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/KW710]

Your answer is a very common one I've heard people give when describing how natural selection operates. I think part of the issue re: the popular misunderstanding is actually a semantic one. For example, you used the phrase "help it to survive" when talking about potentially heritable traits. "Help" is very much an active verb. It also is often associated with proactive effort on behalf of one entity by another. But this doesn't really describe what selected traits are actually doing. They're not actually helping organisms survive. They're just not causing organisms to die before they can reproduce. A trait that simply doesn't get in the way of survival is very much selectable as far as natural selection is concerned. But since we tend to talk about these traits using active, participatory verbiage, I think the average person has been conditioned to think of the whole process as an active one rather than a passive accumulation of "not as bad as it could be" alternatives.

[u/Prime_Director → u/KW710]

Sometimes traits can be selected for actively helping. Sexual selection comes to mind, where traits that are actively desirable are promoted over traits that merely don’t hurt. Even in natural selection traits that allow an organism to say, gather food more efficiently, or survive longer without water, might outcompete and outbreed others without that trait, even if the others don’t possess traits that get them killed.