I used to think this was possible, but lately I have been questioning this. Now I understand we can imagine a scenario where not only the selective pressures don't change, but where the mutations flies back and forth (either through selection or genetic drift) between the same mutations, always reverting back to what it was before.
But can such a situation happen in reality ? I don't think so for the following reasons:
- Selective pressures always seem to change. Even when the environment in an ecosystem does not change, some species will change and this will impact the pressures on the other species.
- Mutation rates per individual per generation are high. Every generation, there is a flood of new mutations coming into the population. Cost of selection obviously limits natural selection from ever keeping up with all these new mutations. Add to that the fact that the majority of mutations have very small effects and so, if they ever reach fixation, it will be through random genetic drift. Which makes it extremely improbable that any species will just have fixed mutations who revert back to the previous mutation, etc.
Now if some things are disputed and/or unclear (which I'm sure some are) go ahead and ask a clarification or tell me what is wrong. Just make sure that the point you are raising hasn't already being raised.
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