Harmful mutations have accumulated during early human migrations out of Africa

@DennisVenema ,
A paper that you might enjoy.


As the introduction of the paper makes clear, this is a case where conflicting results have been batted back and forth for years now.

These are new results. What is your take on them?

Great – now you made me read the paper. It looks good to me. The thing is, the conflicting results (including this one) have been coming from really good, careful researchers who are good at handling both the data analysis and the modeling/theory, so I’m not likely to spot obvious problems that they haven’t. The take of these authors seems reasonable: a lot of the disagreement has probably come from measuring slightly different things in different ways. Plus, the effects are not that large.

My naive expectation originally was that non-African populations would carry more deleterious mutations because of their reduced population size. One of the interesting points of this paper is that it’s not just the smaller population (i.e. the bottleneck), but also the range expansion that contributes to the effect.

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I havent read it carefully yet, but I think that is the key point, Steve.

@glipsnort and @Patrick

When people migrate, they move into a new environment, a new ecological niche, and begin to adapt to that environment. When they move again after many generations, the process begins again. Some of the “mutations” which were helpful in previous niches could be “harmful” in the new niche. Thus it would seem that those who moved the farthest would have accumulated these mildly harmful changes, and maybe a number of good changes also.

Contrastingly, very harmful mutations are found at similar frequencies in all individuals of the world, as if there was a maximum threshold any individual can stand," says Stephan Peischl, a SIB member from Bern, and one of the main authors of the study.

I find this interesting.

Nice quote from the article!

“Significance Human genomes carry hundreds of mutations that are predicted
to be deleterious in some environments, potentially affecting the health or fitness of an individual.”

" We characterize the distribution of deleterious mutations among diverse human populations, modeled under different selection coefficients and dominance parameters."

“Using a new dataset of diverse human genomes from seven different populations, we use spatially explicit simulations to reveal that classes of deleterious alleles have very different patterns across populations, reflecting the interaction between genetic drift and purifying selection.”

" We show that there is a strong signal of purifying selection at conserved genomic positions within African populations, but most predicted deleterious mutations have evolved as if they were neutral during the expansion out of Africa."

In a small band of humans, the weak are weeded out because of the amount of effort to stay alive, thus less deleterious mutations would survive. Also, there have been lots of time for intermingling of intersecting groups to mix up the gene pool. This even happened in Africa. Can the simulations predict all that mixing?

The weak are weeded out more effectively in large bands than small ones. In a small band, the slightly stronger guy may be accidentally stepped on by an elephant. In a larger band, the random chances are averaged across more people.

The topic is harmful mutations that should accumulate in small groups as they traveled out of Africa but the effect is not as large as anticipated. Something is stopping the accumulation. I tried to state that in a small band, harmful mutations might not accumulate because those individuals might not survive to reproduce. If they did reproduce the entire population would be affected and possibly end because of hardships of life. Not sure what that has to do with an elephant stomping one individual, or why a week person is more likely to be weeded out because of angry elephants. Possibly we are using the word week in different ways.

Well, no. This paper finds that they can explain the number that have accumulated.

My point is that harmful mutations are more easily weeded out in a large population, all other things being equal. That’s the way natural selection works.

Now that’s a different idea – that entire bands would be eliminated because they’d accumulated too many bad mutations? I’m pretty skeptical about that having much effect, given the small genetic differences between local populations and the substantial amount of gene flow between them.

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