Freethought & Rationalism Archive

Doubting Didymus · 10-22-2002, 03:25 PM

Specifically talking about wolbachia, (which are bloody interesting little buggers, arent they?), I do not see why this example is incompatible with a gene-centric perspective. Wolbachia is essentially an environmental factor, causing an isolated population and thus instigating speciation. In this sense, wolbachia infection is similar to geographical isolation.

Great! But so what? What is it about wolbachia that causes its host to only be able to breed with other infected insects? It is the heritable features of the wolbachia, (or else the next generation of wolbachia would not inherit the ability).

What is it about the insect populations that make the species a new species? Its the genes of the insect, now become so different from its parent species that it can no longer interbreed, and thus has speciated.

This is why I insist that, if not specifically the genes, the heritable factors of organisms are what biological evolution is concerned with. This does not mean we should ignore the environment, species interactions, or other factors, just that we should be looking at them with a mind to how they effect heritable factors.

Now, I have been under the impression that there are only a few exceptions to genes being the only heritable factors, but I may be wrong. Hence my primary question in this thread, which is: what factors, other than genes, are both heritable and have significant impacts on evolution? So far I have seen cortical inheritance in ciliates and a vague suggestion about heritable centrosomes, which I have not been able to find any more information on.

Peez · 10-23-2002, 04:36 AM

Quote:

Could you please provide a few examples of non-nucleotide inheritance that "make substantial contributions to the form and function of organisms"?
pz:
I presume that you've read D'Arcy Thompson's On Growth and Form? There's a whole book full of 'em, and Thompson doesn't mention genetics or genes except to disparage them (not that I agree with that bit of his attitude, of course).

No, I have not read it, though it sounds like it may be interesting. Meanwhile, could you please provide a few examples of non-nucleotide inheritance that "make substantial contributions to the form and function of organisms"?

Peez

pz · 10-23-2002, 06:16 AM

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Originally posted by Peez:
No, I have not read it, though it sounds like it may be interesting. Meanwhile, could you please provide a few examples of non-nucleotide inheritance that "make substantial contributions to the form and function of organisms"?

All instances of maternal localization of gene products. In Drosophila, for instance, that depends entirely on the proper conformation of the ovary and supporting cells that surround the oocyte.

All instances of axis formation. Symmetry-breaking events usually require extra-genetic imposition of information, whether it is maternal localization, sperm entry, gravity, implantation, etc.

All instances of induction. This should be obvious: if you have two cells with precisely identical genetic content that are going to have different cell fates, you have to have a non-genetic interaction of some sort to initiate those differences.

Does that help?

Now it's your turn. Give me a few examples of genetic inheritance that are entirely gene-autonomous...that is, no epigenetic contribution is required in order to see the phenotype, and the gene/allele is both necessary and sufficient for expression of the property.

(Don't try too hard. I'm not being fair, because I've asked you to support an impossible position.)

Peez · 10-23-2002, 08:03 AM

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RufusAtticus:
The difference here is that because of my linguistic backgroud I am a descriptionist. In that sense I am attempting to describe how the term "evolution" is actually used in the life sciences. A perscriptionist, like you, tries to define how it should be used.[/b]

On the contrary, it is my impression that you are being a "prescriptionist" as you describe it. In any event I attempting to describe how "evolution" is used in the life sciences, specifically evolutionary biology. I am not claiming that it should or should't include certain particular things, only that it doesn't as used by biologists who study such things. Apparently you disagree with some particulars.

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From my perspective, because the allele-centric defination doesn't cover some forms of evolution that are recognized by the scientific community it is incomplete.

Such as...?

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As scientists we should look to "describe" and not "perscribe."

As scientists we should be clear about what definitions we are using, but we should be more concerned with understanding natural processes than arguing about definitions.

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I am not familiar with the example, but if the phenotype is simply modified by the presence of a symbiont then I would not consider it evolution.
Why not?

Because it is not evolution by the definition that I use. This is purely a semantic issue.

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The modification of phenotype is passed from generation to generation.

V infection, along with all the associated symptoms, can be passed from generation to generation.

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If you observed the change of a population from sexual to asexual would you consider it evolution?

I thought that I had already answered this: only if the change was genetic.

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Why should it matter what the underlying mechanism of this change was?

Why should any definition matter? Why should we define green so as to exclude red? Why should it matter what the wavelength was? Of course we can define evolution so as to include phenotypic plasticity, but I am saying that evolutionary biologists do not (certainly none that I know of do).

