From: simonb@extro.ucc.su.OZ.AU (Simon Blomberg)
Subject: Re: Negative selection (really!)
Organization: Sydney University Computing Service, Sydney, NSW, Australia
Date: Sun, 20 Dec 1992 01:26:23 GMT

adpeters@sunflower.bio.indiana.edu (Andy Peters) writes:

>[Disclaimer:  I have missed the beginning of this thread.  If I repeat
>anything below which has been said before, I apologize in advance.]

No disclaimer necessary. You're right on the ball.

>>	I have been asked to explain further my recent comment here
>>that:
>>
>>	Any sort of heavy negative selection (large parasite load) can
>>        cause speciation.  This has been shown in Tom Ray's Tierra stuff.
>>        If some host's dodge one way, and some the other, it can
>>        eventually lead to incompatibility.
>>
>>Tom Ray's work has been published in the Sante Fe Institute's 
>>proceedings for their Artificial Life conferences.  I don't have
>>the paper's handy, so I can't check them right now.  The work
>>I am referring to may be unpublished (I saw it as a demo when
>>I was an undergrad at Delaware)
>>
>>In Ray's system, the organisms are essentially asexual (although 
>>recombination occasionally occurs).  If in his system you start with
>>only a parasite and it's host, new forms of the host quickly arise,
>>(which are immune) to be followed by new forms of the parasite.          
>>(which can parasitize the new form).  Thus what I suggested probably
>>holds true for asexual organisms -- strong negative selection can
>>cause speciation.

>What exactly does "negative" selection mean?  I would argue that
>selection is selection:  some genotypes are more likely to reproduce
>than others.

I would interpret "negative selection", in the light of the rest of
Keith's post, as "disruptive selection". i.e. a bimodal distribution
of a trait is produced by selection against the mean of the original,
unimodal, distribution (see Mayr "Animal species and evolution").

>As for parasites leading to speciation, I suppose it depends on how
>you define "species" in the case of asexual organisms.  The definition
>above seems to be, "any new genotype."  Mind you, I am what W. D.
>Hamilton has referred to as a "host-parasite coevolution freak"
>myself, and I'm glad when anything can be explained in terms of
>parasites :-), but it doesn't seem like any sort of new mechanism has
>been offered here; since "incompatibility" means nothing in an asexual
>population, there's no need for it here.

I agree. Species concepts that are useful for sexual species are generally
difficult to apply to asexual species with any meaning.

>>What about for sexual species?  Well, the same should apply IF
>>the "dodges" used by different variants affect the fitness of
>>hybrids between those variants, such as changes in reproductive
>>structures (macro or micro), alterations in the timing of reproduction,
>>gross genetic rearrangements, etc.  

>Here's where "incompatibility" is more likely to become important.  I
>would argue, however, that any "revolutionary" strategy for avoiding
>parasitism (or any other strong selective pressure) would spread
>rapidly through a sexual population, and not cause speciation unless
>there was significant population subdivision.  If population
>subdivision was being assumed, I apologize for reinventing the wheel.

I agree here, too. Unless enough of the population "dodge" one way, and
enough "dodge" the other _all at once_ in a Goldschmidt "hopeful
monster" sort of way (highly unlikely IMHO), the genes for each strategy
would spread through the population, without causing speciation (unless
there was physical isolation of these subpopulations, as you say).
The most that you would get would be a polymorphism in the population
for the trait that deals with the parasite.

Simon.

-- 
Simon Blomberg                                   simonb@extro.ucc.su.OZ.AU
School of Biol. Sciences, A08
University of Sydney           We demand the right to stop to look even at
NSW   2006  Australia          lizards, who are nobody's property. -JBS Haldane 
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