[Skidoo]

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

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That's about right. As the capacity to reproduce with the reference generation approaches zero, its few remaining descendants are no longer fertile amongst themselves either and so die out. If you're implying that those closer to 100% fertility within generations will find each other and so maintain the group, that's not enough. Any net accumulation of errors not corrected for, which selection doesn't do either, leads to the same inevitable outcome.

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

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There are fewer such individuals as the generations progress due to the said tendency of the genetic basis of reproductive capacity to diverge. Considering only a fertile subset of each subsequent population doesn't remove the constant erosion to the underlying mechanism. Saying divergence is always being selected against doesn't provide a corrective to random change. It only realizes the consequences.

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

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Yeah.

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

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Of course there is: exactly the same process causing reproductive capacity between generations to be reduced.