Evolutionary Biology, exclusion of body parts (wisdom teeth, appendix)

[shawnofdenver]

Wisdom teeth, and the appendix and other such organs used to have a purpose in human's, but now they don't. They now can cause damage and even kill you. I know a couple people who were born without wisdom teeth, and without an appendix. My question is, what causes the body to either know that the body part in question has outlived it's usefulness, or if the parents of someone had it removed (or even just one parent), how does that potentially cause the next generation to be born without those body parts if removing those parts doesn't change the genetic makeup of the person, and therefore not being involved in natural selection.

 

[billvon]

My question is, what causes the body to either know that the body part in question has outlived it's usefulness

If it's harmless, the body doesn't "know" it's useless, and thus is generally preserved for a while. As the person's genome drifts due to random mutations eventually the genes that code for the appendix are lost and the organ is no longer expressed. That could take a long time.

If it's actively dangerous than the people without them will survive and reproduce, and the people with them won't. Thus the genes that code for that structure will be lost more quickly.

or if the parents of someone had it removed (or even just one parent), how does that potentially cause the next generation to be born without those body parts if removing those parts doesn't change the genetic makeup of the person, and therefore not being involved in natural selection.

They don't. Invasive medical care is definitely "anti evolution" in a sense (as is caring for the sick, special diets for people with allergies etc.)

 

[shawnofdenver]

If it's harmless, the body doesn't "know" it's useless, and thus is generally preserved for a while.

Danger to survival is more of a contributing factor? Well, on the face of it the appendix is harmless but appendicitis can kill you and unfortunately is rather common, and to survive, you have to have your appendix removed.

 

[spidergoat]

It could be that appendix infections tend to happen later in life, after one has already reached reproduction age. In that case, dying of infection wouldn't weed out the gene for having an appendix. Or not as much, since your death could affect the reproductive success of your children.

 

[Trippy]

...the appendix and other such organs used to have a purpose in human's...

Speaking to the appendix, it's been recently hypothesized that it does actually retain a purpose, whether it's part of its original purpose, or a co-opted purpose I don't recall, however...

I came across something recently that suggested that people who retain an appendix generally show a quicker recovery time from various forms of enteritis, and the hypothesis is that the appendix acts as a reservoir of normal intestinal flora, which after the illness has passed, reseed the intestinal tract resulting in a quicker recovery.

I'd have to have a bit of a nose around when I got home to provide any links though.

 

[arauca]

Wisdom teeth, and the appendix and other such organs used to have a purpose in human's, but now they don't. They now can cause damage and even kill you. I know a couple people who were born without wisdom teeth, and without an appendix. My question is, what causes the body to either know that the body part in question has outlived it's usefulness, or if the parents of someone had it removed (or even just one parent), how does that potentially cause the next generation to be born without those body parts if removing those parts doesn't change the genetic makeup of the person, and therefore not being involved in natural selection.

I case of wisdom teeth , the primitive man had some additional teeth in the jaw and the modern man have less teeth in the jaw, so at some point the Histone the proteine on which the DNA is raped around have been methylated by the gene for the last jaw teeth section . Then after certain age of maturity an enzyme dimethylated and the missing teeh come out as wisdom teeth. ( pardon my speculation)

 

[billvon]

Danger to survival is more of a contributing factor? Well, on the face of it the appendix is harmless but appendicitis can kill you and unfortunately is rather common, and to survive, you have to have your appendix removed.

Exactly. Thus there is some selective pressure to get rid of it - which is why it is now a tiny organ, on its way to being eliminated. (Note that it is not completely useless; it still serves as a repository for useful bacteria and may play a role in recovery from severe diarrhea. Thus there is some pressure to preserve it, although the pressure to eliminate it is probably greater.)

 

[Buckaroo Banzai]

The organ being tiny does not mean it is "on its way to being eliminated". In fact, if there's indeed selective "pressure" to eliminate it, this pressure decreases as the structure decreases.

It could eventually decrease even further or even disappear through genetic drift only, if its stage of "arrested development" is not something required as a stepping stone to the development of something else. But it could even get larger by the same mechanism. If there was a strong pressure against any increase in its size/favoring smaller size or absence (which is doubtful), it would be counterbalanced though, so in the end it would tend to stay the same size indefinitely, with perhaps only a tiny bias towards decreasing even more/disappearing from the species as a whole.

 

[billvon]

But it could even get larger by the same mechanism.

Useless structures are weakly selected against by conferring an energy penalty to the organism. Put simply, an organism with unnecessary organs will starve to death more often than an organism that does not need to expend energy to create those organs. The smaller they are the less the penalty, which is why most vestigial organs/structures are pretty small.

 

[Buckaroo Banzai]

Yeah, it's thought it was by cutting down costs that we got smaller intestines to begin with. Turns out that we need to spend less time of the day eating than chimps, despite of our brains expending more calories.

However, it can eventually reach a point where the additional maintenance cost is nearly null/small when compared to "noise", not being significant for selective sweeps. Its "rival" state could have something like just one selective death by 400 generations or so.

Natural selection does not care if the individuals require less food to survive/not starve because they have a smaller appendix, smaller muscles, a few less vertebrae, or if they're just smaller overall, or even if they're big, just not yet fully grown, or smaller by being females (or males, in species where males are the smaller ones). Not that the "smaller appendix" or "absent appendix" (or any vestigial structure) wouldn't ever be associated with those other traits or states as well, but as it would be a more recent mutation, not the ancestral state, the ancestral state would be in higher numbers and it's likely that it could be transmitted to the next generations without any noticeable decline in frequency.


But my point isn't that nothing like it could ever happen, that vestigial structures never ever get further reduced by any means or natural selection, or that they won't ever disappear. I'm more stressing that natural selection does not work in a way that would eventually/necessarily make humans evolve into those stereotypical "grey aliens". I'm not implying that anyone here said or thinks that, but it's a misconception many people seem to have when we're speaking of vestigial organs being "destined" to disappear.



And by becoming "larger" by drift I intended to say only slightly larger, a random walk with "steps" that are within the range of normal size variation, not "larger" in the sense of eventually having a cecum again, if that was the impression I gave.

 

[Rav]

Invasive medical care is definitely "anti evolution" in a sense (as is caring for the sick, special diets for people with allergies etc.)

Which essentially means that moving forward, natural selection is probably going to play an almost insignificant role in the future development of human beings in comparison to advances in medical technology.

Unless, of course, we're destined to experience a lot of sudden and significant environmental changes that are beyond our ability to moderate (in terms of their effect on us), which will serve as selective pressures.