There is no general reason to expect evolution to be progressive – even in the weak, value-neutral sense. There will be times when increased size of … - Richard Dawkins

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There is no general reason to expect evolution to be progressive – even in the weak, value-neutral sense. There will be times when increased size of some organ is favoured and other times when decreased size is favoured. Most of the time, average-sized individuals will be favoured in the population and both extremes will be penalised. During these times the population exhibits evolutionary stasis (i.e., no change) with respect to the factor being measured. If we had a complete fossil record and looked for trends in some particular dimension, such as leg length, we would expect to see periods of no change alternating with fitful continuations or reversals in direction – like a weathervane in changeable, gusty weather.