Unit of Selection?

[MarxSchmarx]

Inspired by this thread:

http://www.revleft.com/vb/opinions-s...260/index.html (http://www.revleft.com/vb/opinions-selfish-gene-t81260/index.html)

what do you all think is the PRIMARY unit upon which natural selection acts? By unit, we usually mean level of biological organization. There are a few candidates:

gene (i.e., amino acid site),
gene family,
genome
individual (genome + other crap)
group (kin)
ecosystem

I ask you to justify your answer, and why the others don't work.

I guess I'll start. Although on some level all of these are subject to selection, I decree the gene family (i.e., integrated units of at least one gene) is the primary unit.

It has been posited in the aforementioned thread that this is not so because (1) genes have mutation imposed upon them, and (2) only gene expressions are selected for:
http://www.revleft.com/vb/opinions-selfish-gene-t81260/index.html?p=1171191#post1171191

However, mutation is only good insofar as it generates variation. As long as heritability, variability and differential fitness are available, natural selection will occur. Genes (and by extension gene families) satisfy all these criteria.

Secondly, gene expression is itself subject to selection, often through the control of other genes (e.g. on-off genes in development). Moreover, some genes, like transposable elements or genes involved in miotic drive, can establish themselves remarkably well in the face of little or even negative impact on the individual organism. Moreover, plasticity alone is insufficient to save an allele that is way out there, and if there is a strong environmental effect on a phenotype, than that merely means the selective environment for all genes is relaxed. This means that all genes do OK. Then heritable variation in fitness ceases to exist, which doesn't satisfy a condition of natural selection.

Of course, there are cases of group selection and selection at the level of the individual and even at the ecosystem. However, such examples seem to be the exception rather than the rule.

Yeah I would have had a poll but this seems disabled in the s&e.

[Luís Henrique]

It's the individual, of course.

Genes are only selected through their expression in phenotypes.

Luís Henrique

[MarxSchmarx]

It's the individual, of course.

Genes are only selected through their expression in phenotypes.


Ah but if a phenotype is selected across multiple (potentially unrelated) individuals, the gene that helped determine that phenotype, and not necessarily the individual, spreads.

[Luís Henrique]

Ah but if a phenotype is selected across multiple (potentially unrelated) individuals, the gene that helped determine that phenotype, and not necessarily the individual, spreads.

Yes - genes replicate. But what is selected is the phenotypical expression of genes, not genes in themselves. Which also accounts for the fact that environment is essential for selection - it is what selects individuals.

Luís Henrique

[MarxSchmarx]

Yes - genes replicate. But what is selected is the phenotypical expression of genes, not genes in themselves. Which also accounts for the fact that environment is essential for selection - it is what selects individuals.

Luís Henrique

Rats it seems my reply got lost when the board went haywire. So here's an abridged version:

if it really were the phenotype that is selected, and not the genotype, there are three problems with this.
(1) phenotypic variation is not as heritable as genotypic variation (generally speaking - and narrowly speaking it is not heritable at all, but this is a matter of definitions) - an individual's environment could change so the environment part of the gene x environment interaction is not passed on. Without heritable variation, natural selection can't work.
(2) Environmentally induced variation is part of the selective regime experienced by a gene. A gene that produces a favorable phenotype happened to be at the right place at the right time. Thus enviromental or development variability per se just adds to the gene's fitness landscape.
(3) Selection on phenotypes still doesn't imply selection on individuals. If phenotypes are entirely genetically determined (like blood type) genes are still what are selecte

[Dean]

Inspired by this thread:

http://www.revleft.com/vb/opinions-s...260/index.html (http://www.revleft.com/vb/opinions-selfish-gene-t81260/index.html)

what do you all think is the PRIMARY unit upon which natural selection acts? By unit, we usually mean level of biological organization. There are a few candidates:

gene (i.e., amino acid site),
gene family,
genome
individual (genome + other crap)
group (kin)
ecosystem

Individual. I don't think genes will really wax and wane without the full development dictating its future (i.e. a gene for a disease doesn't start to die off in absence of its full actuation of its 'disease character' or whatever).


In any case, I don't know a lot about this, maybe others would be more insightful.

[zelda]

DNA and natural order.

[trivas7]

what do you all think is the PRIMARY unit upon which natural selection acts? By unit, we usually mean level of biological organization.
Natural selection isn't an outside force that acts upon nature. Rather, It is that very natural characteristic that exhibits change which is natural selection.

