On Wed, 26 May 2004 23:12:56 +0000 (UTC), 
Tim Tyler  wrote:
> Larry Moran  wrote or quoted:

[snip]

>> Hmmm ... I think I see your objection. You want a precise definition of
>> evolutionary change so you can objectively analyze the mechanisms. I've
>> been assuming all along that changes are most easily quantified by adding 
>> up mutations in DNA but you don't seem to accept that particular measure.
> 
> It's a great measure - but not the /only/ one - and as I mentioned in
> another post, it has some definite flaws as a metric of evolutionary
> change.

What are the flaws ... other than the fact that it leads to a conclusion
you don't like?

> I am not my mind.  I am not my body.  I am not my DNA sequence ;-)

What the heck does this mean? I sounds like new-age doublespeak.

>> It was obvious to me. If a new set of limbs is due to 74 different 
>> base substitutions in DNA then that's how you measure it and compare
>> it to, for example, 74 single neutral base substitutions. I think
>> you want to substitute a subjective measure where a new set of limbs
>> has a much higher score because it's more significant to your way
>> of thinking. This is a new idea to me. Do you have a clear quantitative
>> way of calculating the score of morphological changes or is this just
>> a sleight-of-hand way of trying to argue in favor of the supremacy of 
>> natural selection?
> 
> I hope not the latter ;-)

Me too, but I'm not holding my breath.   :-)

> Quanifying phenotypic change is difficult.

That's probably why it isn't very useful. It's too subjective and it
ignores all the other kinds of evolutionary change.

> However, the phenotypes of living organisms still form a tree where
> qualities very along the branches.

You can group species by clades if you concentrate on certain kinds 
of morphological change (shared derived characters = synapomorphies).
This requires you to pretty much ignore most phenotypic change and it 
certainly isn't quantitative. Besides, we have plenty of examples were 
it led to an incorrect conclusion that was fixed by looking at DNA/protein 
sequences. Even more important, it hardly worked at all for bacteria and 
single-celled eukaryotes. 

> Taxonomists don't *have* to look at an organisms' DNA sequence to
> begin to answer questions about which other organisms it is
> most closely related to.

That's correct. It's quite possible to look at a subset of all
evolutionary change in order to work out some of the history. This
applies to morphological change as well as to DNA/protein sequence
change. Do you have a point?

> As for general-purpose metrics - there are several of them.  Length,
> lifespan, mass and number of cell types are all fairly useful means 
> of quantifying evolutionary change.

Now were getting a bit silly .....

[snip]

>> BTW, I'm not conceding that all morphological change is due to natural 
>> selection. Do you have a quantifiable measure of the amount of
morphological 
>> change that's due to natural selection?
> 
> My impression is that this depends in a big way on the effective 
> population size.
> 
> In "small" populations, the founder effect is the main force.
> 
> In "large" populations, the founder effect is of low relevance,
> and selective forces tend to dominate as causes of phenotypic change.

Do you have a quantifiable measure of the amount of morphological change
that's due to natural selection?

> This is *only* if you ignore neutral mutations, of course.

What does this mean? I'm asking for a way to distinguish between 
morphological change that's due to natural selection and morphological
change that's neutral. How can you "ignore" neutral mutations when you
can't even identify them?

> *If* you use a metric that measures raw genome hamming distance,
> random noise is indeed the cause of most changes.

"Random noise" is a loaded phrase that reveals a great deal about 
your biases.

> However, random noise is like radio static - interesting if you are
> wondering how far away the transmitter is - but if you are trying
> to listen to the radio program, it just gets in the way.

I'm interested in molecular evolution and deep phylogeny. Natural 
selection just gets in the way of such studies and may even invalidate
them. Natural selection is sort of like radio static to me - I have
to put up with it but I try to ignore it whenever possible. However,
in spite of my personal interests I would never try and define natural 
selection out of existence just because I'm not very interested in it. 
That would not be scientific.




Larry Moran
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