不會飛的鳥兒何以遍布全球？

趨同進化

不同的物種——甚至在進化上相距甚遠的物種，如果生活在條件相同的環境中，有可能產生功能相同或十分相似的形態結構以適應環境。例如哺乳綱的鯨和海豚、爬行類的魚龍等由于長期生活在水中，因而進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構。此種現象被稱為趨同進化（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？ 1）Rheas live in South America， 2）cassowaries and 3）emus in Australia， 4）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 5）moa， another feathered 6）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 7）convergent evolution， when two species independently evolve the same traits， like hard shells in different kinds of 8）pill bugs or fingerprints in humans and koalas.

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

有這么一個謎：一系列不會飛的鳥類怎么會分布在它們要飛翔才能到達的各個地區的呢？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系，而且都是陸棲走禽。怎么會出現這種現象呢？

對進化學家們來說，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現它們的地方，但多項DNA測試又讓這個問題變得更加復雜難解。

（澳大利亞）阿德萊德大學對象鳥的DNA進行了一項全新的研究。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加斯加，在近幾百年已經滅絕。

研究人員試圖驗證象鳥與恐鳥是否存在親緣關系——后者是分布在新西蘭的另一種長著羽毛的巨鳥，象鳥與恐鳥在大約同一時期滅絕。研究結果表明，與象鳥有著更密切親緣關系的卻是新西蘭的標志性走禽：幾維鳥。研究結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。……