[MarxSchmarx]

Individual. I don't think genes will really wax and wane without the full development dictating its future (i.e. a gene for a disease doesn't start to die off in absence of its full actuation of its 'disease character' or whatever).

What you describe is akin to environmentally induced phenotypic variation. The conditions which determine whether the gene is expressed or not (which is what I think you are talking about) are just another part of the overall selective environment faced by the genes.

Moreover, again I am still confused how the fact that genes are expressed or not has any bearing on whether the individuals become the unit of selection.


Natural selection isn't an outside force that acts upon nature. Rather, It is that very natural characteristic that exhibits change which is natural selection.

Huh?

[Luís Henrique]

Moreover, again I am still confused how the fact that genes are expressed or not has any bearing on whether the individuals become the unit of selection.

Because it is individuals who are selected.

Luís Henrique

[apathy maybe]

You know what, I not only think it doesn't matter shit, I also think that none of you actually know enough to pass judgement on the matter.
However, for the discussion, I point to http://en.wikipedia.org/wiki/Gene-centered_view_of_evolution ans suggest that it might be useful.

[MarxSchmarx]

Because it is individuals who are selected.I am seriously confused, Luís. On the one hand, you say that individuals are the unit of selection because gene expression is what matters. On the other hand, you say gene expression is what matters because the individual is the unit of selection. Doesn't this strike you as circular?

Thanks for the wikipedia link, AM. I think it will help readers of this thread who haven't come across this problem in the past and is quite a helpful addition.

However...

it doesn't matter shitFirst of all, your statement is wrong. If genes are the unit of selection, then the creation of biodiversity is really a by-product of genetic diversity. Therefore, in fields from conservation to agriculture to public health, a gene-centric view should be adopted. If, however, individuals are the unit of selection, then we are wasting our time finding genes that help cabbage grow faster. If groups are the unit of selection, it is more important to conserve the pack of wolves than individual wolves or individual wolf genotypes, etc...

Secondly, my apologies for raising what you view as a non-political question in the SCIENCE AND ENVIRONMENT forum. :rolleyes:


I also think that none of you actually know enough to pass judgement on the matter. With all due respect, AM, what do you purport to know about our educational backgrounds?

For the record, I have delivered lectures on this problem and have spoken personally to the leading proponents of divergent views. I have also sat through hours and hours of tedious, circular discussions on this point, and was curious what other people thought here and why they held those opinions.

So please, such comments add nothing of value to the question at hand and create the impression of uncivil discourse.

[Luís Henrique]

If, however, individuals are the unit of selection, then we are wasting our time finding genes that help cabbage grow faster.

Eh? How that?

We have been selecting plants and animals (and succesfully, too) for millenia, much before we even knew genes exist... How would we do that, if genes were the "unit of selection"?

Luís Henrique

[cyu]

I think it depends on what kind of organisms we're talking about here. Natural selection for viruses will be different from natural selection for humans. After humans developed communication and technology, I think natural selection for genes has taken a back seat to natural selection for social structures that best promote the survival of the members of that social structure.

For example, before advances in medical science, there would have to be genetic selection for immunity to various diseases. After cures were developed, the genetic selection based on disease immunity was replaced by selection on social structures that best provided the cures to the members of that society.

[MarxSchmarx]

We have been selecting plants and animals (and succesfully, too) for millenia, much before we even knew genes exist... How would we do that, if genes were the "unit of selection"?

Does it make a difference if we were conscious that we were selecting for genes?

Wouldn't you say we were selecting for phenotypes (corn that grows faster instead of this or that stalk of corn)? By extension, weren't we selecting for the genes that control those phenotypes? We in fact tried to breed out undesirable correlated traits (like increasing aggressiveness) when they were associated with individuals with preferable traits (e.g. lost of body mass).



I think it depends on what kind of organisms we're talking about here. Natural selection for viruses will be different from natural selection for humans. After humans developed communication and technology, I think natural selection for genes has taken a back seat to natural selection for social structures that best promote the survival of the members of that social structure.

The point is well taken. Therefore, the question becomes, in the majority (60%? 70% 95%?) of cases, what is the unit of selection? Humans are, for the very reasons you mention, exempt from much of natural selection so a gene-centric view doesn't apply as neatly. However, I think such a perspective DOES apply to most plants, insects and single-celled organisms (even the social ones) - in other words, the overwhelming majority of the world's living things.