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  #1  
Old 02-06-2001, 01:27 PM
City Gent City Gent is offline
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I don't think the Staff Report provides much of an answer. The question was why we have five rather than four or six, and the answer was essentially "Because natural selection favored it." This doesn't answer why five is best.

There has to be a distinction between the result of natural selection (five fingers) and judging the adaptiveness of a mutation (why five instead of four), otherwise natural selection becomes tautological: "Which features are the most adaptive? The ones favored by natural selection. Which features does natural selection favor? The ones that are most adaptive." This is a classic creationist critique of natural selection as an explanation for evolution.

I'd like to know, from a "biological engineering" standpoint, why five digits are better than four or six. Would six digits make us clumsy, or be too much for our nervous systems to deal with? Would four digits be too few to perform some vital function, like fighting or making tools?
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  #2  
Old 02-06-2001, 01:42 PM
C K Dexter Haven C K Dexter Haven is offline
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The Staff Report being references is: Why do we have five digits on each limb?

It's a good thing we weren't asked about why we have two sets of two limbs, because then I woulda had to say something about forewarned being forearmed, and so an octopus is twice as well off.
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  #3  
Old 02-06-2001, 02:02 PM
bdemarzo bdemarzo is offline
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It may not mean much, but take notice that most animated and cartoon characters have less than five fingers - four being preferred. Funny, despite this, they still use base-10 math...


- b
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  #4  
Old 02-06-2001, 02:13 PM
Ice Wolf Ice Wolf is offline
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The answer is one word: chance. Our twig on the evolutionary tree hung on better than the others.

In the past there were animals with eight or nine digits (Cambrian explosion and after). But pentadactyls survived the extinctions, the Ice Ages.

Because the part in our genetic code for more than five digits is still there from our distant ancestors, every so often anomalies turn up.
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  #5  
Old 02-06-2001, 02:21 PM
Kamino Neko Kamino Neko is offline
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Quote:
Originally posted by City Gent
I'd like to know, from a "biological engineering" standpoint, why five digits are better than four or six.
That was answered in the column.

Quote:
If you're asking why Therapsids evolved five digits, there isn't necessarily an adaptive explanation. ... Many people forget that "natural selection" means the eventual selective elimination of variants that are at a disadvantage relative to other existing variants. If five digits are no worse than four, then there is no selection against five.
In other words: Five's not better than four or six.

It just wasn't a disadvantage, so there was no reason for it to be selcted against, and those who had it also had other features that DID give an adaptive advantage.

Simple as that.
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  #6  
Old 02-06-2001, 02:48 PM
bungie_us bungie_us is offline
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Chance is clearly a factor, and I don't want to downplay it. But implies a level of randomness that isn't supremely significant in natural selection on a species level. Randomness and chance may be huge at the level of gene mutation, but at the level of gene expression, other factors come into play, such as the applicability of an adaptation to the conditions a critter lives in. If a chance variation isn't just outright fatal, then those other factors get to work on it.

So obviously 4 or 5 or 6 digits isn't outright fatal. You can certainly construct a theory explaining the advantages and disadvantages of any number of digits. And clearly, for the time being, 5 has had edge in humans and related beasts. The question remains, however, why 5? We have the geneology explained in the original column. But I'd like to see some informed discussion about the relative advantages and disadvantages of the number of digits we have, vs., say, the number that the Simpsons and Bugs Bunny have.

I don't expect a definitive answer, since one probably isn't possible. But the original column lacked a certain explanatory gusto, and chance alone doesn't cut it.
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  #7  
Old 02-06-2001, 03:26 PM
Tom Arctus Tom Arctus is offline
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Four just as good as five

I remember shaking the hand of a politician (Don Beyer, then Lt. Gov. of Virginia) and thinking he had adopted a peculiar grip that must have helped him through the long hours of handshaking endemic to the profession. Later I found out he had been born with only four fingers (OK, three fingers and a thumb) on his right hand. The missing digit didn't help him when he ran for the Governorship against now-Sen. Goerge Allen but that had more to do with his position on whether autos should have been taxed as personal property than how many fingers he had, or hadn't.

So at least in the modern world, four's just as good as five. Sometimes the explanation is that there is no explanation.

BTW he didn't use base nine either.
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  #8  
Old 02-06-2001, 04:01 PM
bungie_us bungie_us is offline
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Quote:
Sometimes the explanation is that there is no explanation.
Well, sure. But where's the fun in that?

