Mutation Population

[Southwind17]

So you only want to talk about mutations that are not subject to selection pressures?

Yes, I suppose so, because I think I understand the 'selection' process, which is largely eliminated from the human species, compared to, say, wild animals, that is (or does that demonstrate that I don't understand it as well as I'd like to think I do?! )

If the total population size is also increasing, and six-fingered people aren't selected against, then sure. But if the population stays stable the progress of the mutation (increasing or decreasing in abundance in the population) should also stay relatively stable. Actually it will change basically randomly.

But surely all of the mutations that have occurred in the past that have led to humans being what they now are, and essentially the same in all of us, were not all subject to selective pressures for their replication, or were they?

But of course they aren't biologically identical. You and I are not identical twins. Nor were your parents - and lucky for you because if they were you'd have a decent chance of genetic disease.
There is a certain amount of genetic variation in the population. How is this maintained? It's a good question, but one way is exactly what you referred to - neutral mutations.

I'm sorry, I didn't mean to use that term as a biologist or geneticist might. By 'essentially come to be biologically identical' I mean two arms, two legs, upright walking, useless(?) appendix, limb proportions, hairy parts, etc.).

You don't know many six fingered people, but perhaps this is because six-fingered people are selected against? Perhaps the mutation isn't a neutral one?

Do you think a baby born today with six fingers is unlikely to reach adulthood and procreate?

The fact is that there is a decent amount of genetic variation in the population, and yet most humans still have two arms, two legs, two eyes, functioning red blood cells, livers, etc.
The mutations that would alter those things are selected against. The mutations that make those small differences, like how wide apart your eyes are, or a tiny chance in the shape of your bladder, are not. Or, though they may be selected against, are selected against weakly enough, and occur often enough, that they are still present in a high percentage of the population.

Maybe this is the gist of what's eluding me. Are you able to elaborate on 'occur often enough'? That, to me, suggests identical mutations occurring simultaneoously in different parts of the population. Is that what you mean? If so, is that generally accepted by the evolution gurus?

 

[SomeGuy]

N would logically seem to be the total number of genes at that locus in the whole population.

 

[Southwind17]

PS Southwind, maybe if they can explain this to me it will make sense to you too...

I get the general gist of this (might have to re-read a few times!), but I think you or somebody with a similar understanding might have to translate it into lay-person terms for me! Either that or it's back to the darkened room with a cold, wet towel!

 

[Roboramma]

N would logically seem to be the total number of genes at that locus in the whole population.

That's what I thought, but if that's the case I don't understand this sentence:
"Once the individual mutant gene comes into being, it constitues exactly 1/2N of all genes at it's locus in the population."

 

[Thabiguy]

I don't think it's this simple, because you make a huge assumption, namely that the mutation to which you allude leads to a greater chance of survival.

But that is the entire premise of natural selection. Natural selection is the process that increases the frequency of those heritable traits that increase the chance of survival and reproduction, and decreases the frequency of those that decrease this chance.

What if we're talking about an 'indifferent' mutation?

If a change in genome does not affect the chance of survival and reproduction, it will not be acted upon by natural selection. There are other mechanisms that can lead to increase or decrease of allele frequency in a population, namely genetic drift.

Let's say we use the arguably ludicrous hypothetical example of somebody acquiring a sixth finger on each (or one!) hand. Assuming that person procreates we should, presumably, expect to see the number of six-fingered people increasing over time. But the number of five-fingered people is also increasing, generally directly proportionally.

If somebody acquires a sixth finger as a heritable trait (as opposed to some non-heritable irregularity in prenatal development), and, for the sake of argument, this does not affect the chance of survival and reproduction, then the frequency of the gene encoding that trait in the population will not be increased by natural selection.

Should we expect everybody in the future to have six fingers, given a geological timescale?

No, we should expect this trait to disappear due to generic drift. In a smaller population (say 100 individuals), given enough time and assuming that no further mutations of that kind occur, there would be a 1% chance that at some time in the future everybody will have six fingers, and 99% chance that at some time in the future nobody will.

If not, how did the vast majority of current members of each species essentially come to be biologically identical (I don't know many six-fingered people!)?

Because if a part of population becomes substantially biologically different, you will consider them a different species. Remember that each species must have originated somehow - see speciation. If we consider a simple case, then in order to become a new species, generally a relatively small population would become isolated and develop until they become substantially different from the "parent" species. For example, if they diverge to the point of developing reproductive incompatibility, it means that the isolated population was so small, and/or it developed in isolation for so long, that the trait of not being able to produce offspring with the "parent" species had a chance to become fixated in the population through genetic drift. This was then, by definition, a population bottleneck that would also fixate many other traits that would then be generally shared in all the (presumably numerous) offspring of this isolated population, and we would then consider these traits characteristic for this new species - see founder effect.

 

[Broes]

Imagine that aliens will land on earth... and they will shoot ANY person who does NOT have red hair... then they leave and come back every 10 years repeat the process all over again.
You will see that the percentage of the population with red hair will increase dramaticly in a short period of time.

Now you can replace aliens with any form of (un)natural selection. The population will adept itself and a single mutation in a person might boom very quickly and become a dominant feature of a species. This change might not be as drastic as growing extra appendages overnight but you might look up sickle cell syndrom.

 

[Roboramma]

Yes, I suppose so, because I think I understand the 'selection' process, which is largely eliminated from the human species, compared to, say, wild animals, that is (or does that demonstrate that I don't understand it as well as I'd like to think I do?! )

It's certainly not eliminated from the human species. If there is a difference in the chances of reproductive success (the expected number of offspring) between two individuals based on their genes, then there is selection.
Some people are born with genetic diseases that cause sterility. Others with diseases that are likely to lead to an early death. These are obvious cases of selection.
Others aren't so obvious, but still likely to be meaningful. I would guess that things like sexually selected characteristics - height, facial symetry, etc. are still being selected for.
Fertility is certainly selected for, if there is any variation in the genes that influence it.
Resistance to AIDS.
There are plenty of selective pressures left. Many are probably new ones.

But surely all of the mutations that have occurred in the past that have led to humans being what they now are, and essentially the same in all of us, were not all subject to selective pressures for their replication, or were they?

Probably not - as Taffer pointed out genetic drift can lead to fixation of neutral mutations.

I'm sorry, I didn't mean to use that term as a biologist or geneticist might. By 'essentially come to be biologically identical' I mean two arms, two legs, upright walking, useless(?) appendix, limb proportions, hairy parts, etc.).

I find it pretty likely that those things were selected for.

Do you think a baby born today with six fingers is unlikely to reach adulthood and procreate?

It needn't be unlikely.
It just needs to be slightly less likely than a baby with five fingers. Or, more to the point, the total expected number of grandchildren need only be slightly less.
Maybe six fingered mothers have a harder time, and so end up having slightly fewer children. That's enough to be called selection, and, over time, weed a gene out of the population.

Maybe this is the gist of what's eluding me. Are you able to elaborate on 'occur often enough'? That, to me, suggests identical mutations occurring simultaneoously in different parts of the population. Is that what you mean? If so, is that generally accepted by the evolution gurus?

Well, I don't know how frequent certain mutations are, but they do happen. It's not like a particular mutation happens once and never again.

But I'm not an expert...

 

[Thabiguy]

That's what I thought, but if that's the case I don't understand this sentence:
"Once the individual mutant gene comes into being, it constitues exactly 1/2N of all genes at it's locus in the population."

Actually, N is the population size. In a sexually reproducing species, diploid cells will have two sets of chromosomes, so the number of all genes at a particular locus in the population is 2N. Among all those genes, one new comes to being. Therefore it is 1 of 2N genes, or in other words, constitutes exactly 1/2N of all genes at its locus in the population.

 

[Roboramma]

Thanks! Now I just can't understand why I didn't undestand it before. It's so simple.

 

[MRC_Hans]

Yes, I suppose so, because I think I understand the 'selection' process, which is largely eliminated from the human species, compared to, say, wild animals, that is (or does that demonstrate that I don't understand it as well as I'd like to think I do?! )

Quite a bit of natural selection has been eliminated from humans, but far from all, and at any rate this is a very recent thing, and mostly restricted to develped countries. For half the world to-day and for all of the world just 3-4 generations ago, natural selection very much applies.

But surely all of the mutations that have occurred in the past that have led to humans being what they now are, and essentially the same in all of us, were not all subject to selective pressures for their replication, or were they?

To become a dominant trait, a gene must be selected for. Or at least be neutral.

I'm sorry, I didn't mean to use that term as a biologist or geneticist might. By 'essentially come to be biologically identical' I mean two arms, two legs, upright walking, useless(?) appendix, limb proportions, hairy parts, etc.).

You can be quite sure that all those traits have, or once had, a selection pressure working for them.

Do you think a baby born today with six fingers is unlikely to reach adulthood and procreate?

Not exactly unlikely, but it might have a somewhat lesser chance at procreating, because potential partners might be repelled by it being "weird". You will notice that simply being different is repelling a lot of people. Probably an instinctive avoidance of individuals that might have some genetic fault.

Remember that a selective advantage or disadvantage does not need to be very big to have an influence over many generations.

