I expect this to get argued out in some thoroughness when my thread is promoted. But for now all I'll say is that even if it is a mutation, which I doubt, but say it is, even then what you have is an allele, that's what a mutation produces, right? So this new allele is a mutation that occurred in what, one individual?
Populations evolve, not individuals. But, if this new allele is beneficial and is successfully passed down to some offspring, it will likely give them differential reproductive success. ie. They are more successful at breeding and thus passing the new allele to further generations.
It still has to get passed on to its progeny then, and presumably THEN it gets selected as the light-furred individuals don't survive while the dark-furred ones proliferate.
See, it really doesn't matter if the allele for dark fur was a naturally occurring one that got selected and passed on, or a mutated allele that got selected and passed on. Reduced genetic diversity is the RESULT of the selection process, and that will be the case whether it is a mutation or a naturally occurring allele that is selected, because the other alleles for other colors will be eliminated from the population. THAT's reduced genetic diversity. It doesn't "produce" anything, it's the result of selection.
Reduced genetic diversity is NOT the result. The light fur is recessive. So now, you have a dominant gene for dark fur (call it D) and a recessive gene for light fur (d). The gene for light fur may not be expressed such as in a dark furred mouse with both genes (Dd). If the recessive gene isn't somehow wiped out in the population (which would probably be some kind of close to extinction event - ex. bottleneck) then the mice will most likely retain that variation of the light fur gene in their population and it will be a hidden variation.
Hidden variation happens a lot.
Edit: Just to be clear. Just because a gene is beneficial doesn't mean that it is dominant (ex. blue eyes) and just like the genes that determine eye color, they can be complex and not so straightforward as in my example. Gregor Mendel made his choice of peas because they were not complex.
Edited by DBlevins, : Added for clarification
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