C'mon, how about some wanton theorizing here? Please?
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  #9  
Old 02-06-2001, 04:22 PM
minty green minty green is offline
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Glad I'm not the only one who thought the Staff Report basically dodged the question.

I have no theorizing, wanton or otherwise, on why our ridiculously ancient ancestors evolved five digits instead of four or six. Yet clearly, there must be some sort of evolutionary advantage to five. Otherwise, the four- and six-digit mutations never would have been selected against in the first place, and we'd see a mix of numbers in the general population.
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  #10  
Old 02-06-2001, 04:32 PM
bup bup is offline
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I agree that the question was dodged - sort of.

He says that five is not at a *disadvantage* and then
chance made five win out.

But generally, when traits are not a disadvantage, nor an
advantage, both traits remain in the population.

Like hair color, eye color, and junk.

Why did five digits prevail, and other digitally-numbered
beasts *not make it?*
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  #11  
Old 02-06-2001, 06:47 PM
catmandu42 catmandu42 is offline
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Octal?

Well, I'm not sure that I care much as to why we have 5 digits. We do. They work. Fine with me.
What I want to know is why Octal would be a "more sensible arrangement" as opposed to what we use now. If horses could count, what would they use? Binary?
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  #12  
Old 02-06-2001, 07:53 PM
samclem samclem is offline
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Quote:
If horses could count, what would they use? Binary?
Hey! Whadda ya mean, if ? I seen 'em do it on tv.
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  #13  
Old 02-06-2001, 09:04 PM
foolsguinea foolsguinea is offline
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Well, apparently there's a better chance of a species having one default number of digits-per-appendage. Ancient horse-like skeletons sometimes have three-toed feet, but modern horses and asses uniformly have single-toed feet (solid hooves).

Given that...
I. For any vertebrate species, there is one normal number of digits, and 99% of the individuals of that species have that exact number of digits.
II. This normal number is not constant for all vertebrates, or even all mammals. (E.g., horses have one toe per hoof, but goats have two.)
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Therefore, we assume that there is little, if any, evolutionary advantage to any particular number of digits, but something in the process of speciation has left each species with a single, rarely varying, number of digits, which number is arrived at arbitrarily. That the normal number for humans is five, then, as far as we can tell, and in all probability, is merely chance, and not terribly surprising.
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  #14  
Old 02-06-2001, 09:37 PM
starryspice starryspice is offline
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Well, I'm glad I'm not the only confused one. Basically, the answer replied that five is not worse than four so it prevailed. But that doesn't explain why four (or three) digits didn't *survive*. Why were they worse than five? (this is basically rephrasing what bup said, sorry 'bout that)
Well, here's some illegitimate theorizing. If you look at our pinkies, they don't do a whole lot. But if you move each of your fingers independently, you'll see that the ring finger and the pinkie finger seem connected somehow. (Since I'm a clarinetist, this makes it harder to trill with the ring finger than any other finger.)
So, my conclusion is that the pinkie finger is useful to coordination, though not in trilling, and that this was somehow useful in making tools. Maybe when the ring and pinkie finger work together, they're more precise. (???)
Feel free to correct me, anyone. :-)
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  #15  
Old 02-06-2001, 09:49 PM
Ice Wolf Ice Wolf is offline
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A bit of light reading, everyone:

http://www.biologists.com/serve.cgi?...02/dev5694.pdf

So far, I've found out that in land vertebrates there are only five basic patterns to the Hox genes that control the numbers of digits. While the earliest land vertebrates had eight digits (similar to fish fin structure), this "fanned" method of locomotion became less and less necessary further on in amphibian/reptile evolution.

Maybe, just maybe, a five-toed creature was able to move faster than one with more toes. Dunno. My bet's still on Lady Luck.
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  #16  
Old 02-06-2001, 10:59 PM
Bryan Ekers Bryan Ekers is offline
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seven-eleven

Well, chance or not, if humans had had 8 fingers we probably would have had computer a few years earlier and the octal counting systems wouldn't be just a footnote.

On a related note, a computer professor once pointed out that Christmas was the same as Halloween since the value of "25" in decimal was equal to "31" in octal. In other words:

31 Oct = 25 Dec
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  #17  
Old 02-06-2001, 11:21 PM
asphodel asphodel is offline
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I am a physical education/kinesiology student, and in my human development and biomechanics courses (and a course I took in anthropology) point out that the pinky is useful in tasks requiring dexterity, but the great advantage of having a pinky finger is that it greatly increases the strength with which we can grip. (Try grasping something really tightly without using a pinky). When something is grasped between the palm and the fingers, it is called the power grip. In this position, much force can be applied.

