Faith writes:
Depends on how you are using the terms and it’s not easy to tell. The processes of evolution, which are selective subtractive processes as I’ve been using the term for this purpose, in the process of producing a new subspecies, if using new high frequency alleles, results in loss of competing alleles, which is loss of genetic diversity. This occurs from every population split, but eventually it MAY lead to the state of genetic depletion beyond which further evolution is impossible. It depends on the continuation of selections or population splits. Whether that extreme is reached or not, there should be reduced genetic diversity from population to population to one degree or another.
At the same time, every individual is born with mutations. As I mentioned earlier, for cheetahs we would expect 3 to 5 new genotype alleles
per individual. With a population of just 10,000 individuals, that is 30,000 to 50,000 new alleles per generation. Natural selection can't remove all of them. They will accumulate and increase genetic diversity.
Every population split produces a new set of gene frequencies in the daughter population and possibly also the original population depending on how large it was.
Every generation adds new alleles in both populations. Due to the randomness of mutation, the interplay between mutations, and differences in selective pressures, the unavoidable result of this process is that different mutations will accumulate in each subpopulation causing the two populations to become less and less similar over time.
As an analogy, you are trying to claim that rivers should dry up in a matter of months because they always go downhill. Obviously, you are ignoring the process that causes water to move uphill, what we call precipitation.