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Offline harrypalmer

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Humans & natural selection
« on: 18/10/2005 10:55:45 »
I was wondering to what extent humans are now subject to natural selection.

In the past humans as with other species would have had to adapt to survive, and of course those best suited to certain niches would have survived, but isnt it different now?

Man has the ability to alter his living conditions (to a degree). In addition we are for the most part a compassionate species and care for our sick and disadvantaged. Surely by doing so we are 'interfering' with the normal process of evolution, whereby the fittest or best adapted survive?

Dont get me wrong Im all for compassion, and we still havent got it right across the globe. I am just looking at it from a purely scientific view.

Are we now the exception to the norm on this planet, and the only species no longer evolving purely to survive?

Any thoughts?


 

Offline Ultima

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Re: Humans & natural selection
« Reply #1 on: 18/10/2005 19:57:59 »
Yeah I asked this a while ago. Plus does the media and social trend have an impact :D
So long as the human "ideal" form doesn't change to rapidly in society we might slowly evolve towards it. Mentally I think there is going to be a lot of evolution in the future... saying that though you don't need to be intelligent to procreate, it's probably to the contrary :D.

RISE OF THE CHAVS and stick thin top-heavy people!
 

Offline thefunkyaquarium

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Re: Humans & natural selection
« Reply #2 on: 18/10/2005 20:39:30 »
The thing is, nowadays pretty much everybody can reproduce - even those with genetic defects (as long as they're mild enough for survival).  Poor eyesight, low intelligence, ugliness... they don't stop people from finding partners and having children.

Note also that people who are better off (professional classes, etc) are having fewer children...

So I say natural selection is working the opposite way in humans, and several hundred generations down the line we'll all be weaklings :)
 

Offline harrypalmer

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Re: Humans & natural selection
« Reply #3 on: 18/10/2005 20:55:23 »
Someone I know has raised the possibility that humans may now evolve with an iPod socket.

:S
 

Offline harrypalmer

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Re: Humans & natural selection
« Reply #4 on: 18/10/2005 20:59:18 »
The other thing to consider is whether people in poorly developed countries are evolving differently to us in the affluent West?

We think of ourselves as healthier and stronger, but I suspect we are not.

I remember seeing in Zimbabwe a young local lad pick up a piece of rail track in his teeth that a full grown man from the UK couldnt even lift in his arms.

It was an amazing sight. :)
 

Offline Ultima

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Re: Humans & natural selection
« Reply #5 on: 18/10/2005 22:41:10 »
Yeah everyone in Africa is going to become immune to HIV which means we will be screwed. Except we have the money and technology to steal their genes :D.
 

Offline Simmer

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Re: Humans & natural selection
« Reply #6 on: 19/10/2005 20:53:23 »
We have to be careful not to confuse selective breeding with natural selection.  Ordinary people breeding faster than lawyers or scientists has nothing to do with natural selection; all are human, even the lawyers.

Natural selection applies to mutations, a change in genetic makeup that confers some new trait or ability.  Those mutations are still going on throughout our species even if the selection part isn't operating fully at the moment.  So we are storing up all those new traits, ready to put to the test once times get hard again :D
 

another_someone

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Re: Humans & natural selection
« Reply #7 on: 20/10/2005 00:17:38 »
quote:
Originally posted by Simmer

We have to be careful not to confuse selective breeding with natural selection.  Ordinary people breeding faster than lawyers or scientists has nothing to do with natural selection; all are human, even the lawyers.

Natural selection applies to mutations, a change in genetic makeup that confers some new trait or ability.  Those mutations are still going on throughout our species even if the selection part isn't operating fully at the moment.  So we are storing up all those new traits, ready to put to the test once times get hard again :D



I'm sorry, but I don't see the difference.

Why should selection restrict itself to mutations?

What is generally true is that we have very few truly isolated populations, so we cannot develop the inbreeding required for speciation (i.e. most of the little changes get swamped by the vast number of unchanged genes).  One area where we do have a slightly isolated population is the increased number of people born with inheritable deafness since the development of sign language (which allows deaf people to communicate between themselves, but relatively few hearing people are able to communicate with sign language, thus creating a distinct and separate deaf culture).

I would say the selection part is ongoing within our society, but the selection is geared more towards fitness for our social environment than fitness for the physical environment.  One area where this might be happening is with the pressure towards ever delayed child bearing, that women who have a predisposition to early menopause should be being bred out of our society (it would also have the effect of increasing life expectancy, which is clearly being observed).

quote:
Originally posted by thefunkyaquarium

The thing is, nowadays pretty much everybody can reproduce - even those with genetic defects (as long as they're mild enough for survival).  Poor eyesight, low intelligence, ugliness... they don't stop people from finding partners and having children.