Presumably, this characteristic was selected for during our evolution, as pentadactylism is an primitive trait in placental mammals, and an evolutionary trend in all primates. Strong grip strength is important for arboreal living, which all primates originally did. Tree-dwelling animals would not be able to climb vertically or cling from branches, let alone brachiate, without a very powerful grasp. Primates have evolved a deree of prehension of the hand and foot (except humans) wherby they are able to grasp; a monkey walking along a branch grasps that branch.

The presence of five digits in primates is called a primitive trait - it is primitive since it was originally present in all placental mammals. In many modern placental mammals, this feature has disappeared. Using the example of a horse, it is suited to running on hard ground at high-speed. This results in jarring and does not require forelimb flexibility. Therefore many primitive skeletal elements were selected against and have been lost in the horse, including the clavicle, decreased shoulder flexibility, the two bones of the lower arm have been fused, and the five fingers have been reduced to one, the hoof. Therefore, horses have lost those primitive traits. Horses have a highly specialized locomotor pattern, whereas primates are relatively generalized, we can carry out many forms of locomotion (quadrupedal, bipedal, and several variation thereof).


Also, on the issue of a base-10 numbering system due to having 10 digits - this is not necessarily so. Although the great majority of human societies use a base-10 system for this reason, not all do. In an anthropology class, the professor said that there was a group of people (can't remember the name now) who used a base-8 system because there are eight spaces between our ten digits.
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  #18  
Old 02-06-2001, 11:25 PM
padabe padabe is offline
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Quote:
Originally posted by Bryan Ekers
Well, chance or not, if humans had had 8 fingers we probably would have had computer a few years earlier...
Why? And what are the advantages of base 8?
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  #19  
Old 02-07-2001, 01:24 AM
catmandu42 catmandu42 is offline
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Yeah... That's what I originaly wanted to know in the first place!
And here's another odd question...
Why are our thumbs on the inside as opposed to outside?
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  #20  
Old 02-07-2001, 01:36 AM
asphodel asphodel is offline
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thumbs inside/outside

I know this is a nitpick, but in the anatomical position (the standard position when used to describe the relative locations of body parts), the thumb _is_ on the outside!

FYI - anatomical position is standing, facing forward, feet pointed forward, arms hanging at the side with palms facing forward. In this position, none of your bones "overlap" when viewed from the front. When your thumbs point "in", your radius and ulna (forearm bones) are crossed. When your thumbs point "out" then are lined up straight next to each other.
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  #21  
Old 02-07-2001, 10:08 AM
John W. Kennedy John W. Kennedy is offline
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The actual facts in this matter have only recently been reconsidered. Until lately, it was thought that five digits had always been the norm, and that early Tetrapod fossils with fewer were the exception. It now appears more likely that five was selected for.

It seems to me that five digits may be more or less the most practical number for supporting weight in the splayed postures of primitive amphibians and reptiles, but I'm just guessing. Ever since, it's been the default, with fewer occurring chiefly in fast runners with one or two toes specialized as hooves.

Cartoon characters traditionally have four digits because it's easier to animate, and close enough to five to look natural.
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  #22  
Old 02-07-2001, 10:29 AM
onigame onigame is offline
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Quote:
Originally posted by sparta
the great advantage of having a pinky finger is that it greatly increases the strength with which we can grip. (Try grasping something really tightly without using a pinky).
I'm not convinced by this example. It's true that if I keep my pinky outstretched and try to grasp something with my other fingers, the grip isn't as strong. But I think that's because the pinky is actively acting against the rest of the fingers by me keeping it outstretched. If I immobilize the pinky in the closed position, I feel no difference between that and using it.

Short of amputating, I don't think there's anyway to pursue this example further.
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  #23  
Old 02-07-2001, 11:58 AM
Doug Yanega Doug Yanega is offline
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Since I'm going to have to change the answer anyway (my background research was in error), I'll address your concerns in the revised version; the short of it is that - as I tried to point out - five doesn't HAVE to be "best" from a *design* standpoint. It just has to have no significant difference in *reproductive* value from any other heritable mutant variations. Under such conditions, the only way a four or six-digit mutant will spread is by genetic drift, which is an incredibly long shot.
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  #24  
Old 02-07-2001, 04:11 PM
Irishman Irishman is offline
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I have to agree with onigame. That example just doesn't cut it. If you extend your pinky, it is pulling against the other fingers. I'm not sure the percent increase in grip strength by the pinky, but I'm not convinced it is that much over a hand that doesn't have a pinky. If we had evolved with only four fingers, you would be using all fingers to grip. If extra fingers are more valuable, then why not six, seven, or eight?

