I just want to see the mutation accumulation rate in human populations. Versus the rate at which mutations are selected out. Just wanted to check if the genome is deteriorating


2 Answers 2


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Is the rate at which new mutations occurring higher than the rate at which alleles fix today in humans?

Human populations are exploding! Genetic diversity within a population is a function of the effective size of its population. As the population expansion in human is extremely recent, we ought to expect that our genetic diversity is much lower than what would be expected from our population size and structure. Therefore, yes, genetic diversity is currently increasing, or in other words, more mutations appear than there are alleles that fixed.

Is the genome deteriorating?

I am not sure what you mean by "the genome is deteriorating" but the answer to "Is the genome deteriorating?" is likely no!

Actually, larger populations tend to suffer from less drift load and as such our mean fitness would rather be increasing. However, given the population size, I doubt that would make such an important difference.

On the other hand, medicine is greatly affecting our environment and is reducing the strength of purifying selection. As a consequence, modern populations would eventually be less fit than our ancestors if placed in the environment experienced by our ancestor (that is an environment with little to no medicine). But this will not be true in our current environment.

Prerequisite to understand the answer

This answer felt quite introductory but it actually contains all the following concepts:

  • Allele
  • Allele fixation
  • Genetic drift
  • Effective population size
  • Population structure
  • Drift load
  • Purifying selection
  • Genetic diversity (typically defined as the expected heterozygosity)
  • Effective population size
  • Environment (and even niche construction)

If any of the above concept is unclear to you (and I very much suspect that at least the term "drift load" will be new to you), then you might want to start with an intro course to evolutionary biology and population genetics.


Yes it is of course deteriorating, as can be proven with simple math, see for example Lynch (2009) or this later paper by Lynch (2016).

Thus, although there is considerable uncertainty in the preceding numbers, it is difficult to escape the conclusion that the per-generation reduction in fitness due to recurrent mutation is at least 1% in humans and quite possibly as high as 5%. Although such a mutational buildup would be unnoticeable on a generation timescale, over the course of a couple of centuries (approximately six generations), the consequences are likely to become serious, particularly if human activities cause an increase in the mutation rate itself (by increasing levels of environmental mutagens). A doubling in the mutation rate would imply a 2% to 10% decline in fitness per generation, and by extension, a 12% to 60% decline in 200 years.

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    $\begingroup$ So, this means about 6000 years after the world was created we'll be unfit to do anything? ;) $\endgroup$
    – JollyJoker
    Commented May 23, 2017 at 10:48
  • $\begingroup$ @JollyJoker I am not sure what you are referring to when you talk about the creation of the world but if you refer to either the big bang or the agglomeration of matter that ended making earth what it is, then there was no life 6000 yet years after that event. There was therefore no "we" whatever "we" may refer to in your comment. $\endgroup$
    – Remi.b
    Commented May 27, 2017 at 2:58
  • 1
    $\begingroup$ @Remi.b Just jokingly referring to young Earth creationism $\endgroup$
    – JollyJoker
    Commented May 29, 2017 at 8:42
  • $\begingroup$ @JollyJoker Fair enough. I should have known given your user name! $\endgroup$
    – Remi.b
    Commented May 29, 2017 at 18:53

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