to fly to？ Rheas live in South America， cassowaries and emus in Australia， kiwis in New Zealand and ostriches in Africa. They’re all related and they’re all land-dwelling. How does that work？

That’s kind of a tough one for evolutionary scientists. The dominant answer is that the birds spread to where they’re found today way back before the continents broke up. But DNA tests are starting to make the issue even more puzzling.

A new study from the University of Adelaide looked at the DNA of the elephant bird， one of the biggest birds to have ever existed. It lived on Madagascar and died out sometime in the last few hundred years.

The researchers were trying to work out whether the elephant bird was related to the moa， another feathered behemoth from New Zealand that died out around the same time. Instead， it was more closely related to New Zealand’s iconic kiwi. And it turns out the moa’s closest relative is another chicken-sized bird called the tinamou， which lives—get this—in South America. Another twist： some of them can even fly.

The study’s lead author Andrew Cooper told New Scientist， “ In both cases， the moa and the elephant bird， the nearest relative is on the other side of the world.”

Analysis of the DNA samples showed the elephant bird and the kiwi last shared a living relative around 50 million years ago—way after the continents had already split. So Cooper says the answer to how flightless birds spread all over the world is simple： they flew there.

That would make flightlessness an example of convergent evolution， when two species independently evolve the same traits， like hard shells in different kinds of pill bugs or fingerprints in humans and koalas.

So that’s probably how you solve the riddle of the flightless birds.

有這么一個(gè)謎：一系列不會(huì)飛的鳥類怎么會(huì)分布在它們要飛翔才能到達(dá)的各個(gè)地區(qū)的呢？美洲鴕生活在南美洲，食火雞和鴯鹋生活在澳大利亞，幾維鳥生活在新西蘭，而鴕鳥則生活在非洲。這幾種鳥都有親緣關(guān)系，而且都是陸棲走禽。怎么會(huì)出現(xiàn)這種現(xiàn)象呢？

對(duì)進(jìn)化學(xué)家們來說，這可是一道難題。主流說法是這些鳥類早在各大洲板塊分離之前便分布在今天人們發(fā)現(xiàn)它們的地方，但多項(xiàng)DNA測(cè)試又讓這個(gè)問題變得更加復(fù)雜難解。

（澳大利亞）阿德萊德大學(xué)對(duì)象鳥的DNA進(jìn)行了一項(xiàng)全新的研究。象鳥是世界上存活過的最大型的鳥類之一，一度生活在馬達(dá)加斯加，在近幾百年已經(jīng)滅絕。

研究人員試圖驗(yàn)證象鳥與恐鳥是否存在親緣關(guān)系——后者是分布在新西蘭的另一種長(zhǎng)著羽毛的巨鳥，象鳥與恐鳥在大約同一時(shí)期滅絕。研究結(jié)果表明，與象鳥有著更密切親緣關(guān)系的卻是新西蘭的標(biāo)志性走禽：幾維鳥。研究結(jié)果還顯示恐鳥血緣上最近的近親是一種只有家雞大小的鳥——共鳥 ，而它們呢——聽好了，生活在南美洲。還有一個(gè)意外發(fā)現(xiàn)：有些品種的共鳥 甚至還會(huì)飛。

該研究報(bào)告的主筆安德魯·庫(kù)珀告訴《新科學(xué)家》雜志說：“在這兩個(gè)研究案例中，恐鳥和象鳥血緣關(guān)系最近的近親都遠(yuǎn)在地球的另一邊。”

對(duì)DNA樣本的分析表明，象鳥與幾維鳥最后一次擁有一個(gè)共同的親戚是在大約五千萬年前，而那時(shí)各大陸早就已經(jīng)分離了。因此，庫(kù)珀認(rèn)為走禽能在世界各地分布的原因很簡(jiǎn)單：它們是飛過去的。

這一結(jié)論將使走禽特性成為趨同進(jìn)化的范例——在趨同進(jìn)化中，兩個(gè)不同的物種會(huì)獨(dú)立演化出相同的特征，例如不同品種的球潮蟲都有堅(jiān)硬的外殼、人類與樹袋熊都有指紋等。

這大概就是走禽之謎的答案吧。