Note also that people who are better off (professional classes, etc) are having fewer children...

So I say natural selection is working the opposite way in humans, and several hundred generations down the line we'll all be weaklings :)



What is a genetic defect?

Does not the term 'defect' imply a presumption of some notion of normality - but is not the notion of evolution based on the premise that there is no such thing as normal, only that which survives, and that which fails to survive.  In fact, normality is a notion of conservation, which is the antithesis of evolution.

There are many successful species of animals that are blind, so the fact that some humans are born with inferior eyesight does not make them 'defective', only different.

The fact that professional classes are having fewer children, firstly has to be balanced against the fact that the children of the professional classes generally have a better survival rate, but it also must be remembered that a society that only bred rocket scientists would be a very unbalanced society.

On the broader front, humans are an intensely social animal (more social than any other mammal, and comparable only to some insects), and so you should not be looking at the strengths and weaknesses of the individual, but rather the strengths and weaknesses of the society.  The weaknesses of an individual are irrelevant if they perform a useful function to enhance the survival of their society.  As I indicated above, a society needs only just so many rocket scientists, but it needs a lot more plumbers, labourers, and refuse collectors.
 

Offline Simmer

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Re: Humans & natural selection
« Reply #8 on: 22/10/2005 22:27:14 »
quote:
Originally posted by another_someone
[brI'm sorry, but I don't see the difference.

Why should selection restrict itself to mutations?

What is generally true is that we have very few truly isolated populations, so we cannot develop the inbreeding required for speciation (i.e. most of the little changes get swamped by the vast number of unchanged genes).  One area where we do have a slightly isolated population is the increased number of people born with inheritable deafness since the development of sign language (which allows deaf people to communicate between themselves, but relatively few hearing people are able to communicate with sign language, thus creating a distinct and separate deaf culture).

I would say the selection part is ongoing within our society, but the selection is geared more towards fitness for our social environment than fitness for the physical environment.  One area where this might be happening is with the pressure towards ever delayed child bearing, that women who have a predisposition to early menopause should be being bred out of our society (it would also have the effect of increasing life expectancy, which is clearly being observed).



I take your point about selection still occuring in some respects, in particular your example of selection against early menopause was very interesting.  I'm not convinced about the life expectancy example though, I think that has more to do with diet, sanitation and medicine.

However, I believe that natural selection applies only to mutations because otherwise selection is ultimately inconsequential.  You could selectively breed for minor traits like height, eye colour or some such but, because you are breeding human with human, at the end of the day you will end up with a creature with exactly the same genetic potential as when you started (assuming you didn't inbreed too much).  If you then left that population alone for enough generations it would become indistinguishable from the unselected population in a similar environment. Permanent significant change can only occur through a successful mutation.
 

Offline Titanscape

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Re: Humans & natural selection
« Reply #9 on: 23/10/2005 13:35:39 »
I heard that the present generation has a higher IQ than the last. People can really only have children if they can afford to. People meet at college and University. One needs drive and intelligence to have children.

Pentecostals have a big growth rate, having many children per couple, making one of the reasons the church has grown from a hundred to 500,000,000 in 100 years. Not strictly all are called Pentecostals.

Titanscape
 

another_someone

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Re: Humans & natural selection
« Reply #10 on: 24/10/2005 02:17:58 »
quote:
Originally posted by Simmer

I take your point about selection still occurring in some respects, in particular your example of selection against early menopause was very interesting.  I'm not convinced about the life expectancy example though, I think that has more to do with diet, sanitation and medicine.



The issue about life expectancy has been shown to be true in fruit flies, where fruit flies were bred with the early offspring of the females being killed off, and only the later offspring being allowed to survive.  There was a substantial increase in the lifespan of the fruit flies.

What may be argued is that humans are rather unusual in that we already live so far beyond our reproductive years that the same effect may not necessarily apply to us.

quote:
Originally posted by Simmer
However, I believe that natural selection applies only to mutations because otherwise selection is ultimately inconsequential.  You could selectively breed for minor traits like height, eye colour or some such but, because you are breeding human with human, at the end of the day you will end up with a creature with exactly the same genetic potential as when you started (assuming you didn't inbreed too much).  If you then left that population alone for enough generations it would become indistinguishable from the unselected population in a similar environment. Permanent significant change can only occur through a successful mutation.



There are two different issues here, whether the genetic changes in population are significant, and whether they are reversible.

I did say that without having a small isolated population that is inbreeding you will not get speciation, and without speciation, you will always be able to reverse any genetic change.  Even genetic mutations (such as Huntington's chorea) will not be be able to create a new species unless one has a small isolated population.

On the other hand, many of the genetic differences that exist between populations are more than merely superficial.  Differences in skin colour between races are an adaptation to different amounts of sunlight they are exposed to.  The small and rotund figures of people living in the artic regions again is a reflection of a need to minimise heat loss.  Similarly, diseases such as thalassemia and sickle cell anaemia are adaptations to the level of malaria the populations are exposed to.

http://en.wikipedia.org/wiki/Thallasemia
{quote]
Being a carrier of the disease confers a degree of protection against malaria, and is quite common among people from Italian or Greek origin, since malaria was widespread in those countries at one time. In that respect it resembles another genetic disorder, sickle-cell disease. The disease's geographical association with the Mediterranean sea was responsible for its naming: Thalassa is Greek for the sea. However it is also widespread in the Indian subcontinent.
[/quote]

 

Offline daveshorts

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Re: Humans & natural selection
« Reply #11 on: 24/10/2005 17:16:49 »
I think the really strong selection effect at work at the moment is to be too incompetent to use contreception - this may have worrying long term consequences...
 

another_someone

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Re: Humans & natural selection
« Reply #12 on: 24/10/2005 21:20:55 »
quote:
Originally posted by daveshorts

I think the really strong selection effect at work at the moment is to be too incompetent to use contreception - this may have worrying long term consequences...



It may have long term consequences, just as being easily brainwashed into overusing contraception (the other side of the same coin) may have long term consequences.  Who is to say which of those consequences will ultimately prove positive and which negative (or even how we should judge what is positive or negative)?
 

Offline Simmer

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Re: Humans & natural selection
« Reply #13 on: 27/10/2005 00:39:41 »
quote:
Originally posted by another_someone

[I did say that without having a small isolated population that is inbreeding you will not get speciation, and without speciation, you will always be able to reverse any genetic change.  Even genetic mutations (such as Huntington's chorea) will not be be able to create a new species unless one has a small isolated population.


I think you can get permanent genetic change in a mainstream population, although it takes a while.  A successful mutation that in some way improves the probability of successful reproduction above that of the mainstream will eventually spread to the entire population.

quote:
On the other hand, many of the genetic differences that exist between populations are more than merely superficial.  Differences in skin colour between races are an adaptation to different amounts of sunlight they are exposed to.  The small and rotund figures of people living in the artic regions again is a reflection of a need to minimise heat loss.  Similarly, diseases such as thalassemia and sickle cell anaemia are adaptations to the level of malaria the populations are exposed to.


Depends how you define superficial!  I reckon if you stuck a population of Eskimo's down in Kenya and a population of Kenyans down in Alaska, 10,000 years later you would wonder why you bothered!:)

I think it likely that within every human population there is the genetic potential to adapt to an environment in the same way as any other human population. I would call the mechanism for that selective breeding and reversible if the environment changes again; skin colour and body shape are examples of such adaptions.  I was wrong to call them superficial but they are qualitatively different from the changes that result in the development of a new species.

And I can prove it using the variously shaped and coloured human species, QED! :D
 

another_someone

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Re: Humans & natural selection
« Reply #14 on: 28/10/2005 03:46:47 »
quote:
Originally posted by Simmer

I think it likely that within every human population there is the genetic potential to adapt to an environment in the same way as any other human population. I would call the mechanism for that selective breeding and reversible if the environment changes again; skin colour and body shape are examples of such adaptions.  I was wrong to call them superficial but they are qualitatively different from the changes that result in the development of a new species.

And I can prove it using the variously shaped and coloured human species, QED! :D



There are differences between racial (breed) changes and species changes, but that distinction is different to the issue of reversibility or mutation.

Two different species are defined as two populations that are incapable of interbreeding.  By definition, this means that the members of the new species do not breed with members of the old species.

What you are suggesting is a scenario where, over a long period of time, a large population contains members of a new species and members of an old species, and yet continue to interbreed - yet the very fact of their interbreeding means they cannot be two distinct species.

Nor is there any evidence that a single mutation can create a new species.  On the contrary, we know of many mutations that regularly occur within the human species which do not create speciation (Huntington's is one, and the victims of thalidomide are another).  Even if a single mutation did create someone of a new species, it would have to create a male and a female of that new species, since, if they were truly a new species, they would not be able to interbreed with members of the old species.

Nor is it clear that random mutation is a basis for speciation.  There are many cases where nature has converged upon a similar solution from different directions.  It is clear that if nature relied upon merely random mutations, each solution should be different.  The fact that it is not implies to me that most species must already have the genetic basis for filling a large number of different niches, and much of the speciation must occur by the rearrangement of existing genes rather than the creation of new genes.  This is not to say that random gene mutations do not occur (clearly, they do), only to question whether they are a prerequisite for speciation, or even a mechanism that would itself produce speciation.
 

Offline Ultima

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Re: Humans & natural selection
« Reply #15 on: 29/10/2005 17:00:01 »
quote:
I heard that the present generation has a higher IQ than the last. People can really only have children if they can afford to. People meet at college and University. One needs drive and intelligence to have children.


Nice idea but there is another prerequisite for children, and that's sex. I'm afraid it's the retarded masses that are having children at a young age and continuing to do so throughout their lives. I guess this might not be so noticeable in Australia, but the UK has one of the highest teen pregnancy rates among industrialised countries. Only overtaken by the USA. I know this is a fairly loaded comment but I am not referring to the women who choose to have children at a young age. I am referring to the stupid teens that have no idea what they are doing, or what the long term outcome will be. I have no problem with them having sex; but tbh what chances do their children have if the parents are unable to provide.
 

Offline Simmer

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Re: Humans & natural selection
« Reply #16 on: 29/10/2005 18:56:21 »
quote:
Originally posted by another_someone

Two different species are defined as two populations that are incapable of interbreeding.  By definition, this means that the members of the new species do not breed with members of the old species.

What you are suggesting is a scenario where, over a long period of time, a large population contains members of a new species and members of an old species, and yet continue to interbreed - yet the very fact of their interbreeding means they cannot be two distinct species.


I think it's a cumalative thing, a mutation that prevents interbreeding with the unmutated general population will fail but if it doesn't and confers an advantage then it spreads to the whole population.  It may be that if a section of the main population is isolated for a long period then the accumulated divergence between the two sections make them incapable of interbreeding but usually the significant differences are between the current generation and their distant ancestors.

The main reason I am championing this model so vigorously is that the selection argument so often gets twisted into the elitist "only the best people should be allowed to breed". It's complete rubbish, graduates don't have children measurably brighter than anyone else, for example, and past attempts to selectively breed a noble elite have produced nothing better than haemophilia and receeding chins. :)

As far as the perpetuation of the human genome is concerned it really doesn't matter who the parents are unless they have some new and useful bit of code to pass on - and that virtue is randomly distributed throughout the entire population.

 

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Re: Humans & natural selection
« Reply #17 on: 29/10/2005 19:15:38 »
quote:
Originally posted by Ultima

Nice idea but there is another prerequisite for children, and that's sex. I'm afraid it's the retarded masses that are having children at a young age and continuing to do so throughout their lives. I guess this might not be so noticeable in Australia, but the UK has one of the highest teen pregnancy rates among industrialised countries. Only overtaken by the USA. I know this is a fairly loaded comment but I am not referring to the women who choose to have children at a young age. I am referring to the stupid teens that have no idea what they are doing, or what the long term outcome will be. I have no problem with them having sex; but tbh what chances do their children have if the parents are unable to provide.



It is perjoritive and inaccurate to talk about 'retarded masses'.  That they are the 'masses' would imply an element of normality about them.

As to whether 'stupid teens' actually 'have no idea what they are doing' is also uncertain.  This is something that is often said of them, but it is not something I have heard them say for themselves.

I suspect that in many cases these teens, who don't see themselves making much of their lives, feel that at least in making a baby they are making something.  I suspect it is not always as thoughtless as outsiders accuse them of (although it may be a highly emotionally charged rational - as anything to do with sex often is).

In a country where the population is having difficulty maintaining itself, and with fears that there will in future years not be enough tax payers to pay for the aging population; may it not be said that these girls are performing a public service - at least some kids are getting born.

Furthermore, as I pointed out before, this country has a greater shortage of plumbers than it has a shortage of rocket scientists.
 

Offline Ultima

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Re: Humans & natural selection
« Reply #18 on: 29/10/2005 20:16:27 »
quote:
I suspect that in many cases these teens, who don't see themselves making much of their lives, feel that at least in making a baby they are making something.


That is fairly thoughtless, get a pet! Or even better make something of your life. The tax payer’s money goes on benefits for all of these (more often than not) single parents! WTF is this plumbing thing??? I think it's better for people to enter a trade they would be good at, than have the miss guided view that a degree == job with money. Plus plumbers make an absolute mint compared to scientists. The retardation I am alluding to is not intellectual, it is emotional, spiritual and social!

I very much doubt any of these people have seriously thought about what it might mean to have a child, and how their actions will effect there own lives and that of their child. All the ones I see on my estate back home think it's some sort of game everyone plays!


I am not saying that choosing to be a mother rather than anything else is a problem, it's the fact that these babies are being born to CHILDREN! Most of them you wouldn't trust to look after a dog! Would any adoption agency in their right mind give a baby to a teenager?


I don't know how this would effect any evolutionary process but it will have a dramatic impact on society, if we continue this trend.
« Last Edit: 29/10/2005 20:21:40 by Ultima »
 

another_someone

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Re: Humans & natural selection
« Reply #19 on: 29/10/2005 20:20:51 »
quote:
Originally posted by Simmer

I think it's a cumalative thing, a mutation that prevents interbreeding with the unmutated general population will fail but if it doesn't and confers an advantage then it spreads to the whole population.  It may be that if a section of the main population is isolated for a long period then the accumulated divergence between the two sections make them incapable of interbreeding but usually the significant differences are between the current generation and their distant ancestors.



There is much difference of opinion as to whether evolution develops at a constant and gradual pace, or with long periods of stasis, followed by brief period of rapid change.

If evolution does progress at a steady rate, then we should expect partial speciation.  Certainly, in human terms, the evidence is that populations that probably have not shared a common ancestor for at least 10,000 years, and maybe 30,000 years, have absolutely no disadvantage when interbreeding.

It is interesting to look at the comparison of Neanderthal man and modern man.  Neanderthals lived from 230,000 years ago to about 30,000 years ago (there is still no conclusive proof, and still some debate, as to whether Neanderthals were a different species, or just a different race; but the dominant view is presently that they were a distinct species).  If your notion of gradual change were true, one should expect a gradual change towards modern man over the period of 200,000 years.  In fact, modern man arrived over a fairly short period of time around 120,000 years ago (no-one can say exactly when the change happened because it was a case of one moment none were visible, next moment - in archaeological time -  the new species was there).

Another issue I have with your assessment is the assumption that at any given time a particular species is less than optimal, and thus a new gene would actually produce a better human being.  I know of no evidence for this, and I don't believe it is so.  A species has to adapt because of changing environmental circumstances (it may be loss of habitat, change of weather, change of population density, change of competition with other species), but this is different from assuming that once adaptation has occurred, that the species could be made better adapted to that environment if only it had a new gene to do it with.

That having been said, it is true that the environment is constantly changing (sometimes rapidly, sometimes slowly, but it always changes), and so the stresses and  challenges upon all species is always changing.  Thus, one might say that a new gene may possibly be useful in adapting to a change, but this is different from saying it would make human beings better able to adapt to the environment of yesteryear.

quote:

The main reason I am championing this model so vigorously is that the selection argument so often gets twisted into the elitist "only the best people should be allowed to breed". It's complete rubbish, graduates don't have children measurably brighter than anyone else, for example, and past attempts to selectively breed a noble elite have produced nothing better than haemophilia and receeding chins. :)



I commend your motivation, but not your conclusion.

All that I know implies that you are wrong in stating that IQ is not inheritable.  Although clearly poor diet can constrain IQ, but given an adequate diet, I understand that IQ is largely determined by prenatal levels of testosterone, and is controlled by a gene (or genes) on the X chromosome, and thus inherited through the maternal line.  On the other hand, educational attainment is not always simply a matter of IQ (I know many people with high IQ who got so bored at school that they performed considerably worse than other children with lower IQs).

Where I agree with you is that selective breeding is counterproductive, but not because the traits are not inheritable, but because extreme thoroughbreds are unhealthy, and because in nature there is no such thing as 'best'.  That which may be 'best' in one niche, when the environment changes, even slightly, then that 'best' suddenly becomes a liability.  Human beings, if they have had any virtue, has been in their versatility and adaptability, not in their ability to specialise.  The Giant Panda is a specialist, and is consequently an endangered species because its native environment is under threat.

 

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Re: Humans & natural selection
« Reply #20 on: 29/10/2005 21:06:04 »
quote:
Originally posted by Ultima

That is fairly thoughtless, get a pet! Or even better make something of your life. The tax payer’s money goes on benefits for all of these (more often than not) single parents! WTF is this plumbing thing??? I think it's better for people to enter a trade they would be good at, than have the miss guided view that a degree == job with money. Plus plumbers make an absolute mint compared to scientists. The retardation I am alluding to is not intellectual, it is emotional, spiritual and social!

I very much doubt any of these people have seriously thought about what it might mean to have a child, and how their actions will effect there own lives and that of their child. All the ones I see on my estate back home think it's some sort of game everyone plays!

I am not saying that choosing to be a mother rather than anything else is a problem, it's the fact that these babies are being born to CHILDREN! Most of them you wouldn't trust to look after a dog! Would any adoption agency in their right mind give a baby to a teenager?

I don't know how this would effect any evolutionary process but it will have a dramatic impact on society, if we continue this trend.



Firstly, the notion of teenagers being children is fairly modern.  In the 17th century, 15 year old wives and mothers were not at all uncommon.

Yes, society has changed, and you can say that you are concerned to perpetuate 20th century social norms, and would see teenage pregnancy as a threat to that.  This is reasonable, but the 20th century cannot stay forever.

A far more serious long term threat to society is population implosion.

http://news.bbc.co.uk/1/hi/world/asia-pacific/3708098.stm
quote:

The name Sony summons visions of all things Japanese. Yet its board chairman, Iwao Nakatani, recently called for mass immigration, opening Japan to different faces and influences.
Mr Nakatani is worried because Japanese are living longer, yet having fewer children. The result is a shrinking workforce which threatens economic growth.



What is true of Japan is true of all modern societies.  So you would wish for mass immigration, but would condemn the few who would have children in this country.

I accept that no adoption agency would ever (in the present political climate) give a child to a teenage mother for adoption.  This is a political issue above all else.

I would also agree that it may well be that many of these young mothers (possible for reasons beyond their mere age) would not make the best of mothers; but since many others who may be better mothers are choosing not to be mothers at all, it may well be that we have a choice between a imperfect mothers or no mothers.  Maybe it might be better if we focus on making them better mothers, rather than simply condemn them for choosing to be mothers.
 

Offline Simmer

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Re: Humans & natural selection
« Reply #21 on: 29/10/2005 23:04:20 »
quote:
Originally posted by another_someone


If your notion of gradual change were true, one should expect a gradual change towards modern man over the period of 200,000 years.  In fact, modern man arrived over a fairly short period of time around 120,000 years ago (no-one can say exactly when the change happened because it was a case of one moment none were visible, next moment - in archaeological time -  the new species was there).

Another issue I have with your assessment is the assumption that at any given time a particular species is less than optimal, and thus a new gene would actually produce a better human being.  I know of no evidence for this, and I don't believe it is so.  A species has to adapt because of changing environmental circumstances (it may be loss of habitat, change of weather, change of population density, change of competition with other species), but this is different from assuming that once adaptation has occurred, that the species could be made better adapted to that environment if only it had a new gene to do it with.

That having been said, it is true that the environment is constantly changing (sometimes rapidly, sometimes slowly, but it always changes), and so the stresses and  challenges upon all species is always changing.  Thus, one might say that a new gene may possibly be useful in adapting to a change, but this is different from saying it would make human beings better able to adapt to the environment of yesteryear.

All that I know implies that you are wrong in stating that IQ is not inheritable.  Although clearly poor diet can constrain IQ, but given an adequate diet, I understand that IQ is largely determined by prenatal levels of testosterone, and is controlled by a gene (or genes) on the X chromosome, and thus inherited through the maternal line.  



If we're not careful the next generation will be inheriting this thread too! :)  

For the sake of argument I'm willing to concede that inherited traits may include IQ (really testosterone?) and that human beings suddenly appeared, fully fledged as it were, 120,000 years ago.

Given all that what do you think the mechanism that produced that first human was?
 

another_someone

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Re: Humans & natural selection
« Reply #22 on: 30/10/2005 17:23:11 »
quote:
Originally posted by Simmer

For the sake of argument I'm willing to concede that inherited traits may include IQ (really testosterone?) and that human beings suddenly appeared, fully fledged as it were, 120,000 years ago.

Given all that what do you think the mechanism that produced that first human was?



One thing I have found while trying to find an answer to this question is that I was wrong to say that modern humans evolved from Neanderthal man.  It seems the current consensus is that both are derived, at different times,  directly from H. heidelbergensis.

The more I tried to look for answers as to why it is thought these species diverged, the more confused I became, until I began to even question whether we know whether they are distinct species or not.  They look different, but then a Great Dane looks very different to a Chihuahua.  The common assumption is that these groups, who co-existed for many tens of thousands of years, did not interbreed; and yet even that is not certain.

Because so much is unknown about the exact relationship of early humans, I will avoid directly answering the question about human speciation, but will go try and generalise about speciation.

http://en.wikipedia.org/wiki/Coelacanth
quote:

The first hint that western scientists had of a modern, living coelacanth existed was when Marjorie Courtenay-Latimer, who was curator of a museum in East London, South Africa, was inspecting local fish catches for unusual specimens in 1938.


http://en.wikipedia.org/wiki/Coelacanth
quote:

Coelacanths were believed to first appear in the Carboniferous Period, about 400-350 million years ago.



If we are indeed dealing with the same species (even if they look the same, it would be very difficult to conclusively prove they could interbreed), then we are dealing with a species that survived for over 350 millions years.

http://en.wikipedia.org/wiki/Living_fossil
quote:

The mean species turnover time (the time a species lasts before it is replaced) varies widely among the phyla, but is about 2-3 million years.



If the average is 2-3 million years, and the longest is 350 million years, then the shortest time a species must exist (be created, and then disappear) must be very much shorter than 2 million years.

If speciation were merely the consequence of random mutations over time, causing species to slowly drift away from their original form, one would not expect such widely varying rates of change.

The fact that a species can survive for 350 million years (or even a mere 2 million years) would imply to me that there must be substantial processes in place that conserve the genome of the species, and that some significant external force must be applied overcome this inertia.  The fact that some species can be created, and disappear, in a very much shorter period of time, would imply that the force that can overcome the species inertia is not constant.

We know that on an individual level, there are factors that can alter the probability of mutation (e.g. age of parent, viral infection – and by inference, the effectiveness of the parental immune system, and that might be effected by environmental stresses).  This alone might have an impact that would allow the mechanisms that conserve the species to be overwhelmed by a large number of mutants at times of high environmental stress.  This might then be further exaggerated by significant inbreeding if a small population is cut off from the mainstream population.  Once the mechanisms for conserving the species have been overcome, then it would be a competition between the various mutants as to which will be sufficiently successful to create a new niche for itself in which it can thrive as a new species (possibly several new species – derived from the same parent species - being successful, causing a rapid radiation of new species as is often seen after a mass extinction event).
 

Offline Simmer

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Re: Humans & natural selection
« Reply #23 on: 30/10/2005 22:54:24 »
quote:
Originally posted by another_someone

We know that on an individual level, there are factors that can alter the probability of mutation (e.g. age of parent, viral infection – and by inference, the effectiveness of the parental immune system, and that might be effected by environmental stresses).  This alone might have an impact that would allow the mechanisms that conserve the species to be overwhelmed by a large number of mutants at times of high environmental stress.  This might then be further exaggerated by significant inbreeding if a small population is cut off from the mainstream population.  Once the mechanisms for conserving the species have been overcome, then it would be a competition between the various mutants as to which will be sufficiently successful to create a new niche for itself in which it can thrive as a new species (possibly several new species – derived from the same parent species - being successful, causing a rapid radiation of new species as is often seen after a mass extinction event).


Nice try, another someone - actually a very nice try! :) If the mutation rate does increase in response to environmental stress that would be, as you say, a highly significant evolutionary mechanism.  But before you submit this thread to Nature, is it true? It doesn't seem to affect human mitochondrial DNA, for example, the mutation rate of which is believed to be so regular that it is used to date prehistoric human migration.

A steady rate of mutation can also explain phenomena like coelacanths and other long lived species - a mutation doesn't persist unless it offers some advantage, or at least no disadvantage.  In a well adapted creature in a stable environment most mutations would be less successful than the normal population and eventually be bred out.

Steady mutation rates can also explain the explosion of speciation after a mass extinction.  A regularly occuring mutation that let a well adapted nut-eating bird become a poorly adapted seed eating bird would normally be a disadvantage and be bred out every time it cropped up, unless all the well adapted seed eaters were dead and there was a niche for these mutants to survive in and become adapted to.

Of course all that is just rationalisation of my original standpoint - I don't really know which of us is right (if either!) but at the very least you have convinced me that the there are a lot of possible mechanisms that I hadn't previously considered :)
 

another_someone

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Re: Humans & natural selection
« Reply #24 on: 31/10/2005 16:26:45 »
quote:
Originally posted by Simmer

If the mutation rate does increase in response to environmental stress that would be, as you say, a highly significant evolutionary mechanism.  But before you submit this thread to Nature, is it true? It doesn't seem to affect human mitochondrial DNA, for example, the mutation rate of which is believed to be so regular that it is used to date prehistoric human migration.



Is it true? – I cannot say, I am no expert on the matter, only trying to apply rational logic to what little I know.

I have two answers with regard to mitochondrial DNA.

Firstly, we cannot at all be sure that the rate of change does not alter in times of stress – when have we observed human mitochondria on a population wide basis under stress conditions?

Secondly, to what extent does mitochondrial DNA really give us information about speciation?

