Text 1220, 73 rader
Skriven 2004-12-29 06:21:00 av Perplexed In Peoria (1:278/230)
Ärende: Re: The "fuel" of evoluti
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"Wirt Atmar" <wirtatmar@aol.com> wrote in message
news:cqt5nq$cql$1@darwin.ediacara.org...
> Let me argue with your presumptions, William. Clonal, apomictic
parthenogenesis
> (which are three words meaning much the same thing) is not rare. Indeed, it
is
> the most common of all reproductive modes. For all but a very tiny minority
of
> cells in your body, it is the basic mode of cellular reproduction that you
> employ. It is also the reason that you die.
>
> While the benefits of sexual reproduction may take a little while to become
> apparent, the downside to apomixis becomes readily obvious in just a few
> generations. In the 1960's, Leonard Hayflick demonstrated that human cells
can
> replicate for only about 60 generations, regardless of their chronological
age.
> Replication count determined a lineage's viability, not age.
>
> While we now know that a part of this senescence is preprogrammed by clipping
> teleomeres off the ends of the chromosomes with each replication, that's
almost
> certainly an evolved response to insure to the cell the integrity of the
> information residing on the chromosome.
>
> I mentioned earlier regarding the experiment that every young biologist
should
> perform with a xerox machine. If you do it, you'll quickly find that 60
> replications is a little more than astounding. Using the best xerox machine
> that you can find, you will almost certainly not be able to read your page by
> the 60th replication.
>
> The primary reason why parthenogenetic populations are not common has nothing
> much to do with their demographics, but it does have everything to do with
> their capacity to accurately transmit their genomes in indefinite
replication.

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".

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.

To extend the analogy to sexual reproduction, let us create the sources for
the next generation by cross-splicing together two of the tapes that have
survived the selection process.

The thing is, as I do the math, I don't see how sex (random splicing) helps.
Well, maybe it might help if error-free half tapes are common but error-free
full tapes are rare.  But I don't see how that situation could arise from
any reasonable statistical hypotheses regarding the source of errors.

It might also help if you have "soft" or "truncation" selection which tolerates
a few errors, but ruthlessly weeds out tapes with too many errors.  But, even
so, 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).
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