D.H. Cross

What is the difference between the small incremental mutations that lead to more complex features of a creature (like a wing or eye), and a sudden extreme mutation.

For example, my wife said that there have been people born with a 3rd arm, fully functional. I doubt the "fully functional part". I was wondering if this is true, and if this sort of thing can happen, is it an example of how evolution works? Would people have third arms if it had helped them survive?

I'm just confused about this.

Doubting Didymus

Actually, you have hit apon an area of biology that is quite controversial.

However, I can tell you that there is no real 'difference' between small and large mutations, its just a matter of degree.

There is no doubt in the biological community that small mutaions could be the only kind of successful adaptation, but there is controversy over how often large mutations can affect the course of evolution, if at all.

Personally, I am inclined to think that large mutations almost never find their way into the gene pool, because the bigger the mutation, the less likely it is that the mutation is a net benefit. A third arm is probably more harm than good as it is likely to come with all kinds of negative mutations at the same time.

Richard Dawkins uses the metaphor of small islands representing possible forms that are living, in an ocean of possible forms that cannot survive. Small steps in any direction will keep you on the islands, while giant leaps will probably land you straight in the ocean.

Undercurrent

I think such things have to do with the fact that the form of a creature isn't encoded in the DNA like "place cell #4133324413 at position (10.5, 6.2, 4.6) in the standard coordinate map". If it was, we would expect every small mutation of the genome to produce only a small mutation of the phenotype.

The classic example is the gene in fruit flies that, if disabled, causes legs to grow where the antenna should be. There is another one, that if disabled, causes legs to grow on all segments of the abdomen. This indicates that the part of the genome that encodes development encodes it like "put legs on all of the segments but don't put legs on the abdominal segments and make the ones on the head into antennae." This kind of encoding allows small changes to the genone (e.g. disabling the last clause) to create big changes in the organism (e.g. antennae => legs.)

This also makes sense in terms of evolution. The genome started out as encoding a segmented worm with legs on each segment (i.e. the genome says "put legs on all of the segments") and higher organisms speciallized that by adding genes to specialize the original form (i.e. "but not on the abdominal segments and specialize the ones on the head", rather than reworking the entire organism.

m.

Doubting Didymus

True, but large mutaions like legs instead of antannea are highly unlikely to give a net benefit, and will not get into the gene pool with any notable frequency.

D.H. Cross

It's no wonder evolution is so difficult to understand. I can grasp Darwin's basic ideas, like natural selection, but the inner workings of the genome and random mutations that can produce a third arm or a second head is just...weird. I suppose we are not "programmed" to consider such things, or to identify with them as if it were something quite natural. Of course here the word "natural" becomes a word of debate and varied connotation, but that is another subject entirely.

Would I be correct that a third arm (or second head or whatever) can appear as a mutation on a human only because the genes are there to produce them...it is simply misplaced? This is not the same as a lizard sprouting wings for instance!

Let me clarify my point. I said to my wife that wings did not spring suddenly from birds, but formed over an unimaginable span of time, in small increments. The possiblity of a mutation that would produce a fully functioning wing in a single generation is...well...like about on par with falling up after jumping from a building (unless on the moon). Am I correct? Now this is not to say, I assume, that a bird whose genome already contatins instructions for "wing" cannot be born with a wing coming out of its forehead, right? And so this would be maladaptive, to say the least, and the chances of such a bird being introduced into that gene pool is like...nearly impossible.

THe point? I suppose it is that the difference between a gross mutation like a full arm or a 5th wing or a 3rd eye and the sort of small mutation that happens...is...is...that a complex appendage cannot appear without intermediate steps in development over a peroid of millions of years?

To repeat...human beings did not develop intellegnece over night as a sudden HUGE mutation of the brain, right? Natural selection does not work like that, so I'm told.

And if not, what is this "punctuated equalibrium" that Gould spoke of? (Please excuse my spelling people...). Was he dealing with the sort of mutations that are complex and adaptive in a single generation?

Still confused but getting there,

The Crossman.

Dr.GH

There are genes which control the sequence of development. Minor changes (mutations) of these genes will result in major phenotypic changes.

