Letter: Variation is the key to evolution

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Sir: The response of Peter Winter (Letters, 14 July) to Jonathan Weiner's piece (11 July) on the rapid 'evolution' of Darwin's finches on the Galapagos in response to conditions of stress offers no argument against the view that what was being observed was indeed evolution through natural selection in action.

It is unnecessary to postulate that 'blind chance brought about the right kind of beak in 1977 and again in 1983'. In benign conditions there must already have been birds with a range of beak sizes both smaller and larger than the average. If suddenly there is an advantage in having a larger beak - so that you can deal with the hardest seeds during a drought - however small that advantage may be - then statistically the individuals that have it will be more likely to survive, and breed, hence increasing the average beak size.

The misapprehension of those who cannot accept Darwin is that large changes have to happen in one step, and be caused by a mutation that would have to occur simultaneously in many members of the population in order to propagate itself. What has to be understood instead is just what the finches, and indeed Robin Craig's US immigrant house sparrows (letter, 14 July), demonstrate so clearly: evolution proceeds because of selection pressure that works on variation which is already there.

It is quite easy by extension to accept that when groups of a single species become isolated, and given sufficient time, they eventually diverge so much through adapting to their local conditions that interbreeding is no longer possible and a new species forms. What the birds demonstrate is that the divergence can actually happen surprisingly quickly.

Yours sincerely,

TIM DENNIS

Department of Electronic

Systems Engineering

University of Essex

Colchester

14 July

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