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why do our genomes have so many mutations when themutation rate at each individual locus is low?

Explain why it is so "difficult" for a population tolose deleterious recessive alleles at a given locus.

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Beverley Smith
Beverley SmithLv2
29 Sep 2019

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Related questions

Explain why it is so "difficult" for a population to losedelterious recessvive alleles at a given locus.

Consider a deleterious recessive allele at a frequency of q =0.0016 in the population.
a. How many times more likely is the offspring of a cousin matingto suffer its effects (that is, to be homozygous for it) than is anoutbred individual?

b. Now do the same analysis for recessive alleles at lower (q =0.0005) and higher (q = 0.05) frequencies. For which kinds ofalleles does inbreeding most strongly increase the probability(absolutely, and relatively) of being affected by themutation?

Recently I have found myself in a discussion with a guy thatisn't a creationist, but denies evolution. He cites several studiesthat show problems like this one, but I do not have the knowledgeto refute it properly. Whats the problem with the following:

One problem is that population genetics shows us evolutiondestroys functional sequences much faster than it creates them inany large-genome animal with low reproductive rates, becausedeleterious mutations arrive faster than selection can remove them.This would include just about all mammals, reptiles, and birds--anyof 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 givesus 6-32 deleterious mutations per offspring (most slight). Evenwith the low estimate of 6, every child is less fit than theirparents and natural (or even artificial) selection can only choosethe least degenerate each generation. For every generation where amutation can be selected, random mutations destroy 5-32+ previouslyselected. Beneficial mutations like lactose tolerance (a jammedswitch) take 1000s of years to appear and fixate.

If you know the genome size, mutation rate, and reproductiverate, you can calculate at what rate deleterious mutations arrivefaster than they can be removed:

"It has been estimated that there are as many as 100 newmutations in the genome of each individual human. If even a smallfraction of these mutations are deleterious and removed byselection, it is difficult to explain how human populations couldhave survived. If the effects of mutations act in a multiplicativemanner, the proportion of individuals that become selectivelyeliminated from the population (proportion of `genetic deaths') is1-e^-U, where U is the deleterious mutation rate per diploid, so ahigh rate of deleterious mutation (U>>1) is paradoxical in aspecies with a low reproductive rate." High genomic deleteriousmutation rates in hominids, (Nature, 1999)

They worry about a deleterious rate of U much greater than 1 asbeing prohibitively high and we're talking about it being 6-32+,since we now know the functional genome is much larger than wethought it was in 1999. Taking their Poisson probabilitydistribution and using U=6, that means 1-e^-6 = 99.752% of thepopulation will have to be selected away each generation for onelucky enough to have no deleterious mutations. For two to surviveand maintain constant population size that would require on average806 offspring per female--impossible for most mammals, birds, andreptiles. Actually much more since natural selection is notomnisciently efficient.

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