Try a search on HoX genes. Happy reading. You might also take a look at :<a href="http://www.devbio.com/" target="_blank">Gilbert Developmental Biology, 6-e</a>

and

<a href="http://www.ultranet.com/~jkimball/BiologyPages/W/Welcome.html" target="_blank">Kimball's Biology Pages</a>

Search on "HoX." These are both rather large sites and I can only say that the portions I have reviewed are, within the limits of my competence, accurate representations of current knowledge.

[ July 14, 2002: Message edited by: Dr.GH ]</p>

Hallucigenia

In Darwin's view, natural selection was granted the vast majority of creative power in evolution. While it was acknowledged that sudden, extreme mutations could produce a new trait or form or whatever, such "leaps" must be necessarily rare, lest mutation become the creative force, and natural selection be relegated to the role of executioner. Thus, Darwin posited that mutations, by and large, must be small in effect, thus allowing natural selection to "create" by virtue of slow accumulation of the most beneficial mutations over vast periods of time.

Quote:

And if not, what is this "punctuated equalibrium" that Gould spoke of? (Please excuse my spelling people...). Was he dealing with the sort of mutations that are complex and adaptive in a single generation?

Punctuated equilibirum was developed as a theory to posit methods of speciation, not a novel mechanism for such. The driving force behind PuncEq is still Darwinian natural selection. The twist comes in that, whereas Darwin posited speciation by virtue of the aforementioned slow, steady accumulation of mutations (aka, "anagenesis", or speciation by transforming lineages), Gould & Eldredge put forth the theory that most speciation occurs rapidly, when lineages splinter off to form sub-populations (aka "cladogenesis", or speciation by splitting). This, then, explains the "sudden" appearance within the fossil record of most forms: the speciation event, while still generally longer than the lifespan of a human, is geologically quite rapid, thereby limiting the opportunities for fossilization of the intermediate forms.

[Removed extraneous quote tag]
[Then corrected a typo]

[ July 14, 2002: Message edited by: Hallucigenia ]

[ July 14, 2002: Message edited by: Hallucigenia ]</p>

RufusAtticus

D.H. Cross,

Mutations of large effect are less likely to be incorporated into the population than mutations of small effect. However, this is relative. "Large" and "small" can differ depending on how far the population is away from the local fitness maximum.

Let's say the optimum length of a cogworm is 12 inches and fitness increases as the worms approach 12 inches.

Say cogworms are currently around 3 inches long. At this point a mutation that increases the length by 4 inches would have greater fitness than one increacing it by two feet.

However, if cogworms are currently around 10 inches long a muation that adds or subtracts 4 inches would have less fitness than one adding just 1 inch.

It appears that as a population nears a local maximum, the mutations successfully incorporated have smaller effect.

This isn't to say that evolution is goal oriented or produces optimum individuals. It's that typical selection mechanisms do tend to make the population more adapted to a given environment. In the previous example, "optimum" refers to the point in phenotype-space from which no adaptation can improve.

Punctuated Equilibrium has little to do with mutations. Punctuated Equilibrium is simply an application of population biology and evolution to paleontology. What Gould and Eldrigde realized is that the broken nature of the fossil record is exactly what we should expect from how we know nature works. Before punctuated equilibrium, most people thought that the fossil record was incomplete because we hadn't found enough fossils. Punctuated equilibrium actually explains that the fossil record will be incomplete no matter how many fossils we find. This is due to how speciation occurs.

Speciation in most instances is the result of isolated populations aquiring novel traits and then replacing the larger population. This replacement can and does happen suddenly, geologically speaking. As such, fine-grained species to species transitions should be rarely preserved in the fossil record.

Puncutated Equilibrium is not a new theory of evolution. It is in fact the application of evolutionary theory to paleontology.

~~RvFvS~~

DireStraits

For example, my wife said that there have been people born with a 3rd arm, fully functional. I doubt the "fully functional part".

DiI was wondering if this is true, and if this sort of thing can happen, is it an example of how evolution works? Would people have third arms if it had helped them survive?

Vorkosigan

I'm a little curious about how "small" and "large" are to be defined here? By the percentage of genes affected? The morphological change? I mean, if my kid developed the ability to sense UV radiation, wouldn't that be a bigger change than a third arm, even if the morphological changes in the eye were small?

In any case, we know of this kind of massive change in evolutionary history: for example, the fact that our left brain controls our right side is due to such a violent mutation, no?

Vorkosigan