The Selective Brake
The critics of the Darwinian theory of evolution often comment on the “negative” effect of Natural Selection. How can a destructive force (elimination of the less-fit) generate anything?
This argument can come in two flavours. The obvious one stresses the apparent paradox. Selection destroys, limits variation, so how can it generate anything that wasn’t already there? This always reminds me of Michelangelo’s famous saying about his sculptures: they are already there; I only remove the unnecessary bits. Has anyone any doubt about the creativity of Michelangelo?
The more subtle argument relays on the “braking effect” of selection on the speed of evolution. This comes from a true fact. Since not all the mutations are beneficial for the survival, then not all of them are preserved; in fact a tiny fraction of all possible or actual mutations is selected favourably, the vast majority is selected against. It has been mathematically proved that the maximum speed of evolution (that is, the rate of fixation in a population of the new mutations) is equal to the mutation rate. Then selection is actually braking the speed of evolution, isn’t it?
Well, yes. But you may also want to consider what happens to individual mutations. If selection didn’t play any role, mutations of any sign will become fixed at constant rate. It’s even possible that a beneficial mutation may be lost due to genetic drift. However, if selection is taking place, any advantageous mutation will spread in the population faster than what it would have by chance alone. Positive selection speeds up evolution for every individual advantageous mutation.
If by evolution we mean change, selection is indeed a brake. But it is a biased brake since it only stops the fixation of deleterious mutations. For the other kind, the “good ones”, selection accelerates their fixation.
But we may think of evolution as a succession of improvements (as indeed it is: every population is better fit for its environment) and then if it weren’t for selection, the rate of evolution would be much slower.
This argument can come in two flavours. The obvious one stresses the apparent paradox. Selection destroys, limits variation, so how can it generate anything that wasn’t already there? This always reminds me of Michelangelo’s famous saying about his sculptures: they are already there; I only remove the unnecessary bits. Has anyone any doubt about the creativity of Michelangelo?
The more subtle argument relays on the “braking effect” of selection on the speed of evolution. This comes from a true fact. Since not all the mutations are beneficial for the survival, then not all of them are preserved; in fact a tiny fraction of all possible or actual mutations is selected favourably, the vast majority is selected against. It has been mathematically proved that the maximum speed of evolution (that is, the rate of fixation in a population of the new mutations) is equal to the mutation rate. Then selection is actually braking the speed of evolution, isn’t it?
Well, yes. But you may also want to consider what happens to individual mutations. If selection didn’t play any role, mutations of any sign will become fixed at constant rate. It’s even possible that a beneficial mutation may be lost due to genetic drift. However, if selection is taking place, any advantageous mutation will spread in the population faster than what it would have by chance alone. Positive selection speeds up evolution for every individual advantageous mutation.
If by evolution we mean change, selection is indeed a brake. But it is a biased brake since it only stops the fixation of deleterious mutations. For the other kind, the “good ones”, selection accelerates their fixation.
But we may think of evolution as a succession of improvements (as indeed it is: every population is better fit for its environment) and then if it weren’t for selection, the rate of evolution would be much slower.
posted by BioMaxi at 4:29 pm
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