When discussing finger linkages, it seems to me that the 3rd and 4th fingers are stronger linked than the 4th and 5th. Try extending your pinky with all other fingers bent. Then try extending your ring finger with (a) pinky extended and middle finger bent; (b) pinky bent (trapped to palm) and middle finger extended. I find I have better range of motion moving with middle finger than pinky. I recall a comment by someone who does a lot of rock climbing who said something to the effect that on small hand holds, use the middle and 4th finger rather than pointer and middle, because those two fingers are linked.
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  #25  
Old 02-07-2001, 05:45 PM
panamajack panamajack is offline
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This is an additional question, and more generically about genetics, but since there's already three open threads, I thought I'd pose it here.

I've always heard that polydactyly is a dominant trait (or at least that the 6-finger allele is dominant). Since I know very little about genetics, my questions are :
Is this even correct?
Does the dominance of an allele provide any information about an organism's evolution? Should the dominant trait win out over time in an otherwise equal situation?
If it should, perhaps the polydactyl mutation hasn't been around long enough? Would it always have been dominant, or is it possible for that to change over time? I suppose in that case it might be considered a different mutation (recessive polydactyly vs. dominant polydactyly). Have two different versions like this possible/observed (on any gene)?
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  #26  
Old 02-07-2001, 06:54 PM
catmandu42 catmandu42 is offline
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Well, in cats polydactylism is considered a dominant trait, or so I've been informed. Why there aren't more of them if this is true is a matter of speculation. I think they just aren't bred as fanaticaly as the "pure" breeds because "cat fancier" magazine or whatever says not to. In humans, I would suspect that having 6 digits on each hand would probably win you a lot of bar bets, but wouldn't exactly endear you to the ladies.
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  #27  
Old 02-07-2001, 08:35 PM
Chronos Chronos is offline
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Dominance and recessiveness don't have any direct connection to the commonness of a neutral gene. Dominant just means that you'll show the trait if you have the gene; it does not mean that you're more likely to pass it on to your offspring. The textbook example in humans is hair color: Dark hair is dominant over light hair (there's a slew of other genes responsible for hair color, as well, but this is a reasonable simplification). If a person has dark hair, they might have two dark genes, or one of each, and if they have one of each, then they can still pass on a light-haired gene and possibly have blonde kids. If a person is light-haired, then they must have two light genes, and so all of their kids will have at leastone light gene.

This does not apply if the gene is not neutral; that is, it has either a positive or negative effect. If a "bad" trait is carried on a dominant gene, it will very quickly die out, since anyone having the gene will have the trait, but genetic diseases can hide out in recessive genes for a very long time, where the organism won't even notice them.
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  #28  
Old 02-08-2001, 07:08 AM
bungie_us bungie_us is offline
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Quote:
Originally posted by Irishman
I have to agree with onigame. That example just doesn't cut it. If you extend your pinky, it is pulling against the other fingers. I'm not sure the percent increase in grip strength by the pinky, but I'm not convinced it is that much over a hand that doesn't have a pinky. If we had evolved with only four fingers, you would be using all fingers to grip. If extra fingers are more valuable, then why not six, seven, or eight?
This response is part anecdotal, part random speculation. One of my roommates in college used to work cannery ships as a summer job. One year he had an accident on the ship and lost his right pinky. That was it for that year, of course, but he went back the next year . . . and was home again in a week. Even though he'd fully recovered (short of growing a new finger) and had done a lot of physical therapy, he struggled with controlling his knife. He definitely felt it was a grip strength problem. I don't know that this proves anything, but I thought I'd throw it out there.

Now for random speculation. Let's, at least for the moment, assume that pinkies ARE important from a grip strength standpoint. Why not more fingers? Well, additional anatomical features require additional energy to develop and support them. You gain one kind of benefit from adding features, but for a price. At some point you have diminishing returns. The benefit might not increase as fast as the cost. So maybe six fingers are slightly better, but more than slightly more expensive. Throw in catmandu42's unenthralled ladies, and five fingers ends up winning. This is probably grossly oversimplified, but you can see what I'm driving at. (A brick wall?)

