Individual vs Gene selection.
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Dylab said:
Individual selection refers to the selection of individuals, while gene selection refers to the selection of genes. Does this qualify for a "Duh"?
Seriously, though. The "obvious" unit of natural selection is at an individual level; an individual organism mates or not, feeds or not, and dies or not. When that organism dies, his copies of the genes are no longer available for reproductive purposes, which is typically bad.
If you look a little closer at why he died (or more happily, why he survived as long as he did), though, you may find that a few of his genes had a big effect on this. In a statistical sense, we can talk about a "gene for such-and-such" that improves the chance of an organism surviving and reproducing. If we view the organism population as a set of genes, we can look at the gene distribution and see how a positive gene increases in frequency and a negative gene decreases. (Obviously, the positive gene increases in part because it helped its carriers -- note plural -- survive and reproduce as individuals.) But it's convenient to look at the frequency increase as a process of competition between genes, and the "better" gene wins. At least, wins more often.
I think the difference between individual and gene selection is best shown in the sociobiology of altruism. Species (and individuals, of course) desplay altruism when they help others at a cost to themselves. If natural selection happened at the level of the individual, altruism would be a phenomenon which ran counter to adaptiveness! (After all, risking one's own reproductive potential is bad enough, without adding in that you are helping another to increase theirs!) But...at the level of the gene...we see that the species which demonstrate altruism are species in which genetically related extended families live in close proximity to one another. Helping a neighbor (whether you are human, turkey, prairie dog...each have been used as examples by evolutionary biologists) is helping a genetic relative. If you slightly risk your own reproductive capability (and of course, not all helping of others is a significant risk to one's own reproduction), but at the same time help those who share your genes, then an altruistic trait (if it is genetically prompted) gets passed on...through selection by gene, not individual.
Mercutio said:
There's a different angle to this that shouldn't be overlooked. The unit of selection actually is the individual. Natural selection cannot and does not act directly on the gene. It always acts on the net result of the genes (+ environment), the phenotype. This is hardly efficient because it really needs to act on the underlying gene(s) but cannot get to it (them).
One of the angles here is what you wrote. Altruism is selected for because the sacrificing individual has ensured that genetically close individuals have survived even though he didn't. The other angle, though, is that every individual selected for is actually a big bag of genes and each attempted gene selection actually selects the whole bag. All those other genes are hitchikers. (I'm using this in the colloquial sense here, and somewhat ignoring the technical meaning of "hitchhiker.") With all these hitchhikers, each individual selection causes a churning in the other genes. This churning is one of the driving forces for the great variety we see in every population.
Art Vandelay
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"Selection" refers to the general trend from earlier generations to later ones. If a gene is increasing in frequency, thenthat is a case of positive gene selection. If it is decreasing, that is negative gene selection. Since the probability of any generation having an exact copy of an individual from a previous generation is basically zero, "individual selection" doesn't have much meaning. Individuals are not selected; each is born exactly once, and each eventually dies. Only the genes vary in terms of survival rates.
Which brings up just one more point, a small but important one. With the notion of natural selection, the level of analysis is now the population, rather than the individual. Certainly the action takes place through individuals, but we no longer think of a prototypical species member (with variability from that ideal viewed as "error"), but rather a population that varies tremendously on many different traits (to strain your metaphor from above, phenotypic traits that are both hitchhikers and drivers). This variability is no longer seen as error, but as a characteristic of the population, and a crucial component of natural selection.BillHoyt said:
The change in gene frequency is not necessarily due to selection. The selection you describe here is called directional selection. It is one of three (directional, disruptive,balancing.) If we have selection, we will get a change in gene frequency. But the converse is not true. A change in gene frequency can be due to mutation or drift or hitchiking (the technical sense this time).Art Vandelay said:
But what we are discussing here is at the molecular level. Selection cannot get directly to the genes, and must go through the individual. That is the basic point; it is forced to dump, directly or indirectly, the baby with the bathwater in every individual case of selection.
Art Vandelay said:
Only if you take an extremely narrow view of individual "survival." Although every organism "eventually dies," they will vary wildly in how long that takes, and perhaps more importantly, in how many offspring they create in the meantime.
But, more to the point, individuals are in fact the only organisms "selected" as genes themselves cannot die. An organism with a lethal-in-womb mutation will die as an individual, irrespective of the state of its other genes. As BillHoyt put it, "every individual selected for is actually a big bag of genes and each attempted gene selection actually selects the whole bag."
This is closely related to the notion of the "spandrel" in evolutionary biology, a trait that evolves in the absence of genetic selection pressure. Although (by definition) genetic selection pressure is absent for a such a spandrel, individual selection can nonetheless put evolutionary pressure on the spandrel gene.
