How evolution get rid of unnecessary stuff?

Submitted by CoolAppz t3_xzysh0 in askscience

We all know about how evolution goes by preserving the genes that generate specimens that are fit for the environment, as the non-fit die and their genes disappear but what the other way around?

I mean this. Suppose nails. For wild animals, nails are necessary for defense, ripping food, etc., but with humans, as we have developed tools, are nail are fragile and if they ever existed on the form closer to the one now, even with Neanderthals, I don’t see them being that useful, for the same usage they have with wild animals, specially toe nails.

Do evolution get rid of stuff that is particularly irrelevant for survival? If so, how?

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EmperorGeek t1_irpbbva wrote

It doesn’t always. Take your Wisdom Teeth for example. Most human mouths don’t have room for all the teeth we used to have.

Over time, if there is a selection bias where people who have wisdom teeth don’t breed as often, they will eventually disappear, but other than that, there is no selection bias for people without wisdom teeth, so they will continue to be an issue some of us.

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kittenTakeover t1_irrf47p wrote

There is a selection bias against neutral traits. It's called opportunity cost since maintaining the trait typically requires forgoing other possible mutations, that could be beneficial. For this reason traits that lose their usefulness tend to disappear over time.

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NewBromance t1_irpp0nw wrote

Does this mean wisdom teeth where being selected against until we got technologically advanced enough to be able to remove teeth? Is that why some people don't have any trouble with wisdom teeth at all or even come through.

I've always sort of wondered because somehow all 4 of my wisdom teeth came through fine with no problems.

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iayork t1_irpd2w2 wrote

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.)

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NewBromance t1_irppgvo wrote

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?

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konwiddak t1_is1fubu wrote

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.

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NewBromance t1_is1g673 wrote

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?

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konwiddak t1_is1vwfb wrote

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.

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Lord_Nokia1234 t1_irpalza wrote

Sometimes, yes. However, most of the time it does not. These are called vestigial organs. For example, some humans have a muscle below their wrist that was once used to help with climbing trees, but has of course been rendered useless.

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TerpenesByMS t1_irpxhba wrote

Nifty enough for inheritance, it is much easier to turn a trait off than to acquire it in the first place. This makes it easier & faster for vestigial traits to wither or disappear if there is a substantial advantage to be had by losing it - compared to developing whole new adaptations - in terms of number of generations. Evolution is lazy, path of least resistance usually wins.

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professor-of-things9 t1_iruqjom wrote

Hmm. Your post makes me instantly think of our abilities to talk, and use language!

It accompanied a lowering of our larynx that makes us… incredibly vulnerable to death by choking. By any measure, this would be incredibly disadvantageous.

And yet, here we are, still talking, thousands of years later. Amongst other things, we use language to… teach each other the Heinrich maneuver.

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