What prevents evolution?

[Rduke55]

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How do you figure this "need" could prevent mutations of the sort I'm referring to?

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C'mon. It doesn't prevent the mutations.

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Meanwhile, mutations detrimental to fertility accumulate more or less homogeneously within the prospective new species concurrent with its divergence from the original group.

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No. Adaptively neutral mutations accumulate homogeneously. Mutations detrimental to fertility are selected against.