Friday, December 08, 2006

Anonymous said...

Where can I find more information on the role of adaptation in evolution? Many adaptations are explained away in what seem to be 'just so' stories, when in fact they could just as easily be explained by a phenotype being predominant purely by association at the genome level to some sexually selected trait - and then the organisms in the new population exploit the phenotype in such a way that afterwards it looks like adaptation drove the evolution. It just seems to me that, for example, 'Longer fingers for tree climbing' is hardly the kind of thing that would affect reproductive success - which HAS TO happen for a change in a population's genome. My 'gut feeling' is that evolution just randomly speciates, with crazy variety, and THEN niches are exploited; then in retrospect paleontologists infer reproductive success for adaptations -when in reality nothing of the sort happened.
So where can i go to validate/debunk this gut feeling? who is doing work to test the mechanisms of adaptation and selection and writing books for the layperson? is convergent evolution 'proof' of the selective power of adaptation, or just statistical certitudes given enough time, in a pretty much random process exploring all morphological spaces?



That’s quite a question, but you have picked up a few misconceptions there. I’ll try to pack it all into a reasonable reply post, but do send us another comment if you want more (and leave us your contact details).

Reproductive success is by no means the be-all and end-all of success in terms of passing on your genes. Animals must survive long enough to reproduce, and few survival traits are linked to sexual selection or to sexual characteristics. Quite the reverse in fact if you look at how sexual selection operates. Look at this recent paper (abstract here) on the changes in limbs in lizards. Limb length changed twice in less than a year in response to the introduction of a new predator. A change in limbs and behaviours allows only some lizards to survive to reproduce, so their mating success is directly tied to this, if not their ability to compete successfully for mates.

You have also used a very strict definition of 'evolution', assuming that it must be an active trend. Traits may appear that have no obvious function (like red hair in humans, or a 6th finger), but if they do not affect the survival of an animal, or its reproductive success then (all other factors being equal) there will be nothing to stop it from passing on its genes and this trait may spread through a population by genetic drift. Reproductive success does not ‘have to happen’ to change the genoype of a population. However, these traits have still evolved, as they did not exist in earlier generations. They may subsequently become 'functional' (red hair might be deemed more attractive, or a 6th finger might help climbing) but this does mean that evolution has not occured.

Novel adaptations can certainly drive evolution however, see some examples here. Obviously the success of groups like birds and bats is in response to their ability to fly and the evolution of flight has allowed them to exploit new niches and speciate and diversify as a result. With much of this work being done by the Gants on Darwin’s finches. At times of great change (after mass extinctions say, or the initial colonization of land) then niches are available to be exploited, and otherwise unsuccessful or uncompetitive adaptations may (temporarily) become established (evolution may go a little nuts) but in times of stability, the better adapted organisms will come to dominate (e.g. the rise of the dinosaurs in the Jurassic). Niches are plastic things and as environments change (over space and time) and organisms, evolve, adapt and compete, they will shape their own niches and those of other species around them.

As for convergent evolution. There are a limited number of ways of solving certain problems (e.g. feeding on termites) with certain body plans and so it is somewhat of an inevitability. In some cases (e.g. birds, bats and pterosaurs) a remarkable problem (flight) can be solved in very different ways, but in other, conservatism of shape is clearly the best solution (whales, penguins, icthyosaurs, tuna) and so evolution will eventually stumble on the ‘best’ solution and shapes will converge.

You also asked about books, take a look around. A quick search on Amazon for “evolution” and “adaptation” found a few dozen popular and more technical science books. A good start would be the Dawkins books The Blind Watchmaker and Climbing Mount Improbable.

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