Though, finally, I have to say that I know at least two women who would have found a use for an extra finger or two, if I'd had them. I'd have won more than a bar bet.
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  #29  
Old 02-08-2001, 10:23 AM
Captain Amazing Captain Amazing is offline
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We have five digits on each hand because otherwise, our gloves wouldn't fit.
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  #30  
Old 02-10-2001, 04:51 PM
Derleth Derleth is offline
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In re. Dr. Lecter, having six fingers obviously does not compromise mental ability, and if that does not aid in survival I don't know what does.



Seriously now, my theory is chance with some coordination issues thrown in. Five fingers might not be much easier to control than six, or much harder than four, but the limited amount of neurons you can put to controlling the fingers on one hand would limit the amount of fingers per hand. Maybe this barrier would not be firm (few things are at this level of abstraction) but it would exits. So with maxiumum brainpower per hand to set the upper boundary, minimum dexterity and grip strength to set the lower boundary, and a liberal amount of blind chance, five is just the number of fingers most humans wound up with.

Besides, if we didn't have an odd number, how could we flip someone the middle finger?

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  #31  
Old 02-10-2001, 04:54 PM
Derleth Derleth is offline
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Find-and-replace

Replace the sentence "Maybe this barrier would not be firm (few things are at this level of abstraction) but it would exits." with "Maybe this barrier would not be firm (few things are at this level of abstraction) but it would exist." and slap me upside the head.
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  #32  
Old 02-12-2001, 01:18 PM
tracer tracer is offline
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Birds!

What I wanna know is, why do birds only have FOUR digits on their feet, if they evolved from the same pentadactylic critters that we did? Huh? Hmm? Huh?
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  #33  
Old 02-12-2001, 01:41 PM
sagitta sagitta is offline
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grumbles

First I would like to remind sparta that we are talking about _comparative_ anatomy i.e. the anatomy of organisms in general. In vertebrate comparative anatomy the thumb is considered to be on the inside. It is only human anatomy which considers it necessary to twist peoples' limbs around before dissecting them. I don't think there's any good reason for this, it's just a historical quirk of the medical profession.

More importantly, I take issue with several of Doug's statements. I note in passing that he quite reasonably glosses over the conflicting definitions of Tetrapoda (traditionally vertebrates with digits but controversially defined by Gauthier as Amniota+Amphibia). But I can't tell what he means by 'land tetrapods'.

>the evolution went--as far as we can tell--very rapidly from fins to five toes, possibly with seven and/or six toed intermediates.

There is no evidence at all for a transition from fins to five toes. The best evidence suggests a move from fins with a fan of bones to fins containing many digits. An unnamed fish (http://sln.fi.edu/qa98/biology/journals/part14.html) has 8 in the forefin (hind fin unknown), but this may have been variable.

Closer relatives also had more than five digits per limb. See http://phylogeny.arizona.edu/tree/eukaryotes/animals/chordata/terrestrial_vertebrates.html for a cladogram of tetrapods and their relatives. Note that sucessively closer outgroups include Ichthyostega (unknown, 7), Acanthostega (8, 8) and Tulerpeton (fore 6, hind 7). Digit numbers are unknown for the other outgroups.

It is extremely unlikely that these early forms all independently acquired extra digits. It is much more parsimonious to suppose that the first toed animals had more than five, but the number may have been quite variable. Later tetrapods reduced the number to five - this may (speculation) have been associated with the move to land. Some, notably amphibians, later reduced the number further. Only one clade - ichthyosaurs - permanently increased its digit count.

>There simply haven't been any such mutations on the line leading to primates.

Polydactyly is a very common mutation among tetrapods. It occurs fairly frequently in humans and many other mammals. It doubtless occurred millions of times in the early mammals which gave rise to primates.

>Under neutral selective conditions, the only way a four or six-digit mutant will spread is by genetic drift, which is an incredibly long shot.

