不同的物种——甚至在进化上相距甚远的物种，如果生活在条件相同的环境中，有可能产生功能相同或十分相似的形态结构以适应环境。例如哺乳纲的鲸和海豚、爬行类的鱼龙等由于长期生活在水中，因而进化出与鱼类相似的体型;澳大利亚的袋食蚁兽、非洲的土豚、亚洲的穿山甲和南美洲的食蚁兽都具有相似的生活方式和适于捕食白蚁的相似生理结构。此种现象被称为趋同进化（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.
　　有這么一个谜：一系列不会飞的鸟类怎么会分布在它们要飞翔才能到达的各个地区的呢？美洲鸵生活在南美洲，食火鸡和鸸鹋生活在澳大利亚，几维鸟生活在新西兰，而鸵鸟则生活在非洲。这几种鸟都有亲缘关系，而且都是陆栖走禽。怎么会出现这种现象呢？
　　对进化学家们来说，这可是一道难题。主流说法是这些鸟类早在各大洲板块分离之前便分布在今天人们发现它们的地方，但多项DNA测试又让这个问题变得更加复杂难解。
　　（澳大利亚）阿德莱德大学对象鸟的DNA进行了一项全新的研究。象鸟是世界上存活过的最大型的鸟类之一，一度生活在马达加斯加，在近几百年已经灭绝。
　　研究人员试图验证象鸟与恐鸟是否存在亲缘关系——后者是分布在新西兰的另一种长着羽毛的巨鸟，象鸟与恐鸟在大约同一时期灭绝。研究结果表明，与象鸟有着更密切亲缘关系的却是新西兰的标志性走禽：几维鸟。研究结果还显示恐鸟血缘上最近的近亲是一种只有家鸡大小的鸟——共鸟 ，而它们呢——听好了，生活在南美洲。还有一个意外发现：有些品种的共鸟 甚至还会飞。
　　该研究报告的主笔安德鲁·库珀告诉《新科学家》杂志说：“在这两个研究案例中，恐鸟和象鸟血缘关系最近的近亲都远在地球的另一边。”
　　对DNA样本的分析表明，象鸟与几维鸟最后一次拥有一个共同的亲戚是在大约五千万年前，而那时各大陆早就已经分离了。因此，库珀认为走禽能在世界各地分布的原因很简单：它们是飞过去的。
　　这一结论将使走禽特性成为趋同进化的范例——在趋同进化中，两个不同的物种会独立演化出相同的特征，例如不同品种的球潮虫都有坚硬的外壳、人类与树袋熊都有指纹等。
　　这大概就是走禽之谜的答案吧。