Text 1324, 118 rader
Skriven 2005-01-04 20:31:00 av Perplexed In Peoria (1:278/230)
Ärende: Re: The "fuel" of evoluti
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"Tim Tyler" <tim@tt1lock.org> wrote in message
news:crc4mn$21a2$1@darwin.ediacara.org...
> Perplexed in Peoria <jimmenegay@sbcglobal.net> wrote or quoted:
> > "Tim Tyler" <tim@tt1lock.org> wrote in message
news:cr754q$dhl$1@darwin.ediacara.org...
> > > The /reason/ sex can be beneficial at cleansing deleterious mutations
> > > from a population is that deleterious mutations can act togther
> > > *synergetically*.  In other words, having mutation A and mutation B
> > > can sometimes be a /lot/ worse than having either alone.  This is
> > > what is sometimes known as "escalating damage" - e.g. see
> > > Mark Ridley's Mendel's Demon, p. 117.  As Mark notes:
> > > "sex only helps purge mutations if successive mutations do
> > > escalating damage to the body" - p.123.  However there are some
> > > good reasons to think that "escalating damage" really does
> > > happen - at least some of the time.  One such reason has to do with
> > > redundancy and backup systems - where single defects can be recovered
> > > from and only *combinations* of defects are fatal.
> >
> > Yes! I fully agree with the Ridley quote.  What he calls "escalating
> > damage" is a prerequisite to any model attempting to justify sex
> > as an error correction mechanism.  And while there is some empirical
> > evidence in favor of "escalating damage", there is also some evidence
> > against it.
>
> Sometimes it happens, sometimes it doesn't.

OK.  But this conversation began with your claim that sex promotes
mutation repair.  Period.  Now it seems that you are admitting that
sex only promotes mutation repair *under some circumstances*.  I have to
consider that as progress.

[snip]

> > Certainly, in biochemistry, you see mechanisms which only work if
> > every one of several genes is doing its job.  Each such mechanism is
> > a case of "de-escalating damage" and thus an argument in favor of
> > asexual reproduction as an error control mechanism!
>
> I don't /think/ that is correct.  If such cirumstances were universal,
> sex would not be favoured.

McGinn would have a field day with your circular logic here (except that
I'm pretty sure that McGinn is on your side on this issue).  You claim
that sex promotes mutation repair and offer your simulation as evidence.
And then you justify the assumptions of your simulation by saying that
if those assumptions weren't true then sex would not be favored.

> However, asexual reproduction would do no
> better at correcting errors.  Basically both the sexual and asexual
> individuals would fail as soon as they encountered such a mutation.

Not necessarily.  If the mechanism is useful, but not essential - for example,
the ability to digest lactose - then the mutation is not fatal.  My point
is that you don't have escalating damage if a single mutation is all it
takes to destroy the mechanism.

Incidentally, after thinking about it, I think that a better name for this
phenomenon would be "saturating damage", rather than "deescalating damage".

> Asexuality might spread in such a population by avoiding the costs of sex.

Definitely.  And on the other hand, one advantage of sex/recombination is that
it can directly repair some kinds of mutation damage; for example, single
strand breaks and uv dimers.  There is a trade-off between these two issues.

However, notice that both of these issues have a direct effect on individual
fitness.  Our original topic - sex's possible contribution to helping
selection to remove mutations from a population - has nothing to do with
individual fitness.  Your argument is that sex exists because it shuffles
mutated genes into one basket, so that selection can remove them from the
population more efficiently.  This argument inevitably invokes selection
at the population or species level.  I am not completely allergic to such
arguments, especially since sex is, in some sense, a population level trait,
rather than an individual trait.  However, I strongly suspect that direct
individual-selectionist factors such as the cost of sex vs dimer repair
will be much more important that species-selectionist issues such as the
increase in the variance in mutations.

> Since "deescalating damage" is rather irrelevant to the issue - and
> escalating damage favours sex - the issue boils down to how common
> escalating damage is.
>
> > I'm not sure whether the most common situation is escalation or
> > de-escalation.
>
> Don't worry, nor is anyone ;-)

You seem to be saying that escalating damage favors sex, but that saturating
damage does not favor asex.  Do you really believe this?  It would be easy
enough to modify your simulation to find out.  I believe that the dividing
line between escalating and saturating damage is "independent damage"; that
is, each mutated locus creates, say, a 10% chance of dying in a generation.
You could easily modify your simulation to check this borderline case by
computing chance of survival as 0.9^mutation_count.

You could model escalating damage by assigning an 11% chance of death to the
second mutation, 12% to the third, etc.  And you could model saturating
damage by assigning a 9% chance of death to the second mutation, 8% to the
third, etc.  Or choose any other escalation/deescalation "schedule" that
suits you.  I would prefer, though, that you avoid sharp thresholds.

I would be willing to bet the standard sbe stake ($10,000 I believe ;-)
that the simulation would show that saturating damage favors asex, escalating
damage favors sex, and independent damage favors neither.

In any case, I consider the proper use of simulation models to be the
investigation of exactly this kind of question, rather than as a club
to promote acceptance of some particular theory of the origin of sex.
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