Any variance in the frequency of such spandrels would thus be the result, not of gene selection, but of individual selection.
Yes, and this viewpoint has been a long time coming. Now the arguments in theoretical population genetics circles are about what John Gillespie calls "the Great Obsession" of trying to understand all the forces that cause this great variety in every population.Mercutio said:
Thanks guys for the informitive posts.
Wouldn't the altruistic gene example also be an example of group selection? How is it different?
How do the opinions of scientists differ. I remember reading a Stephen Jay Gould essay about how the assymetry in genetic relationship of kin results in the social structure that bees exhibit. Isn't this an example of Gene selection? But I have read that Gould did not agree with Gene selection. How does someone like Gould and someone like Dawkins differ in oppinion on the subject.
Wouldn't the altruistic gene example also be an example of group selection? How is it different?
How do the opinions of scientists differ. I remember reading a Stephen Jay Gould essay about how the assymetry in genetic relationship of kin results in the social structure that bees exhibit. Isn't this an example of Gene selection? But I have read that Gould did not agree with Gene selection. How does someone like Gould and someone like Dawkins differ in oppinion on the subject.
Art Vandelay
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BillHoyt
Drkitten
I would consider those to be forms of selection. “Selection” has a connotation of being goal oriented, but that does not apply in evolution.
Drkitten
But no matter how long they live, or how many offspring the create, there will still be only one instance of that individual. The frequency of a particular individual does not vary; each one exists only once.
Perhaps not “die”, but they can be eliminated from existence.
I don’t understand what you’re saying here. To what does the term “individual selection” refer, and how can it apply evolutionary pressure without creating genetic selection pressure?
The trouble with debates on what does or does not constitute "selection"or "evolution" is that it's easy to mean subtly different things by those terms.
Selective forces operate on many levels. It may be easier to see if the forces are artificial. CRD often used examples from animal husbandry .
Right now , humans, sheep and cattle are among the most "successful" large mammals- species and/or genus level selection, if you like, at the expense of seals, whales, bison, etc.
Within cattle, selection for certain genetic traits is evident. That's intra-specific. Is it individual selection? Yes. Choose a bull and use his semen to fertilise selected heifers. Is it genetic selection? Yes.
That's what you are selecting WITH. What you select FOR is milk yield etc. The selective force is human intent.
With the introduction of genetic engineering we are bypassing Billhoyt's (otherwise correct) observation that selection can't act directly on the genome. It can, now.
Is this "Natural Selection?" For my money, yes, it is;- it sure ain't supernatural. But it's largely a matter of semantics.
Kim Sterelny's book "Dawkins vs Gould" is a fair resume of the much hyped difference between the two scientists. Dan Dennett , in "Darwin's Dangerous Idea", goes so far as to accuse Gould of not being a Darwinist at all. He is scathing of the whole "spandrel" model. (To my mind he nitpicks a tad on the architectural meaning).
The bottom line is that all natural selection is gene selection. And if you're a gene, I'm sure that's of interest. But to us organisms, a wider view seems natural. You work at the level of relevance to you. Why limit your perspective?
Selective forces operate on many levels. It may be easier to see if the forces are artificial. CRD often used examples from animal husbandry .
Right now , humans, sheep and cattle are among the most "successful" large mammals- species and/or genus level selection, if you like, at the expense of seals, whales, bison, etc.
Within cattle, selection for certain genetic traits is evident. That's intra-specific. Is it individual selection? Yes. Choose a bull and use his semen to fertilise selected heifers. Is it genetic selection? Yes.
That's what you are selecting WITH. What you select FOR is milk yield etc. The selective force is human intent.
With the introduction of genetic engineering we are bypassing Billhoyt's (otherwise correct) observation that selection can't act directly on the genome. It can, now.
Is this "Natural Selection?" For my money, yes, it is;- it sure ain't supernatural. But it's largely a matter of semantics.
Kim Sterelny's book "Dawkins vs Gould" is a fair resume of the much hyped difference between the two scientists. Dan Dennett , in "Darwin's Dangerous Idea", goes so far as to accuse Gould of not being a Darwinist at all. He is scathing of the whole "spandrel" model. (To my mind he nitpicks a tad on the architectural meaning).
The bottom line is that all natural selection is gene selection. And if you're a gene, I'm sure that's of interest. But to us organisms, a wider view seems natural. You work at the level of relevance to you. Why limit your perspective?
Peter Soderqvist
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Gene selection and individual selection is complementary!
