Re: Genomic Instability?

Perplexed in Peoria wrote:
"Alan Meyer" <ameyer2@xxxxxxxxx> wrote in message news:eqsprn$17cm$1@xxxxxxxxxxxxxxxxxxxxxx

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.

I have to disagree. As to the production of novel genes by recombination,
that rarely happens. Mutation is much more common. In fact, the number
of point mutations per generation in humans is a couple times larger than
the number of crossings-over.

Furthermore, you are wrong to suggest that the gene shuffling which
takes place among weakly linked genes (those on different chromosomes, say)
provides an important source of variation for NS to work on. Variation
is indeed created, and there may even be differential survival and
reproduction of the variants. But variants created by recombination
of this sort will inevitably be destroyed by recombination in further
generations - the kind of variation you are talking about is not
_heritable_ variation, and hence has little value for evolution. Most
of the 'progress' made by selection gets undone as the linkage
disequilibrium is relaxed. On the other hand, mutations continue to
breed true for many generations.

Whoah. Just because variants generated by recombination are
sometimes transient in nature, that doesn't mean they have
'little value for evolution'. Those variants act as an important
defense against parasites. The ability to resist parasites is an
extremely valuable characteristic.

As for the idea that sexually created combinations
cannot persist because they will inevitably be destroyed by
further recombination - that seems mistaken. Such variation
most certainly /can/ be inherited.

Offspring typically have a 25% chance of inheriting
two unlinked genes from one of their parents. For such
a combination to monopolise the next generation, about
75% of offspring would need to die. For it to merely
persist - by being present in half the offspring - only
50% of the offspring would need to die.

That would be a lot of death for a population of modern
humans - but it's peanuts for many fish, spiders, plants,
etc, where often > 99% of offspring typically don't make it.

Also, unlinked genes are not the only type of variation
created by recombination. Linkage cannot simply be ignored.
Genes are often within linkage reach of dozens of other genes.
These have a substantially reduced chance of being divided again
by recombination straight away. That's a lot of heritable
sexually-created combinations for you right there.
--
__________
|im |yler http://timtyler.org/ tim@xxxxxxxxxxx Remove lock to reply.

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