Peez

pz · 10-23-2002, 08:21 AM

Quote:

Originally posted by Peez:
Of course we can define evolution so as to include phenotypic plasticity, but I am saying that evolutionary biologists do not (certainly none that I know of do).

Do you know any of those wild and crazy evo-devo rascals? Or even those sneaky new guys, the eco-devo crowd? (The other day, someone who was reviewing my long term research plans suggested that I was turning into an eco-devo guy. I don't know if that was good or bad, yet.)

I suppose that in a way you are right. The operational definition of what an evolutionary biologist does has been gradually crystallizing a bit, to the point where biologists who study other aspects of evolution feel like they are treading on people's toes by calling themselves "evolutionary biologists", and they've started inventing awkward new names for what they do, trying to find one that fits more comfortably.

Peez · 10-23-2002, 08:44 AM

Quote:

pz:
All instances of maternal localization of gene products. In Drosophila, for instance, that depends entirely on the proper conformation of the ovary and supporting cells that surround the oocyte.

Please explain further, this just sounds like the environment influencing the expression of genes.

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All instances of axis formation. Symmetry-breaking events usually require extra-genetic imposition of information, whether it is maternal localization, sperm entry, gravity, implantation, etc.

Once again, this sounds like environmental effects of the expression of genes.

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All instances of induction. This should be obvious: if you have two cells with precisely identical genetic content that are going to have different cell fates, you have to have a non-genetic interaction of some sort to initiate those differences.

Of course. Nobody is arguing that the environment is unimportant.

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Does that help?

I am sorry, but no. These do not seem to be examples of non-nucleotide inheritance that "make substantial contributions to the form and function of organisms". Perhaps I have missed something, and an elaboration would help.

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Now it's your turn. Give me a few examples of genetic inheritance that are entirely gene-autonomous...that is, no epigenetic contribution is required in order to see the phenotype, and the gene/allele is both necessary and sufficient for expression of the property.

First, I need to clarify: are you referring to epigenesis (foetal development) or epistasis (interactions among loci influencing phenotype)? From the context I will go with the latter, but please correct me if I am wrong.

You are setting up a straw man. I have never stated or implied that interactions among loci are not important, nor have I claimed that an allele is expressed entirely independant of the environment or of other alleles. The issues of epistasis and phenotypic plasticity are well-recognized by evolutionary biologists, but do not change the fact that it is virtually only genes that are inherited. Again, maternal effects are also well-recognized, but fade after one or a very few generations.

Some simple examples of nucleotide inheritance that "make substantial contributions to the form and function of organisms": nucleotide sequence of the transfer RNA for tryptophan, primary structure of the alpha subunit of haemoglobin, structure of the cell wall of a plant cell, the number of fingers on each hand, and attraction to certain chemicals in bacteria.

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(Don't try too hard. I'm not being fair, because I've asked you to support an impossible position.)

Perhaps so, but in any event it is not my position so I will not bother to try to support it.

Peez

[ October 23, 2002: Message edited by: Peez ]

Peez · 10-23-2002, 08:55 AM

I would like to ask a question about "evolution", with an eye to clarifying the positions held here:

What would be an explanation for the evolution of birds from "reptiles" that does not require changes in allele frequency?

I do not know whether or not this is a reasonable question to ask, but if it is reasonable then I think that the answer(s) might be enlightening.

Peez

Doubting Didymus · 10-23-2002, 03:54 PM

It looks to me like both PZ and PeeZ are using the same argument.

"Show me an example of a feauture that genes can produce on their own"

"Show me an example of an evolutionary progression that can occur without genes"

Both of these are very obvious straw men.

Pz, I want you to clarify your position on heredity, please. See my post on page three, about wolbachia, if you will.

pz · 10-23-2002, 05:21 PM

Quote:

Originally posted by Peez:

Please explain further, this just sounds like the environment influencing the expression of genes.
[ .. ]

Once again, this sounds like environmental effects of the expression of genes.

Hmmm? You asked me to provide "examples of non-nucleotide inheritance that 'make substantial contributions to the form and function of organisms'". There they are -- they are not genetic, they are inherited, and they clearly make substantial contributions to the form and function of organisms. How can you now say "These do not seem to be examples of non-nucleotide inheritance that 'make substantial contributions to the form and function of organisms'"?

Saying that it "just sounds like the environment influencing the expression of genes" is a rhetorical game. It would be like me saying that genes are nothing but passive responders to the dictates of the cytoplasm. It's an attempt to trivialize something that really makes a profound contribution to the biology.

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Again, maternal effects are also well-recognized, but fade after one or a very few generations.

They do?? That's going to be a great surprise to developmental biologists. When did the configuration of follicle and nurse cells in the Drosophila ovary fade away?

