Evolution always cuts down the genetic diversity, no matter what.
This isn't correct. It's not evolution that reduces genetic diversity it's isolation. If a small group of organisms become separated from the main group they will have less diversity than the majority - obviously. Taken as a whole though, nothing has changed.
The two groups may then diverge as a consequence of selection or drift. Eventually the two groups will no longer interbreed and speciation will have said to have happened.
Overall, there's now more diversity than before - two seperate species must be more diverse than one single species when added together.
If you imagine a thousand creatures were a group of 50 get seperated, that main group is unlikely to lose any diversity at all.
But it's very likely that the smaller group will have relatively low diversity within its own species because of bottleneck effects - but this is not a given - a fast breeding organism with a high mutation rate may recover diversity very quickly.
But over long periods of time mutations must increase diversity - we see it happening.
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