Maybe this is the gist of what's eluding me. Are you able to elaborate on 'occur often enough'? That, to me, suggests identical mutations occurring simultaneoously in different parts of the population. Is that what you mean?

All depends. Some mutations will spread quickly, others slowly. This depends on a lot of factors, such as whether they affect a dominant gene, whether thay have strong selective pressures working on them, etc.

If so, is that generally accepted by the evolution gurus?

What do you mean by "evolution gurus"? No matter how much creationists try to make it look like that, evolution is not a religious movement, and it has no need for gurus. All it needs is scientists.

Hans

 

[Soapy Sam]

Yes, I suppose so, because I think I understand the 'selection' process, which is largely eliminated from the human species, compared to, say, wild animals, that is (or does that demonstrate that I don't understand it as well as I'd like to think I do?! )

Disease resistance and ability to withstand famine are genetic factors. If you starve, or die of cholera at twelve, you don't reproduce. This is rare in New York, but still very much a fact of life in other places.

But surely all of the mutations that have occurred in the past that have led to humans being what they now are, and essentially the same in all of us, were not all subject to selective pressures for their replication, or were they?

Yes, they were, but to varying extents. The hypothetical example I suggested earlier is illustrative. A mutation that happens commonly, but produces a protein trivially different is effectively selection neutral, until such time as an environmental condition arises where the difference in proteins is NOT trivial- for example, one form is more resistant to a disease or parasite.

Do you think a baby born today with six fingers is unlikely to reach adulthood and procreate?

No, if the extra finger is the sole change induced by a mutation. It probably won't be though. Also- always remember, the environment of humans is mostly other humans. If a superstition arises against polydactyly, he's in trouble- and superstition IS an environmental factor.[/quote]

Maybe this is the gist of what's eluding me. Are you able to elaborate on 'occur often enough'? That, to me, suggests identical mutations occurring simultaneoously in different parts of the population. Is that what you mean? If so, is that generally accepted by the evolution gurus?

See my post above. Selection only works where there is variation to select.
Even if one single human is born with some bizarre mutation making him incomparably "better" than the rest- and it doesn't happen to make him sterile, or totally homosexual or restrict him from breeding in any other way- that mutation still won't spread fast, unless he happens to be a potentate with 200 wives. But a mutation which occurs spontaneously and frequently , so it is always present in the gene pool can spread fast if environmental conditions change. If genes take their cues from environmental factors- say growing a shaggy coat in cold weather- so long as that gene is already in the population at a reasonable frequency, a ten year cold snap could rapidly push that gene to 100% presence, because the short hairs are all frozen- or moved south in a potential speciation split.

 

[DavidS]

The random variations that might by chance be "better".

I agree with jimbob. DavidS, if you read my post carefully you will see that I wrote "... one of its essential ingredients ..." (emphasis added).

Your emphasis supports my reading of the original post. The nit that I pick is not whether random variations are the essential ingredient of evolution, but whether their random nature is at all essential to the process.

Yes, yes, random variation is one way to get a variations that might, by chance, better survive the selection process. My point is that evolution operates on the variations without regard to whether they are random or "magically directed by design".

The real strength of the theory of evolution by natural selection is that directed variation is not at all essential to its operation. Because even random variations filtered through natural selection will ultimately yield populations of improved fitness, the apparent fitness of any organism is, of itself, no evidence that those variations were other than random. Of course, neither does it provide evidence that those variations were random, but absent evidence to the contrary there's no reason to postulate any other mechanism.

 

[dacium2007]

I think the point here is that humans have not been around long enough to mutate much. It appears almost every human (according to dna analysis) has come from north africa around 60,000 years ago. This is practially zero time on an evolutionary scale. Its quite likly that humans as we know them have never had a genetic mutation that has swept over the entire population, only minor changes (which is the reason why there are different races/looks to different people as the mutations only spread locally).

Mutations that spread through a whole species can take millions of years. It is a tiny advantage that can prevail in the end of millions of years. Consider for example if one mutation made a person better phsyically at hunting than other. If is forseeable that on average, the better hunter is going to not die as often as the poorer one (all else being equal). It is forseeable that over millions of years (hundreds of thousands of generations) that he will eventually take over the entire population. And this is exactally what happened. Species like homo erectus took over all of asia, homoantecesso took over all of euro/africa. If it weren't for the geographical seperation that had occured, its is likly one would have takne over the population of the other, given enough time. It appears to be us (homo spiens) who first spread to other continents (after the seperated), by the time we did only neanderthal man was left and we bested him with greater intelligence.

