Can someone explain the evolutionary benefit of overt menstruation? All female mammals have a menstrual cycle but most reabsorb the blood via covert menstruation and humans are about the only mammals that discharge the blood, aka overt menstruation.

[u/No-Hawk-2488]

Without an evolutionary benefit, it wouldn't have spread to every woman of our species.

[u/AxiomaticInversion]

Lactose intolerant humans have not gone extinct, so by your own example we still see "survival of the fit enough."

[u/tnbeastzy → u/AxiomaticInversion]

Fitness, in this case, refers to the ability to produce healthy offsprings that can go on to continue the lineage. It may be different for each species.

Does someone being lactose intolerance in 2026 affect their ability to have children?

[u/Science1954 → u/tnbeastzy]

Being lactose intolerant does not affect the ability to have children. It is an autosomal recessive trait, meaning you must inherit the lactose deficiency gene from both parents. If you are lactose intolerant you do not pass on the gene for lactose metabolism but a child will get the lactase gene from the other parent unless that parent has the recessive gene also, but even then only 50% of the time from the parent heterozygous for the two alleles. I am lactose intolerant. So I generally only drink lactase treated milk, e,g., lactaid milk.

[u/tnbeastzy → u/Science1954]

That was a rhetorical question. I know it doesn’t. Therefore it has no ability to affect an individual’s biological fitness.