Thursday, September 24, 2009

Flightless birds: why are they found on islands?

Whilst on holiday (see previous post) I've been reading Jerry Coyne's excellent "Why Evolution is True".
Here's a tip: next time you encounter an advocate of "intelligent design" or, perish the thought, creationism, ask them if they've read his book  - or something comparable. Ask if they know about 'devolution' (my term ) which is the loss of function, with vestigial traits still visible - like that  otherwise inexplicable "floating"  pelvis and hind limb of the whale.  If they haven't, then refuse to enter into any discussion - probably futile in any case - until they have apprised themselves of the impressive marshalling of evidence in Coyne's book.

I should probably stop here - not presuming to be on anything like equal terms with someone who has spent his entire career researching the mechanisms of evolution (yes, the latter is possible, after a fashion, using his experimental model of fruit flies with their relatively short generation times). But I can't resist floating an idea that may or may not be codswallop. It concerns "flightless birds" - an example of that so-called 'devolution'.

The kiwi (NZ) - archetypal flightless bird

Here's Coyne's rationale:

"The long and the short of it is this: flight is metabolically expensive, using up a lot of energy that could otherwise be diverted to reproduction. If you're flying mainly to stay away from predators, but predators are often missing on islands, or if food is readily obtained on the ground, as it can be on islands (which often lack many trees), then why do you need fully functioning wings? In such a situation, birds with reduced wings would have a reproductive advantage, and natural selection could favor flightlessness. Also wings are large appendages that are easily injured. If they're unnecessary, you can avoid injury by reducing them. In both situations, selection would directly favor mutations that led to progressively smaller wings, resulting in an inability to fly."

Well, maybe, but wings confer such an extraordinary advantage over us terrestrial landlubbers that one has to seek an equally extraordinary reason why birds would divest themselves of such an advantage - even if  in thrall to random mutation and natural selection.

Here's my explanation - for what it's worth.

Suppose the first bird to land on an island had been blown way off course by strong winds, and had landed exhausted on its new home with abundant plant life and insects, but no snakes and other reptiles, or other predators- notably birds and mammals. Suppose too the bird had found a mate earlier, and was carrying eggs, duly laid in the new home.

The fledglings would have developed into fully-grown birds in due course. What then? Would they have stayed in their place of birth - or would they have been adventurous, and ranged further and further over the ocean in search of their more 'natural' habitat, ie that imprinted on inherited memory from mother?

Those that explored further afield might be fortunate and make landfall. Alternatively, and more probably,  they sadly would not make it, and would fall exhausted into the sea,  or hopefully perhaps turn back in time and return to their island home-sweet-home.

Over the millennia, natural selection would favour birds that were content to "stay put". Mutants that had progressive loss of flying ability would gradually predominate. Flight-capable birds would continue to suffer attrition, at least where the island was concerned. In the absence of predators, the flightless mutants would gradually displace the gypsy wanderers, provided there was sufficient food available at ground level - seeds, insects etc.

Maybe Mother Nature pioneered the joys of indolent vegetating, with old birds and/or their partners marooned on islands,  long before late middle-aged expatriate human beings arrived on the planet...

No comments: