Re: Selection Pressure vs Mutation

Al wrote:

On Oct 12, 7:17 am, "Sam" <s...@xxxxxxxxxx> wrote:

Let's see if I can ask this in a way that makes sense.


Failed there.
I think the problem is a fairly common abuse of the word mutation.
Did you mean a spontaneous alteration of genetic matter? Or do you
mean any variation of genetic material? Because any population has a
fair spread of option two regarless of morphology changes. Most
evolution happens without random, spontaneous, mutations. But
generally if it's passed from generation to generation it's usually
DNA based (or RNA).
There are several things that can change morphology that aren't DNA,
but they don't tend to inherit very well. EGs above include
thalidamide, but many other things do other horrid things, such as
tobacco, excessive alcohol, coccaine, hell, even vitamin or mineral
defficiencies.
The thing is, these embryonic environment issues aren't inheritable
except in very vague ways. So they don't generally accumulate. Your
example of size changes on islands etc, generally starts using
existing size variation within the population, and is selection for
smaller individuals, within the existing population. But then all
evolution is selection of existing traits within a population.
Sometimes the available genetic material to select from might
diversify due to externally created mutations, but the evolutin is
based on the selection.

I will just add that if multiple genes contribute to adult size, it's
possible through selection, without adding any new mutations, to evolve
considerably past the size range of the original population. Consider a
simplified model with 10 genes in a haploid genome, each with a T (tall)
and t (short) allele. Your height is the twice the number of T alleles
plus the number of t alleles. Say the optimum height is 15. On the
average, you would benefit from having half your alleles be T and half
t; doesn't matter which. Some individuals would have more, some less.
Very few individuals would have all T or all t (approximately 1
individual in a thousand, before selection; after selection, hardly
any). Now suppose the optimum height changes to 10, because you're now
on an island. Now the optimum mix of alleles is all t. T alleles will
gradually be eliminated from the population, and the average height will
end up being less than the minimum height in the original population.

.

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