iayork t1_irpd2w2 wrote on October 10, 2022 at 12:36 AM

There are three possibilities for irrelevant structures, as far as evolution is concerned:

they’re useless, but completely harmless.
they’re useless, but passively harmful - they use energy or resources that could be used elsewhere.
they’re useless, and actively harmful.

The effect of the third possibility is obvious. Individuals that disrupt that harmful thing do better, reproductively. Mutations that prevent the harmful thing from forming are selected, just likely any other positive selection, and the disrupted version spreads through the population.

The second option may be harder to visualize, but it leads to exactly the same outcome. Forming an organ you don’t need uses resources; maintaining a non-helpful organ uses energy. If you can divert those resources and that energy to, say, earlier reproduction, you’ve got a positive selection trigger that will spread through the population.

If something is useless, but harmless, then there won’t be any positive selection in disrupting it. (Also, it’s possible in the second case that the positive selection is so weak that it won’t spread particularly quickly, and that comes down to this case.).

But mutations happen all the time. We are all born with dozens of mutations, almost all harmless or nearly so. Those who are born with harmful mutations have them negatively selected. Harmless mutations just hang around, because they aren’t selected against.

If a mutation hits our now useless structure, there’s no selection against that mutation - neither negative nor positive. If there’s no negative selection against mutations, then very gradually, just through genetic drift, the structure will slowly accumulate mutations and gradually, just through drift, become slowly and randomly inactivated.

Nails are a bad example here because they do have positive value to us - not ripping apart food, but supporting touch and gripping, for example. A better, classic example is the vitamin C production system in fruit-eating primates. Because fruit is rich in vitamin C, there was no value to those primates in keeping the system, but it was also pretty much harmless, not using significant resources. Over time, genetic drift allowed mutations to accumulate in the enzymes involved and the system stopped working - causing no harm, but no benefit either.

(Some people argue that inactivating the vitamin C pathway does have a small positive value, but the principle is there anyway.)

NewBromance t1_irppgvo wrote on October 10, 2022 at 2:16 AM

Where does humans inability to make vitamin C fit into all this? I know we get vitamin C from our diet so it rarely becomes a problem unless you're on a ship or low food quality environment - and then scurvy happens. So there was no selective pressure to keep it working really, but was there a selective pressure to stop it working like point 2.

I.e. do animals rhat can synthesis vitamin C have to pay "an upkeep" to maintain the bodies ability to do so or something?

konwiddak t1_is1fubu wrote on October 12, 2022 at 4:37 PM

I don't believe we think there was a selective pressure to get rid of it. However mutations and differences don't necessarily happen in isolation - it could have been that the ape who had the original mutation might have happened to be exceptionally strong/smart/fertile (or plain "lucky") and therefore it was advantageous for his genes to propagate despite the flaw.

NewBromance t1_is1g673 wrote on October 12, 2022 at 4:40 PM

Almost like the gene piggybacked of another gene.

Ape has a gene that means he can't make vitamin C, but that doesn't matter because he gets it all from his diet. However he also has a bunch of useful genes that make him more successful and so him and his descendents do very well and thr defective vitamin C gene goes along for the ride?

konwiddak t1_is1vwfb wrote on October 12, 2022 at 6:22 PM

Pretty much, the vitamin C gene could have been completely unconnected to the success of this genetic lineage. We don't really know, this is just a theory - it could have also been that there was a forest fire and only one family of apes survived.