Why is it necessary for there to be variation in population in order for evolution by natural selection to occur?

There needs to be variations in population in order for natural selection to occur because the entire point of evolution by natural selection is only the best variations will survive. So if there were no variations then there would be no natural selection because the animal's survival rate would be the same no matter how many times they reproduce because there are no different variations being introduced into the species. However, if there are variations then the animal's survival rate could be impacted because of the variations, for example, white mice would be easier to find for predators on a dark surface, while a darker mice would be harder to find for predators on a dark surface, thus, allowing the darker mice to prevail as they have a higher survival rate and the species will slowly evolve into the darker mice. But, if there were no evolution, in this case, then no matter what happens, the white mice would not be able to evolve into a darker mice because there are no such thing as variation. That is why it is necessary for there to be variation in order for evolution by natural selection to occur.

most community's take hundreds of years to become stable

during the process of dna replication, brdu can replace thymidine and incorporate into the newly synthesized dna, cell proliferation thus can be reflected in the extent of brdu incorporation.