Blackwell Publishing

Molecular evolution and neutral theory - Why are Kimura's arguments flawed?


Hard and soft selection

We should distinguish between what is called hard and soft selection. When natural selection is actually substituting one gene for another, it does so by means of extra mortality, on top of the background mortality which exists anyway. This is called hard selection. If selection requires extra mortality, it will tend to depress the population size. The rate of evolution, therefore, has an upper limit: if selection is too strong it will depress the population size to zero.

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