Genetic drift is not an incredibly long shot. It is an extremely powerful evolutionary force. 'Silent' mutations, which are thought to be virtually unaffected by selection, can become fixed in populations extremely rapidly (in geological terms). It is not known whether polydactyly per se has any advantages or disadvantages. It can be caused by otherwise harmful mutations, but other forms appear benign.
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  #34  
Old 02-12-2001, 06:36 PM
samclem samclem is offline
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sagitta Holy snikes! Thanks for the post. Nice way to enter the board. Good info. Is this your field?
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  #35  
Old 02-13-2001, 12:30 PM
Mycroft.xxx Mycroft.xxx is offline
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Why it's not 6

The offshoot which had six fingers was deselected by a swordsman who kept repeating the same line, "My name is Inigo Montoya, You killed my father, prepare to die."
The movie "The Princess Bride" chronicles the first such incident. Thus having six fingers became an undesireable trait.
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  #36  
Old 02-13-2001, 01:36 PM
Chronos Chronos is offline
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Wow, a Heinlein-inspired name, and a Princess Bride-inspired first post! Stick around, Mycroft.xxx, you'll fit right in at this place
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  #37  
Old 02-14-2001, 07:29 AM
C K Dexter Haven C K Dexter Haven is offline
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Heinlein-inspired? Mycroft? ... Somehow I think it goes back a wee bit further, old chap. If we were dicsussing oxygen, I'd say it was element-ary.
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  #38  
Old 02-15-2001, 03:15 PM
sagitta sagitta is offline
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thanks

Thank you, samclem. you're quite right, I am a professional annoying smartass. Also I used to study biology, and I was sufficiently interested to do a little web research.

tracer: the story of birds' toes is quite interesting. Fossils suggest that it happened like this: early dinosaurs started walking on their hind legs. Theropod dinosaurs soon reduced the first and last toes to small claws, so they were walking on just 3, a bit like some early horses. Some theropods enlarged the first toe to make a strong claw (but not dromaeosaurids ('raptors'), whose sickle claw is the 2nd toe). Later theropods also lost 'fingers' 4 and 5.

Birds evolved from theropods, probably but not certainly from dromaeosaurs. They reversed the first toe to point backwards-handy for perching, but by then the fifth toe had already disappeared. Most birds have 4 toes on their feet, and three digits fused together inside the wing. From studies of chick embryos the remaining fingers look more like 2, 3 and 4 than the 1, 2 and 3 of theropods, but I recommend siding with the fossils on this one.

A little polydactyly trivia: I remember reading of some graves of important families excavated in the Middle East. I think it was Egypt, but before the time of the Pharaohs. most of the skeletons had exrta fingers and/or toes, and the number tended to increase over time. The archaeologists thought that this family might have been revered for their polydactyly, marking them out as rulers.

Anne Boleyn, the 2nd wife of Henry VII of England, had 6 fingers on one hand, but they didn't do her much good. It was seen as a sign of witchcraft, and court gossips whispered that she had seduced the king by unnatural means. When she became inconvenient to Henry, she was beheaded on trumped-up charges of adultery.

Any other famous personalities with extra digits?
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  #39  
Old 02-16-2001, 05:34 PM
Doug Yanega Doug Yanega is offline
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Sagitta wrote:

>More importantly, I take issue with several of Doug's statements. [snip] But I can't tell what he means by 'land tetrapods'.

It becomes clear below what I meant, and why I specified this.

>>the evolution went--as far as we can tell--very rapidly from fins to five toes, possibly with seven and/or six toed intermediates.
>There is no evidence at all for a transition from fins to five toes. The best evidence suggests a move from fins with a fan of bones to fins containing many digits.

And from fins with many digits, to limbs with many digits, to limbs with five digits, all in a brief geological interval *is* what I implied above. I didn't say DIRECTLY from fins to five digits, I said RAPIDLY.

>It is much more parsimonious to suppose that the first toed animals had more than five, but the number may have been quite variable. Later tetrapods reduced the number to five - this may (speculation) have been associated with the move to land.

And in what way is this any different from what I said? If the first tetrapods living on land had more than five digits, then they represent the intermediates I referred to. The idea that they reduced the number to five in association with the move to land is exactly why I specified LAND tetrapods, as opposed to aquatic forms like Acanthostega and semi-aquatic forms like Ichthyostega.

>Polydactyly is a very common mutation among tetrapods. It occurs fairly frequently in humans and many other mammals. It doubtless occurred millions of times in the early mammals which gave rise to primates.

Doubtless it has, and now you're just nit-picking, because I didn't say "succesfully perpetuated mutation".

>Genetic drift is not an incredibly long shot. It is an extremely powerful evolutionary force. 'Silent' mutations, which are thought to be virtually unaffected by selection, can become fixed in populations extremely rapidly (in geological terms).