Since natural selection works directly on the organism's fitness, but the gene is indirectly the basic unit of selection, because the genes has programmed the organism's fitness, and is directly the basic unit of inheritance of these adaptive traits, but the organism carries out the instruction so natural selection has something to work upon!
Since natural selection works directly on the organism's fitness, but the gene is indirectly the basic unit of selection, because the genes has programmed the organism's fitness, and is directly the basic unit of inheritance of these adaptive traits, but the organism carries out the instruction so natural selection has something to work upon!
You may, but they aren't.
I agree. Our language does not even offer many graceful ways to avoid implying intent.
But many instances of the genes which make up that individual.
I'm not sure I quite get that either.
The way I understand the 'spandrel' idea, certain design features develop only as side-effects of other features. Lacking functionality of its own, such a feature remains neutral; any selective pressure acting on it is that which it 'inherits' from its 'parent' design feature -- that is, unless it is co-opted for a particular use. (A cowboy won't get circumsized because he needs a place to keep his chaw while he's in church).
Art Vandelay said:
In the case of spandrels, "evolution" usually occurs via genetic drift, usually on the basis of selection pressure exerted on adjacent genes. Adjacent, in this sense, can even be literal, as in adjacent sections of DNA within the same chromosome. Although there's some shuffling of genes within chromosomes at each new generation, the chances of a split between any two genes is rather small. Thus, if I have adjacent genes for X and Y, the odds are quite good that my children will have the same adjacent genes. If there's (genetic) selection pressure against Y, then there will implicitly be selection pressure against X as well, because many carriers of X also carry Y. (Obviously, X and Y are metasyntactic variables here, not sex chromosomes.)
This is one of the reaons that genetic drift occurs, because the organisms selected for by genetic pressure are likely to carry a slightly non-representative sample of all the other traits. Perhaps all the people with resistance to malaria also happen to have green eyes. In this case, the next generation of people will have more green eyes than the last, not because green eyes per se is pro-survival, but because resistance to malaria is pro-survival and most of the people with malaria resistance also had green eyes.
This is one of the reasons that breeding in small pools (for example, purebred greyhounds) is tricky. Many of the individual animals with wanted traits also carry the genes for unwanted traits at the same time (hip dysplasia, for example). By "selecting" a particular sire, you are implicitly getting all of his genes -- the good, the bad, and the ugly. You don't select "genes," you select "individuals," hopefully with a predominance of good genes over bad ones. But the selection is done on the basis of the individual organism, and if all of the individuals with the coat color you want have the gene for bad hips as well, then there's no way to "select" just the coat gene.....
I hope this isn't too nitpicky.
As I understand things, what you have described is what P.W. Hedrick first referred to as as 'genetic hitchhiking', slightly different from 'genetic drift'. The latter refers to nucleotide base substitutions which result in synonymous codons (and therefore identical proteins), or which result in amino acid substitutions having little or no actual phenotypic effect.
Art Vandelay
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To be clear, are you denying that they are subsets of selection, or are you denying that they overlap? For instance, suppose genes A and B have the same effect of fitness. However, gene A often mutates into gene B, while gene B hardly ever mutates. Would you not say that B will be selected over A?Dymanic said:
Yes, that's my point: selection applies to genes, not individuals.
So why will he get circumsized?
drkitten
I can see how, when evolutionists abstract evolutionary phenomena, they would categorize this as selection for the other gene, but there seems to be an undercurrent of "the gene is cheating". No gene acts in a vacuum; any attempt to "give credit" to the gene "truly responsible" for the selection will eventually bog down in arbitrary determinations. When all is said and done, the only truly objective standard, and the only one that matters in the grand scheme of things, is that a gene is selected to the extent that its representation is increased in the gene pool. The "fitness" of the gene includes everything that allows it propogate, including resistance to mutation, tendency to arise from mutations of other geners, association with other genes, meteors that hit several individuals carrying competeting genes, etc.
That's right, Peter; they are complementary. But since the biology can't directly get at the genes, it has to work on the phenotype. It is "trying" to get at the genotype, but doesn't have the direct means to get there.Peter Soderqvist said:Gene selection and individual selection is complementary!
Since natural selection works directly on the organism's fitness, but the gene is indirectly the basic unit of selection, because the genes has programmed the organism's fitness, and is directly the basic unit of inheritance of these adaptive traits, but the organism carries out the instruction so natural selection has something to work upon!
Ernst Haeckel commented on gene/individual selection in his birthday essay in Science (I think 2 July 2004). What he said was pretty much the same as BillHoyt's 1st post, to the effect that natural selection acts on the product of the genes, rather than directly on genes themselves.