A few quick comments on this whole deleterious versus beneficial versus neutral mutation argument:
Mutations can be non-neutral, i. e., have biological effect, and be neither beneficial or deleterious or can be both beneficial and deleterious. For example, suppose a sub-population acquires a mutation that slightly reduces the expression of growth hormone, while another sub-population acquires a different mutation that augments the expression of that hormone.
There are now three populations that are on average short, "standard" height (the unmutated wild population), and tall. Which is the most advantaged height (for species survival)? The short population will have lesser food needs and the taller population may have greater food foraging capability. Usually, each sub-population will tend to adapt to areas that best fit their capabilities. For survival of the species the best strategy is to display a range for each trait to allow maximum use of resources and to be pre-adapted to changes in the environment. And this is what we generally see.
So, mutations in opposite directions can all be beneficial, particularly at the species level.
There was a thread a while back that got into this rate of mutation question that had several references. I recall a sort of consensus that each cell duplication in mammals incurred from a couple of dozen to a few hundred mutations with a lot a variability. Also, lets not forget about the homologous recombination that occurs during meiosis in which homologous chromosomes are literally reshuffled together and that often induces genetic changes.
Finally, while writing this, I produced a few billion new skin cells, I'm sure some of those had mutations in the insulin gene and/or its control region. But since that gene is silenced in skin cells, weren't those neutral mutations in junk DNA? I understand that we are really talking about mutations in germ line cells and not in somatic cells, but those germ line cells have pseudo-genes, endogenous retroviral (ERV) genes and a lot of other unexpressed DNA fragments. I would think that these DNA segments would qualify as junk.
Finally, finally, has everyone noticed that, after throwing his ignorant little stink bomb into the classroom, SHEKINAH seems to have left the building. But this has become an interesting thread that is much better off without him/her, so lets keep it going.
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