I was thinking about what the impact modern medicine might have on human evolution based on a couple assumptions.

If we assume that:

  1. modern medicine has massively cushioned the selection pressure against people with inborn errors of metabolism, weak immune systems, disabilities, and genetic defects more broadly.
  2. that these people, due to modern medicine, are able to live past reproductive age.
  3. and that these people (broadly speaking) go on to produce offspring at the same or marginally lower rate as those without these defects.

could we expect to see these defect-causing alleles increasing in frequency in the now and more so in the future? or maybe at some point, given the constant improvements in modern medicine, these alleles essentially become fixed in subpopulations or maybe even the global population?

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    $\begingroup$ Can you please provide a reliable source for the massive cushioning modern medicine has provided these individuals? Thanks. $\endgroup$ Jun 11 at 11:56
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    $\begingroup$ I can't find any study, but surely its reasonable to assume it. And I don't think there will even be studies in this topic for the near future anyways, given that there is no benefit from researching this. But that doesn't mean its perfectly resonable. for instance, as a higher percentage of people who would've died from infection are kept alive at least up until reproductive age. $\endgroup$
    – AnethOthbo
    Jun 11 at 22:43
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    $\begingroup$ This question is strongly associated with the pseudoscience of eugenics, and specifically the idea of "dysgenics", supposing that modern interventions "weaken" the gene pool. I don't suggest this is your intention, I merely point out the connection. For more information I'd google "medicine dysgenics", which brings up scholarship such as this. Trivially... sure, there is probably some effect. Substantially, it's not clear that this has much effect at all. $\endgroup$ Jun 12 at 10:47
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    $\begingroup$ I believe @MaximilianPress has put it best. I also think you do not give credit to those with inborn errors of metabolism; most would not likely choose to procreate knowing that they are passing a fatal gene to their offspring. Anyone can claim anything on the internet. Did you know that I can fly? It’s an autosomal dominant mutation, obviously, because all of my children are good flyers as well. And the sun has no effect on our wings. $\endgroup$ Jun 12 at 12:59
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    $\begingroup$ Finally, speaking as a physician, of the more than 1000 IEMs, only a metaphorical handful (not a massive cushion) can be treated with the individual reaching adulthood able to reproduce (for a variety of reasons). Some IEMs aren’t diagnosed until mid to late adulthood, so are probably common enough. Frame of reference is everything. Someone with a treatable IEM might win a Nobel prize someday for something that changes the world for the better. You never know. $\endgroup$ Jun 12 at 13:30

1 Answer 1


Yes, in principle, but it could take a very long time. Consider phenylketonuria, which is a severe genetic disease that is completely preventable by excluding phenylalanine from one's diet. Suppose that PKU is now completely neutral due to our ability to screen for PKU alleles and counsel carriers appropriately (not to avoid reproducing, which would induce selection against PKU, but to make sure that their children are tested and put on an appropriate diet).

How long will it take PKU to fix due to genetic drift?

From here, the time to fixation

$$ E(T) = -4N_e [p \ln p + (1-p) \ln (1-p)] \textrm{ generations} $$

where $N_e$ is the effective population size and $p$ is the starting frequency (this counts fixation at either 0% or 100% but should be a lower bound for fixation at 100%).

The highest frequency of PKU in any population in the world is around 70% (Gundarova et al. 2018); starting from here, the fixation time would be $0.63 N_e$ generations. Even in a very small population (e.g. $N_e = 200$), this will take hundreds of generations/thousands of years ...

The time scale might be much shorter if there were positive selection, i.e. some selective advantage to carrying the allele that also/formerly caused disease. This is known, or speculated, to be the case for many deleterious recessive alleles (sickle-cell, thalassemia, cystic fibrosis ...). However, in many of these cases the selective advantage is against infectious diseases such as malaria whose effects have also been mitigated by modern medicine ...

Gundorova, Polina, Rena A. Zinchenko, Irina A. Kuznetsova, Elena A. Bliznetz, Anna A. Stepanova, and Aleksander V. Polyakov. “Molecular-Genetic Causes for the High Frequency of Phenylketonuria in the Population from the North Caucasus.” PLoS ONE 13, no. 8 (August 1, 2018): e0201489. https://doi.org/10.1371/journal.pone.0201489.


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