Here we genuinely differ in opinion. Drift is a negligible evolutionary force in any population of respectable size. It only becomes "powerful" in populations composed of, say, less than 50 male/female pairs. An excellent analogy is trying to flip a coin so it comes up heads every time; the odds of flipping nothing but heads only become significant when you have a VERY small number of flips. Genetic drift is almost exactly the same: random shuffling of alleles in the population until one of the two alternative alleles manages to - at some point - eliminate the other. Moreover, it has to be the initially *rare* allele which wins in the end. This is NOT a powerful force, by any means. It is almost never invoked as being meaningful except in cases where one or a few individuals of a species become totally isolated from the remainder of the species, called "the founder effect" - a situation which may occur in many speciation events, but evolution and speciation are not synonyms; speciation is just one special case of evolution. Let's put it this way: natural selection is a major force in evolution, and drift is a major force in speciation. The VAST majority of evolutionary change occurs via the action of natural selection on mutations, and does NOT involve speciation.
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Old 02-16-2001, 06:58 PM
JillGat JillGat is offline
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I'm just impressed with how many 1st and 2nd time posters this SR inspired.
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  #41  
Old 02-22-2001, 04:07 AM
Gaspode Gaspode is offline
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OK, this is just wild theorizing and I am definitely not an engineer, but how about this.
Each limb in a tetrapod has to be able to support body weight in a stable manner. The minimum number of contact points with the ground to provide stable support is three to form a tripod. Therefore the first terrestrial tetrapods had to have at the bare minimum of three toes. Since nature seems to always select for some redundancy where possible four toes would have become the norm to compensate for accidental amputations etc. The dew claw/toe in tetrapods is located quite a way behind the other digits and is designed primarily to give sideways support to prevent sliding. It doesnít contribute a lot to stability during forwards movement. Once the dewclaw had moved to its current position it couldnít contribute to stability and so had to be either replaced by another digit or more likely another digit was retained to make up the four necessary for stability +redundancy.
Now I know the heel can be used as the third leg of a tripod when standing motionless, but during movement the heel is lifted from the ground, so three really is the bare minimum number of toes.
Why only one redundant toe might have been selected rather than 2 or three I canít hypothesise except that obviously the use-it-or lose it principle applies. If accidental amputations of two toes was uncommon enough it didnít confer significant advantage then only one redundant digit would be needed.
Yes I concede this is a WAG based on some science and made up entirely to fit the facts.

Catmandu42
Why are our thumbs on the inside as opposed to outside?
Simple, because our thumbs are modified dewclaws. Dew claws are designed to allow an animal to apply sideways grip. This is applied in carnivores when hunting to allow them to hold their prey, and in arboreal mammals to allow them to grip tree limbs. It is also used by males to grip females during copulation. In other tetrapods the dew claw is held much closer to the ground and is used to provide traction that prevents the legs from moving on unstable surfaces.
Amongst these animals the legs are splayed, making it easy for the legs to fly out but almost impossible for them to collapse in, hence the dew claw developed on the inside. As a result of all these pressures the dew claws are located on the inside of the limbs and since thumbs are modified dew claws the thumbs are there as well.
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Old 02-24-2001, 02:24 PM
Doug Yanega Doug Yanega is offline
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Gaspode wrote:

"Why are our thumbs on the inside as opposed to outside? Simple, because our thumbs are modified dewclaws."

Look at the ancestral tree tracing back from primates. You'll notice that none of our ancestors all the way back to the amphibians had dewclaws. None. That doesn't lend much credence to your theory. To say that the dewclaw is HOMOLOGOUS with the thumb is fine (i.e., have a shared ancestry), but not to say that the thumb is derived *from* a dewclaw. There is simply no evidence for that.
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Old 02-24-2001, 03:41 PM
blokheadj blokheadj is offline
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5 is not necessarily the better number

