What prevents evolution?

[Rduke55]

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what would stop the same tendency from operated within A1?

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Yes, because the group drifts together.

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However, if a mutation carries with it a tendency to reduce fertility without otherwise improving adaptation (and there's little reason to assume otherwise)

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OK! Now I'm seeing why you're confused.
This is not the case. We are not talking about these mutations reducing fertility on its own or as a whole. Mutations cause infertility between the groups because of the effects these changes have on gene alignment etc.
Their genomes become incompatible with each other.

You can have mutations in a group that increase fertility within that population yet reduce the fertility with the other group. In fact, you would expect this to happen.

So this:

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Thus, the same process pulling A1 from A also pulls apart A1 and prevents true speciation.

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is wrong.