How did ratite species (ostriches, emus etc) evolve?

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Offline svenrufus

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I just watched Attenborough's Big Birds show from the Natural World, and he gave an account of the evolution of the Ratites (ostriches, emus etc) which doesn't make sense to me, so I wonder if anyone else can enlighten me?

He said that it had once been assumed the fact that the large flightless birds were found on numerous continents was due to early adoption of flightlessness on Gondwanaland before continental drift seperated them and evolution made them diverge, already as a group of flightless birds. I get that idea.

However recent DNA analysis shows that a Ratite which can fly, the Tinamou in S America, fits in the family tree in such a way that indicates that actually the whole group of Ratites spread around the world as flying birds and later all independently developed their flightless habits. The main evidence for this which Attenborough offered was that there were no examples in the world of flightless birds that had re-acquired the ability to fly.

That seemed like a pretty circular argument to me, and I watched the rest of the show wondering what was more likely: a) that a single species of a flightless group should reacquire the ability to fly or; b) that a geographically widespread group of flighted birds should all at the same time lose the ability to fly, while other birds from other groups did not exploit the niche which accomodates flightlessness at the same time.

No matter how hard I puzzle on it, it seems to me - in the absence of other unstated reasons for why reacquiring flight is so implausible - that one species doing something exceptional is easier to accept than almost all of the species from one group and none from other groups* should independently and concurrently abandon flight.

If it is the case, that Ratities developed a flightless habit after separation, what was it about the Ratites as flighted birds that made it almost inevitable that they would then go on to lose the ability to fly?

* I know there are a few other flightless species here and there, but relatively few, and not enough to deflect from my main point about the concentration of this evolutionary path being pursued within the Ratite clade.
« Last Edit: 24/06/2015 22:25:05 by chris »