Re: An introduction

dkomo wrote:

John Harshman wrote:


forcythe@xxxxxxx wrote:



My understanding is that we see much more change as a result of new
combinations of existing genes than from the individual mutation within
a single genome. Mutation may keep feeding into the possible new
combinations of genes, but, hypothetically speaking, we have plenty of
variability stored up to continue evolution without mutation for quite
some time.


True, for some values of "quite some time". But eventually this engine
will grind to a halt.


Really? Why? I assume you're referring to the fact that all the
allelles with the highest fitness values in the population will
eventually become fixed in the absence of mutation. That's Pop Gen 101.
But...

But what about sexual recombination? Frankly, I've seen very little
discussion anywhere about the effects of recombination on allelle
frequencies under natural selection.

No?

Here's what I think will happen to a large population even if there is
no mutation. I'm going to neglect drift here to keep things simple.
And I'm going to assume that a substantial fraction of phenotypical
traits of the animals or plants in that population are *polygenic*.

When selection acts, it acts upon phenotypic traits, not directly on
alleles. So if those traits are mostly polygenic, entire sets of
alleles will under selection together. But the population mates
sexually, so the recombination will continually be producing new
combinations of alleles for each polygenic trait. It seems to me that
it will be impossible for the allele frequencies to become fixed under
these conditions.

Why does it seem that way to you? I imagine you could set up a model of
gene interactions in which that would happen. But would it be a
realistic model? The simplest model of polygenic traits is additive, and
in such a model the "best" alleles at each locus should rapidly become
fixed if the greatest development of the trait is indeed optimum. It's
not clear to me what would happen if the optimum is somewhere in the
middle. But I suspect that some of the "good" alleles would become fixed
and others would be lost (fixing the "bad" alleles) in a real system;
any balance would, I think, be unstable.

So the population, in a sense, will have continuous
variation in terms of phenotypical traits even in the absence of
mutation and drift.


How fast it will do so depends on how strong the
selection is.

It appears that the engine will run indefinitely.

That may appear to you, but you don't seem to have thought about it very
hard. Diversity can be maintained under selection, but it's not trivial.
Frequency-dependent selection is the easiest. If selection changes in
time and space, that might maintain diversity, though it's not
necessarily a stable system. Heterosis would increase diversity at a
single locus. Complicated gene interactions can in some cases act to
maintain diversity too. But it's not a given by any means.

.

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