Quote:

Some simple examples of nucleotide inheritance that "make substantial contributions to the form and function of organisms": nucleotide sequence of the transfer RNA for tryptophan, primary structure of the alpha subunit of haemoglobin, structure of the cell wall of a plant cell, the number of fingers on each hand, and attraction to certain chemicals in bacteria.

Please explain further, this just sounds like the the expression of genes responding to the environment. I would also suggest that all of those are implicitly dependent on a very specific cellular and organismal milieu and are as dependent on the environment as the environment is on the genes involved.

To pick one example of particular interest to me, do you really believe that the number of fingers on the hand is defined by gene action alone?

RufusAtticus · 10-23-2002, 05:28 PM

Quote:

In any event I attempting to describe how "evolution" is used in the life sciences, specifically evolutionary biology. I am not claiming that it should or should't include certain particular things, only that it doesn't as used by biologists who study such things. Apparently you disagree with some particulars.

What you are describing is how "population geneticists" study evolution. The majority of evolutionary biologists are "population geneticists" in some sense or another. (At my university, the two are virtually synonymous.) However, I think that pop-gen people are narrow sighted to say that the only way to investigate or verify evolution is through changes in gene pools.

I'm not talking about phenotypic plasticity. Infection by a cytoplasmic parasite is a trait of the population. In fact, it a trait that is transmitted to offspring. And in the case of Wolbachia it has significant pleiotropic effects on phenotype. Likewise, when a bacterial population acquires a plasmid from its environment, it is a trait that is passed on in the cytoplasm and has significant pleiotropic effects on phenotype. Why is infection by a plasmid considered evolution, but infection by Wolbachia not?

What is so important about nucleic acids that only they can cause evolution? Sure changes in nucleic acid composition probably makes up most of the evolutionary events in the past three billion years or so, but does that mean that only changes in nucleic acids should be considered evolution?

Let us take a look at one mechanism of evolution applied to this issue. Selection acts upon the variability in a population. If that variation has a heritable component, then the effects of selection will be transmitted to future generations. Why then should it matter whether the heritable variation is in the genome or cytoplasm? The same kind of argument can be made for drift and migration.

That is why I have a problem restricting evolution to changes in nucleic acid makeup.

~~RvFvS~~

[ October 24, 2002: Message edited by: RufusAtticus ]

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10-22-2002, 03:25 PM	#71
Doubting Didymus Veteran Member Join Date: Jul 2002 Location: East Coast. Australia. Posts: 5,455	Specifically talking about wolbachia, (which are bloody interesting little buggers, arent they?), I do not see why this example is incompatible with a gene-centric perspective. Wolbachia is essentially an environmental factor, causing an isolated population and thus instigating speciation. In this sense, wolbachia infection is similar to geographical isolation. Great! But so what? What is it about wolbachia that causes its host to only be able to breed with other infected insects? It is the heritable features of the wolbachia, (or else the next generation of wolbachia would not inherit the ability). What is it about the insect populations that make the species a new species? Its the genes of the insect, now become so different from its parent species that it can no longer interbreed, and thus has speciated. This is why I insist that, if not specifically the genes, the heritable factors of organisms are what biological evolution is concerned with. This does not mean we should ignore the environment, species interactions, or other factors, just that we should be looking at them with a mind to how they effect heritable factors. Now, I have been under the impression that there are only a few exceptions to genes being the only heritable factors, but I may be wrong. Hence my primary question in this thread, which is: what factors, other than genes, are both heritable and have significant impacts on evolution? So far I have seen cortical inheritance in ciliates and a vague suggestion about heritable centrosomes, which I have not been able to find any more information on.

10-23-2002, 08:55 AM	#77
Peez Veteran Member Join Date: Apr 2001 Location: Canada Posts: 5,504	I would like to ask a question about "evolution", with an eye to clarifying the positions held here: What would be an explanation for the evolution of birds from "reptiles" that does not require changes in allele frequency? I do not know whether or not this is a reasonable question to ask, but if it is reasonable then I think that the answer(s) might be enlightening. Peez

10-23-2002, 03:54 PM	#78
Doubting Didymus Veteran Member Join Date: Jul 2002 Location: East Coast. Australia. Posts: 5,455	It looks to me like both PZ and PeeZ are using the same argument. "Show me an example of a feauture that genes can produce on their own" "Show me an example of an evolutionary progression that can occur without genes" Both of these are very obvious straw men. Pz, I want you to clarify your position on heredity, please. See my post on page three, about wolbachia, if you will.

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