What prevents evolution?

[Alex-db]

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because dna changes over time, A is "more similar" to B than to C.

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The DNA can't somehow coordinate its changes to retain fertility WITHIN each generation while at the same time producing infertility BETWEEN generations. What do you suggest as an additional mechanism, remembering the objections I have already raised?

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I haven't read that book, but I guess it says something like this, since it seems fairly straight forward:

Assume the parents are representative of an 'average' of the offspring. So 90% reproductivity with the parents means an average of 90% with each other as well. Say the true range is 80% to 100%

Since reproducing requires reproduction, in this question of inheritance it is only sensible to consider the members of a generation who actually reproduce.

Therefore:

When considering a generations' parents, the reproductive % between those parents appears to tend towards 100%, even though the range within that generation was 80-100% and had a difference from its parents of 90%.

(THE ONES THAT COULDN'T REPRODUCE DIDN'T REPRODUCE!)

This no logical basis for expecting reproductivity within a generation to reduce.

This is actually beautifully intuitive if you take a moment to understand it. The simplicty is what makes evolution almost certainly inevitable.