Hello Donald,
I think that a bathtub metaphor may be helpful.
Roughly, if evolutionary mechanisms have a bathtub full of existing heritable variation (aka polymorphism) to act upon for diploid organisms like us, new mutations add only a single drop of new water (one-millionth) to that bathtub each generation. This does not hold true for haploids such as bacteria.
So if mutation magically stopped tomorrow, this tiny ratio suggests you wouldn’t know it by looking at the products of natural selection and drift for thousands of generations.
(10 hr later) To add more, the water in the bathtub is reduced by inbreeding.
Some examples from humans: the child of first cousins would have a bathtub that is 7/8ths full. The child of a brother and sister would have a bathtub that is 3/4ths full.
Interestingly, we have many strains of fully inbred lab mice (repeated brother x sister matings) that essentially have empty bathtubs. They wouldn’t survive more than a generation or two in the wild.