This is kind of an aside but seems to be an issue here as well. I'm surprised that no one's brought this up but I recall learning in three seperate biology classes that five digits is actually a recessive trait in mammals. Six digits is a dominant trait. The reason we don't all have six digits is because of a genetic mutation that causes the recessive trait to manifest.
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Old 02-24-2001, 07:38 PM
Gaspode Gaspode is offline
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Okay, I've realised I'm using the word dew claw completely incorrectly here.
I'm referring to the 'thumb' digits that diverge from the feet at a distinctly more proximal location than the other digits.
Anyone know the name, I'm sure I did know it but I've obviously forgotten.
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  #45  
Old 02-25-2001, 12:28 PM
Colibri Colibri is offline
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Quote:
Originally posted by Gaspode
Okay, I've realised I'm using the word dew claw completely incorrectly here.
I'm referring to the 'thumb' digits that diverge from the feet at a distinctly more proximal location than the other digits.
Anyone know the name, I'm sure I did know it but I've obviously forgotten.
Well, the "thumb" digit does not "diverge" from the feet at a more proximal position than the other digits relative to the carpals/tarsals (wrist/ankle bones). It may look that way in some taxa, like in humans. The digits in vertebrates are numbered I-V, with the innermost, corresponding to the human thumb, being digit I. In most vertebrates, it differs from the other digits in having fewer than 3 phalanges (joints). The "primitive" condition in tetrapods (at least those beyond the more-than-five-digits-stages) is a phalangeal fomula of 2-3-4-5-3. In mammals that has been reduced to 2-3-3-3-3 as a primitive condition, as is found in humans.

"Dewclaws" properly refers to the reduced digits that do not support weight (and often don't touch the ground) in many artiodactyls and carnivores. In artiodactyls, IIRC they represent digits II and V (I being lost almost completely), while in dogs I think it is digit I.

blokhead, traits don't spread in a population depending on whether they are dominant or recessive, they spread depending on whether they are advantageous or deleterious. There are lots of deleterious alleles that are dominant.
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Old 02-26-2001, 07:13 PM
blokheadj blokheadj is offline
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I disagree

You said: blokhead, traits don't spread in a population depending on whether they are dominant or recessive, they spread depending on whether they are advantageous or deleterious. There are lots of deleterious alleles that are dominant.

Well I'm not sure I agree with you there unless you also think that more people have Brown eyes(a dominant trait in humans) because its advantageous. I can't see why brown eyes would be more advantageous than blue.

Although traits also manifest based on whether they are advantageous or deleterious(that's just typical of evolution) they also do spread based on whether they're dominant or recessive. If that weren't so then we'd all have 6 fingers and brown eyes. And I've never had six fingers so I can't say that it would be deleterious. You might think that but it may not be true.
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Old 02-26-2001, 07:34 PM
Colibri Colibri is offline
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Sigh. blokhead, I wan't to insult you but you clearly do not have even a basic understanding of either genetics or evolution. The statements I made are so fundamental to the subject I don't really feel it is necessary to defend them. I would suggest you do some reading in the subject before you decide you "think you disagree" with me.
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  #48  
Old 02-26-2001, 07:52 PM
blokheadj blokheadj is offline
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Fine lets agree to disagree then.

I DON'T SIMPLY THINK IT...I KNOW I DISAGREE. Is that any more clear? And actually I might say the same to you. I am not a geneticist nor do I pretend to be but I do know a thing or two about it. I did not simply state that you were wrong merely that it's not entirely correct. So let's just agree to disagree and leave it at that.
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Old 02-27-2001, 08:52 AM
Colibri Colibri is offline
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Sorry, blokhead, the mission of this board is to fight ignorance, and your statements are based on ignorance. It is not merely that I disagree with you, the facts disagree with you. My statements are based on information that can be found in any elementary discussion of evolutionary genetics, while yours are evidently based on half-remembered or misinterpreted information from high-school or undergraduate biology classes.

To clarify: dominance/recessivness can influence the rate of spread (or elimination) of an allele in a population, but not whether or not it spreads. The main factor influencing whether a trait spreads or not (in large populations) is natural selection (including sexual selection) although other things such as mutation pressure can come into play in rare instances.

Now go do some homework on the subject before I bury you in Hardy-Weinburg equations.
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  #50  
Old 02-27-2001, 12:15 PM
Doug Yanega Doug Yanega is offline
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Sorry to interrupt this little war you guys are having, but you're both a little off base. Allow me to rephrase so things are technically correct:

For an allele that is NOT selectively neutral, dominance/recessiveness can influence the rate of spread (or elimination) of an allele in a population, but not whether or not it spreads; that is determined by selection. The rate is faster is the allele is dominant. For an allele that IS selectively neutral, dominance/recessiveness is unimportant, because genetic drift is indeed an essentially random process (assuming no pleiotropy, etc.)

If things like brown eyes or six fingers are dominant but selectively neutral, their spread or elimination can only occur by chance.

P.S. It's Hardy-WeinBERG.
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