Recently I have found myself in a discussion with a guy that isn't a creationist, but denies evolution. He cites several studies that show problems like this one, but I do not have the knowledge to refute it properly. Whats the problem with the following:
One problem is that population genetics shows us evolution destroys functional sequences much faster than it creates them in any large-genome animal with low reproductive rates, because deleterious mutations arrive faster than selection can remove them. This would include just about all mammals, reptiles, and birds--any of our would-be ancestors over the last 300m years.
Specifically, humans get 60 to 160 mutations per generation, 10-20+% of our genome is sensitive to substitution, and this gives us 6-32 deleterious mutations per offspring (most slight). Even with the low estimate of 6, every child is less fit than their parents and natural (or even artificial) selection can only choose the least degenerate each generation. For every generation where a mutation can be selected, random mutations destroy 5-32+ previously selected. Beneficial mutations like lactose tolerance (a jammed switch) take 1000s of years to appear and fixate.
If you know the genome size, mutation rate, and reproductive rate, you can calculate at what rate deleterious mutations arrive faster than they can be removed:
- "It has been estimated that there are as many as 100 new mutations in the genome of each individual human. If even a small fraction of these mutations are deleterious and removed by selection, it is difficult to explain how human populations could have survived. If the effects of mutations act in a multiplicative manner, the proportion of individuals that become selectively eliminated from the population (proportion of `genetic deaths') is 1-e^-U, where U is the deleterious mutation rate per diploid, so a high rate of deleterious mutation (U>>1) is paradoxical in a species with a low reproductive rate." High genomic deleterious mutation rates in hominids, (Nature, 1999)
They worry about a deleterious rate of U much greater than 1 as being prohibitively high and we're talking about it being 6-32+, since we now know the functional genome is much larger than we thought it was in 1999. Taking their Poisson probability distribution and using U=6, that means 1-e^-6 = 99.752% of the population will have to be selected away each generation for one lucky enough to have no deleterious mutations. For two to survive and maintain constant population size that would require on average 806 offspring per female--impossible for most mammals, birds, and reptiles. Actually much more since natural selection is not omnisciently efficient.