But this leads to considerable total population variation. Yes, if even one allele changes, that alters allele frequencies, so a modest amount of evolution has occurred. Recessive has higher equilibrium frequency.
Recessives are hard to select against when rare because most are hidden in heterozygotes.
The fact that deleterious recessives are maintained is a major cause of inbreeding depression. This means that it is not a force tending to produce adaptation. Giraffes don't get more mutations for longer necks where the trees are taller and harder to reach. It can be fairly strong if there is a lot of migration. But if migration is very large, we would just consider it as one big population.