What prevents evolution?

[Rduke55]

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At some point inthe future either population A or poulation B will constitute 100% of the population. This is genetic drift due to randomness in the environment, as long as there is a differnece in reproductive success there will be change, it requires a mechanism to stop what would otherwise become a mathenmatical certainty given enough permeations.

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One mechanism that stops this completely hypothetical process from occurring in the real world is the increasing infertility among the members of the genetically drifting population that accompanies their individual divergence from the original stock.

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So...speciation prevents evolution?
Extinction prevents evolution?

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Speciation causes infertility.

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I think you're backwards there.

Also, how do individuals become infertile with each other?

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Mutation, of course.

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Please elaborate.