Text 1253, 102 rader
Skriven 2004-12-31 10:54:00 av Perplexed In Peoria (1:278/230)
Ärende: Re: The "fuel" of evoluti
=================================



"Tim Tyler" <tim@tt1lock.org> wrote in message
news:cquoj6$u8q$1@darwin.ediacara.org...
> Perplexed in Peoria <jimmenegay@sbcglobal.net> wrote or quoted:
>
> [brutal snip]
>
> > I am unconvinced by your Xerox experiment that sexual reproduction does a
> > better job of preserving information than does asexual reproduction.  One
> > problem with your analogy is that a Xerox degrades the source in "analog"
> > fashion, whereas biological reproduction degrades the source in "digital"
> > fashion.  A better analogy might be an old fashioned paper tape
reader/punch
> > that occasionally leaves (and misreads) "hanging chads".
>
> Sex /does/ help.  It helps by combining deleterious mutations in
> certain individuals.

Yes, it might do that.  But it also might break up combinations of
deleterious mutations.  If recombination is assumed to be random,
then it does both.

> When these die or fail to reproduce, they take
> out a load of mutations from the population with them.  Conversely,
> sex creates relatively unmutated individuals from the ancestral gene
> pool.

And I would argue, conversely, that it also breaks up relatively unmutated
genomes.

[snip]
> Analog/digital is a side issue to this argument.

Wholeheartedly agree!!

> > Clearly, to have any hope at all of preserving the original information, it
> > must be the case that any one tape in the population will sire on average
> > more than one error-free descendent.  To complete the analogy, we need a
> > selection process that is biased toward chosing error-free descendents as
> > the sources for the next generation of copies.
>
> Yes, but a way of increasing the variance of the number of mutations in
> individuals would help selection do its job in this area.
>
> If the deleterious mutations are concentrated in some individuals,
> fewer members of the population need to fail for the gene pool to
> remain in good working order.

I fully agree that IF recombination increases the variance in number of
mutations per individual, then your argument works.  What I don't
agree with is that recombination *will* increase the variance, unless you
make some totally artificial assumptions regarding the source of
mutations.  For example, if your mutation process inserts *exactly* one
mutation per individual (low variance), then recombination obviously
will redistribute the mutations so that you have (in the simplest model)
25% with no mutations, 50% with one, and 25% with two mutations.  The
variance per individual has increased.  But the initial assumption
regarding the statistics of the mutations is unrealistic, IMO.

> I did a computer simulation of this once ...
>   http://alife.co.uk/eosex/

I would be interested in seeing the code or a list of modeling
assumptions.

> > ... it is difficult to see sex as a win in the face
> > of the fact that it doubles the inherent error rate (by doubling the
> > size of the genome).
>
> That's surely not a vepy good description of what having paired
> chromosomes does to the error rate.  It's not much like having a
> genome of twice the size.  It's more like having a backup copy
> of your existing genome.
>
> Besides, "having paired chromosomes" and "sexual recombination" are
> logically independent:
>
> Sexual recombination involves exchanging your genetic material
> with that of other individuals.
>
> Having paired chromosomes in each cell is /one/ way of implementing
> that - but it is not the /only/ way.

I agree with the distinction between diploidy and recombination.
I only mentioned the doubling of genome size because it is commonly
associated with recombination and because one occasionally sees diploidy/sex
justified as a technique for removing two recessive-lethal genes from
the population with a single organism-level selective death.  Yes, it
does do that, but only at the cost of doubling the number of mutations
per organism generation - which is why I raised the point.  But I am
perfectly happy (in fact, I would prefer) to keep our discussion at the
haploid organism level and to have paired chromosomes only as a transient
phenomenon during recombination.
---
ū RIMEGate(tm)/RGXPost V1.14 at BBSWORLD * Info@bbsworld.com

---
 * RIMEGate(tm)V10.2á˙* RelayNet(tm) NNTP Gateway * MoonDog BBS
 * RgateImp.MoonDog.BBS at 12/31/04 10:54:59 AM
 * Origin: MoonDog BBS, Brooklyn,NY, 718 692-2498, 1:278/230 (1:278/230)