Mitochondria is inherited solely from the mother, yet the definition of a species depends on the way sexual reproduction behaves.  Mitochondria are not directly affected by sexual reproduction, since it has no input from the paternal line.

In particular, in sexual reproduction, there is much shuffling around of genes between the chromosomes of the two parents.  This shuffling around can cause all sorts of transcription errors, and problems with improper pairing of chromosomes.  In all of this, the mitochondria remain unaffected, and thus far more conserved and stable.  This is what made mitochondria such an easy target for tracing a maternal lineage, but it also makes it very different in nature to the chromosomes that are within the cell nucleus.

Another anomaly that arises is from a similar project that traced the human maternal line through mitochondria, but this traced the paternal line through the Y chomosome.

http://news.bbc.co.uk/1/hi/sci/tech/999030.stm
quote:

The most recent ancestor of all males living today was a man who lived in Africa around 59,000 years ago, according to an international team of researchers.
The scientists from eight countries have drawn up a genetic family tree of mankind by studying variations in the Y chromosome of more than a thousand men from different communities around the world. The Y chromosome is one of the two sex chromosomes (X and Y) which only men carry (women carry two X chromosomes).
The new research confirms the Out of Africa theory that modern humans originated in Africa before slowly spreading across the world.
But the finding raises new questions, not least because our most recent paternal ancestor would have been about 84,000 years younger than our maternal one.


http://news.bbc.co.uk/1/hi/sci/tech/999030.stm
quote:

This Out of Africa hypothesis has been confirmed by studies of mitochondrial DNA, the segment of genetic material that is inherited exclusively from the mother.
Based on these studies, our most recent common ancestor is thought to be a woman who lived in Africa some 143,000 years ago, the so-called Mitochondrial Eve.



Since the nature of a species is that the mate of an individual of a species must also be from the same species, this leaves three possibilities:

Either the clocks are wrong, and these two ancestors did live at the same time.

Or, mitochondrial Eve was not human.  All the trace can really tell us is that all living human females are related to mitochondrial Eve, they cannot tell us what species mitochondrial Eve actually was.

Or, Y chromosome Adam was not the first human male.  All the trace can tell us is that all living males are related to Y chromosome Adam, they but by inference, we must also be related to his father, his grandfather, his great grandfather, etc.

None of this really tells us very much about where the boundary between one species and another occurred.  In other words, if we were able to travel back in time, which of our ancestors could we successfully mate with, and and which point in time would we first come across an ancestor we are no longer able to mate with?

quote:

A steady rate of mutation can also explain phenomena like coelacanths and other long lived species - a mutation doesn't persist unless it offers some advantage, or at least no disadvantage.  In a well adapted creature in a stable environment most mutations would be less successful than the normal population and eventually be bred out.



This is half the answer.

The fact is that most mutations do not cause speciation.

Humans have invaded a very wide variety of environments, and one way or another, have adapted to those environments.  Some of those adaptations have been through mutation and selection, which has brought about racial differences, but it has not brought about speciation.

The more we discuss this, and the more I think this through, the more I begin to question whether speciation has anything really to do with fitness for purpose.  Speciation is first and foremost a barrier to sexual reproduction, and not of itself an adaptation to the physical environment.  It is a means of preventing genes from one population from leaking into the gene pool of another population.

Is this barrier merely a by product of extreme racial/breed divergence, or it some separate mechanism to do with genetic competition through a form of genetic branding (in the way corporations brand themselves as a way of giving themselves an identity and distinguishing themselves from their competition).  If it were merely a consequence of extreme racial/breed divergence, should not a lesser degree degree of racial separation begin to impose partial barriers to sexual reproduction?  Is there any evidence for such a partial barrier to exist between human races, or breeds of dogs?

Certainly, the physiological differences that can exist between races/breeds are substantial enough, and sometimes even greater than might exist between two species.  I am not saying that physiological differences caused by environmental adaptations do not often go hand in hand with speciation, but that would be because animals that live in different environmental niches will often be sufficiently physically isolated from each other (even if not today, then probably at some time in the past) to allow speciation to occur; but is speciation merely a product of that adaptation process, or a parallel mechanism?


quote:

Of course all that is just rationalisation of my original standpoint - I don't really know which of us is right (if either!) but at the very least you have convinced me that the there are a lot of possible mechanisms that I hadn't previously considered :)



Indeed, neither of us can know the real truth (nor in fact do most of the experts – but they probably have a little more information to work with than us), but it has been a useful exercise even for me to challenge my own ideas, and have to think them through to their logical conclusions.
 

The Naked Scientists Forum

Re: Humans & natural selection
« Reply #24 on: 31/10/2005 16:26:45 »

 

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