Re: Genomic Instability?


"g" <gillawton@xxxxxxxxxxxxx> wrote in message news:eqefnu$1jno$1@xxxxxxxxxxxxxxxxxxxxxx

"Alan Meyer" <ameyer2@xxxxxxxxx> wrote in message
news:eqd78k$1091$1@xxxxxxxxxxxxxxxxxxxxxx
On Feb 4, 3:19 pm, "g" <gillaw...@xxxxxxxxxxxxx> wrote:
...
How can random mutations come up with a linear progression from four toes
to
a hoof in front and three toes to a hoof in the back? When we think long
and hard about it, what were the odds of a whole SERIES of mutations
coming
up with a hoof? Just random chance, huh?
...

Didn't you already answer this one yourself? It's not random chance,
it's natural selection.
...
If so, I was not aware of it. You see, I have a problem with any model
which explains how something came to be, without explaining how it
occurred... and then answers the question of how we know it came to be, by
way of the model, is because, why gee whiz... because we see the RESULTS, do
we not?
......

Maybe the problem here is that you're having trouble figuring out
where all the variation comes from that natural selection operates
upon.

Bear in mind that _mutation_, i.e., changes in DNA base pairs
during DNA replication, is only one source of genetic variation.
In bacteria it's probably a major source. In plants and animals
that use sexual reproduction, it's a minor source.

Humans have 23 chromosomes pairs. The selection of which
individual from the father's pair (i.e., from his father or his mother)
and which individual from the mother's pair (from her father or
her mother) is random. Thus there are 2^23, or over 8 million
possible variations in chromosome selection in each gamete.

But even that is only part of the story since each chromosome
pair undergoes one or more "crossovers" during meiosis producing
a random mix of DNA from each chromosome of a pair. This
introduces even more variation than that introduced by the
random selection of which chromosome goes into the gamete.
These crossovers can occur in the middle of a gene leading
to novel alleles never before seen.

I'm not sure that's what you're questioning, but if it is, you can
see, I think, that natural selection has a great deal of variability
it can work on. In the horse case, I don't think we need to
imagine cosmic rays or other genetic damage as the reason
why some horses wind up with a bigger middle toe than others.
I am thinner than my father. My son is taller than I am. My
daughter is heavier than my wife - and so on and so on. Our
children don't look exactly like us or like each other. If big
feet or big hands turned out to have serious survival value,
there's plenty of natural variation among humans from which
to select those characters. So too, I think, for horses and
their toes.

The bacterial case is different - though even among bacteria
it is now known that sex-like gene transfers occur. Bacteria
have a higher rate of mutation than humans because they
do not have the same replication proof-reading mechanisms
that eukaryotes have. Also, with such rapid reproduction
(mitosis can be repeated in as little as 20 minutes with some
bacteria under ideal conditions), the number of reproductive
events is many orders of magnitude higher than for horses
or humans, and the corresponding availability of mutations
is higher and the effects of mutation are seen more rapidly.

Alan



.

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