 

[Taffer]

Hey Taffer, I remembered reading this, and so looked it up before writing my last post. Found the relevant section in that old and probably out of date textbook - Sociobiology - where he says the same thing. He goes in to why but I don't quite understand it. I remember spending about twenty minutes trying to understand exactly and making sense of it when I first read it, but now it escapes me:

Here's the relevant quote:


Could you explain this to me?
Specifically I don't understand this part: "Once the individual mutant gene comes into being, it constitues exactly 1/2N of all genes at it's locus in the population."
Why?
Maybe I'm unclear as to what N is...

PS Southwind, maybe if they can explain this to me it will make sense to you too...

* I don't know how to write greek characters.

What Thabiguy said. It is really a poor explanation, to be perfectly honest. A quick google turned this up:

Neutral mutants fix in proportion to their representation. Thus, a new mutant, with an allele frequency of 1/2N, has a 1/2N chance of fixing in that population. (Somebody’s gotta be the ancestor of everybody -- if everything is equal (i.e. neutral) then each of the 2N alleles has an equal chance, hence the 1/2N.

A basic assumption of molecular clocks derives from this fact. New mutation should arise in populations at a rate proportional to the population size, i.e., the number of new mutants at a locus per generation in the population is 2Nu. The probability that each of these is fixed is approximately 1/2N. Thus the rate (over a long period of time) of fixation of new mutations should be 2N u/2N = u, which is independent of population size.

Linky

 

[drkitten]

I don't think it's this simple, because you make a huge assumption, namely that the mutation to which you allude leads to a greater chance of survival. What if we're talking about an 'indifferent' mutation?

Then the chances of it becoming fixed are much smaller. It could still become fixed if on average, I had more kids survive and reproduce (and my kids had more kids, and so on); the two-children per widget is just a long-term average thingy.

Let's say we use the arguably ludicrous hypothetical example of somebody acquiring a sixth finger on each (or one!) hand. Assuming that person procreates we should, presumably, expect to see the number of six-fingered people increasing over time. But the number of five-fingered people is also increasing, generally directly proportionally. Should we expect everybody in the future to have six fingers, given a geological timescale?

No, we shouldn't; the odds are against it. But the fact that the odds are against something doesn't mean that it won't happen, just that the smart money doesn't expect it to happen.

 

[Paul C. Anagnostopoulos]

But surely all of the mutations that have occurred in the past that have led to humans being what they now are, and essentially the same in all of us, were not all subject to selective pressures for their replication, or were they?

If the mutation changes the phenotype in any discernible fashion, there is probably some pressure on it. Also, keep in mind that one mutation can ride piggyback on another one with more obvious pressures on it.

~~ Paul

 

[Southwind17]

What do you mean by "evolution gurus"? No matter how much creationists try to make it look like that, evolution is not a religious movement, and it has no need for gurus. All it needs is scientists.

Sorry, I used 'guru' with a loose, non-religious intent, essentially to mean 'authoritative'. I figured it might provoke a reaction though, and should have chosen a better word.

I don't believe that all scientists are necessarily correct mind!

 

[Southwind17]

Your emphasis supports my reading of the original post. The nit that I pick is not whether random variations are the essential ingredient of evolution, but whether their random nature is at all essential to the process.

Yes, yes, random variation is one way to get a variations that might, by chance, better survive the selection process. My point is that evolution operates on the variations without regard to whether they are random or "magically directed by design".

The real strength of the theory of evolution by natural selection is that directed variation is not at all essential to its operation. Because even random variations filtered through natural selection will ultimately yield populations of improved fitness, the apparent fitness of any organism is, of itself, no evidence that those variations were other than random. Of course, neither does it provide evidence that those variations were random, but absent evidence to the contrary there's no reason to postulate any other mechanism.

That's clear to me now DavidS, thanks. I didn't intend to infer any emphasis on the word 'random'. My post should have been interpreted as reading:

... if biological evolution relies on mutations (mutations that just happen to be randomly generated) ...

but given what you've said I suppose it would be acceptable just to refer to 'mutations' and forget about the randomness altogether.

Part of my inclination to be mindful of randomness, though, stems from a co-running 'evloutionary' thread where such randomness is pivotal to the discussion, which pertains to the similarity, or difference, between natural evolution and human design.

 

[Southwind17]

This is great feedback guys - thanks. Beats wading through books trying to find the answer (not that I'm averse to a little reading and research!).

Anybody want to rent a darkened room with cold, running water?!

 

[MRC_Hans]

Sorry, I used 'guru' with a loose, non-religious intent, essentially to mean 'authoritative'. I figured it might provoke a reaction though, and should have chosen a better word.

I don't believe that all scientists are necessarily correct mind!

No worries.

Scientists can be wrong. Of course.

Hans