In this view there is a superabundance of new harmful mutations throughout the population, and these can easily be moved to fixation merely due to a population bottleneck, nothing else is required. But here is the problem. Let me clarify Glenn's statement for our readers. A typical gene is 1000 nucleotides in length. With a mutation rate of one in 1E7, the typical gene would experience one mutation in 10,000 generations.
Some alleles require multiple substitutions.
Then he makes a huge mistake. He goes on to claim this is too low to explain the observed high levels of genetic variation. But Glenn misinterpreted the situation. To see his error let's again take the above population of one million adults. Since they are diploids, there are two million instances of the given gene.