Comments on: Rethinking Sahelanthropus

By: mystery_man

mystery_man — Wed, 07 Mar 2007 16:07:49 +0000

I will even add to my comment above and go as far as to say that it does not even require genetic drift for these neutral mutations to occur. All it needs is a genetic mutation to occur, be that genetic drift, radiation, copy errors during duplication of genetic material, whatever. The point is, this mutation occurs randomly to beging with. It is not until it is subject to the effects of the ecosystem that it is selected for or against. It does not have a purpose or reason to come about in the first place. Normally these random changes to the genome would be shaped by natural selection but nuetral mutations as I have said are not subject to this. They have no reason to be there, yet have no reason to be changed or supressed by any sort of selective pressure and so they remain until that is no longer the case and selection changes them. In this way, there is no need for a reason for the feature to be there, merely that it is not selected against and the feature has a chance to spread through the genome.

By: DWA

DWA — Wed, 07 Mar 2007 14:56:37 +0000

Well, mystery_man:

I don’t think that there’s a selective pressure mechanism that operates through some sort of selective pressure to exert what we might call selective pressure on organisms to adapt according to selective pressure.

I think on a per-word basis I have you beat. It might even make sense if I read it.
But seriously, folks. As I said in a post above, I agree with you. Some things happen and just kind of stay there because nothing selects either for or against them. Can’t it be said that the incredible range of appearance differences among humans exists because our cultural evolution has insulated us from much of the selective pressure other species face?

I count five uses. Pretty good there.

By: mystery_man

mystery_man — Wed, 07 Mar 2007 14:51:46 +0000

And I appreciate the thoughts on aesthetics, but that is not what I meant. It was more of a joke that evolution itself doesn’t look at it and think it’s ugly, so I will get rid of it.

By: mystery_man

mystery_man — Wed, 07 Mar 2007 14:45:10 +0000

How’s that for fitting “selective pressure” as many times as possible into a paragraph?

By: mystery_man

mystery_man — Wed, 07 Mar 2007 14:43:43 +0000

Not neccesarily so, Kittenz. I appreciate your thoughts, but I have to disagree with you there. I will put it another way. Not all features develpo purely due to selection. In a species genome, there will sometimes occur manifestations of certain mutations that are random and occur by statistical chance. This is called genetic drift. It does not require a “reason” as you put it. Sometimes these mutations will be beneficial and will, due to selective pressures, become dominant traits aiding the survival of the species. Others are detrimental, which will obviously fall out of the gene pool. Others are referred to as “nuetral mutations” that have no positive or negative effect on species survival or mortality. In my theory above, the brow ridge could be considered one of these. None of this is happening in a vaccum, mind you, these are random manifestations of traits caused by chance genetic drift. In absense of selective pressures, a species genome undergoes a steady accumulation of these neutral traits. In effect, if this random mutation causes no selective pressure to get rid of it, then it just ends up being “there” and propogates through the genome as any other feature would. Without any particular selective pressure to remove it, it remains. It requires no reason at all, merely that the nuetral genetic mutation not be influenced by selective pressures within the environment.

By: kittenz

kittenz — Wed, 07 Mar 2007 13:14:47 +0000

I agree with what you said, mystery_man, everything that remains is not always useful. But there had to have been some “reason” for the feature to have evolved in the first place.

There are also “reasons” that some things semm to be “aesthetically pleasing”. Many of the reasons are not always readily apparent. We might just think “Wow, (he or she) is nice-looking” - but what is it that makes a human think that another is attractive? Most of the outward signs of “beauty” or “attractiveness” are also indicators of good health - which means that that individual should produce healthy offspring, so “combining genes” might be advantages for the survival of one’s own offspring.

Nothing evolves in a vacuum.

By: Ceroill

Ceroill — Wed, 07 Mar 2007 04:26:12 +0000

Perhaps a better example of a useless vestigial feature is nipples on male mammals.

By: mystery_man

mystery_man — Wed, 07 Mar 2007 02:46:42 +0000

I must add that the brow ridge is such a prominent feature in so many primates and early humans that I doubt this is the case here. It would seem odd for this feature to just happen to “be there” in so many different species. It was just a thought.

By: mystery_man

mystery_man — Wed, 07 Mar 2007 02:41:03 +0000

Kittenz- True, we may not always know the reason for a development, but evolution does not always lead to more complex forms. I guess you are right that some of these organs can lead to some different purpose so perhaps my example of snakes was a bad choice. But my point is that not everything that rermains is always useful. If a feature is benign and does not work against the animal in any way and therefore gets passed on, then a feature that serves no purpose can be passed on. The environment does shape the creature but in a way that adapts it to best survive in the environment, these features get passed on because of selective pressures. If there is something that is not impeding that, then there is no particular reason why it should have to dissappear. Evolution does not say “don’t need that” and then get rid of it because it isn’t aesthetically pleasing. I did not say they evolve to be for no reason, but rather if they are just there and never had to be selected out by the environment, and it is not detrimental to the organism in any way, who is to say that it should have dissappeared?

By: kittenz

kittenz — Tue, 06 Mar 2007 22:39:31 +0000

Actually boas’ and pythons’ spurs do have a purpose, mystery_man. They are used during mating. Both males and females have spurs but as a rule the spurs of male snakes are larger and better developed. Each spur has a claw, very similar to the claw of a small dog. The snakes have some control over the claws and can move them.

Features that serve one use can become useful for something entirely different over time, the way snakes’ spurs have done. Characteristics and organs that no longer enhance survival can become vestigal, like the appendices and brow ridges of modern humans.

I disagree that features can evolve for no reason. We don’t always figure out the reasons, and we may very well be wrong about some of those that we think we have figured out, but environmental factors shape all animals.

It’s true that not all features are necessary for survival, but any feature which makes it a bit more likely than an animal will survive, to produce offspring which also survive, is likely to be passed on and become a feature of that species.

Of course, it’s also possible that the ancient females just thought that males with heavy brow ridges were better looking :).