Population genetics help

hi everyone, just had a quick question, why would harmful mutations accumulate more in smaller populations compared to large ones? and why would an isolated population created via the founder effect have fewer harmful mutations? thanks :slight_smile:

Hey @nep good questions!!

Harmful mutations tend to accumulate in smaller population for a few reasons:
Smaller populations tend to have a higher rate of inbreeding than larger populations, and this means that there is a higher chance of homozygosity for deleterious recessive alleles. Another reason that this can occur is due to a phenomenon called Genetic Load. The Genetic load, or the overall burden if deleterious mutations in a population, tends to increase in smaller populations due to reduced opportunities for purifying selection.

An isolated population created via the founder effect might have fewer harmful mutations for a couple of reasons:
Some of the initial harmful mutations may have been lost by chance due to genetic drift. The small population might have undergone a purifying selection process where strongly deleterious alleles are removed by selection.

Hope this helps!

Hi nep

Good questions!

As Sarah pointed out, the main reason for this is a phenomenon known as inbreeding depression, which is often observed in a smaller population (after a bottleneck effect). Since individuals within the population are likely to breed with each other more often, deleterious/lethal (recessive) alleles can accumulate faster, and thus have a higher chance of being being passed on.
Say a recessive allele is responsible for a harmful/disadvantageous trait. A heterozygote would be protected from it (because they have the dominant allele). However, if you have two heterozygotes (who have the recessive allele) mating, there’s a chance it’ll be passed on to the offspring to produce more carriers.

This may only be true in the short term (as per reasons that Sarah pointed out). In the long term, a smaller population will almost always end up with inbreeding depression and thus have a higher chance of accumulating more harmful recessive/lethal alleles.

Watch out for exam questions that also get you to discuss linked genes and unlinked genes in terms of variation.
Linked genes reduce variation in a population as they get inherited together, while unlinked genes are better for the population as they have a chance (via independent assortment and segregation) to be passed on separately, thus increasing variation.

Hope that helps clarify things a bit more !

thank you for elaborating on my answer, you were able to word it much better than I was able to, you also helped me to understand it a better myself.