From: robison1@husc10.harvard.edu (Keith Robison)
Newsgroups: sci.bio
Subject: Re: Negative selection (really!)
Date: 17 Dec 92 20:17:16 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.]

>In article <1992Dec16.144720.18569@husc3.harvard.edu> robison1@husc10.harvard.edu (Keith Robison) writes:
>>Isn't just my signature convincing enough?  :-)
>>
>>	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I am referring to may be unpublished (I saw it as a demo when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.

Agreed. If one genotype is experiencing negative selection, another
is being positively selected.  What I should have said is "extreme
selection".


>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.

Of course, many species in this world don't fall neatly into 
"sexual" and "asexual", and perhaps it's in that realm where these
effects might be most relevant.

>>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.

>>Keith Robison
>>robison@biosun.harvard.edu 
>-- 
>--Andy

Again, this phenomenon of selection-driven sympatric speciation would require
the following conditions:

	(1) Multiple possible beneficial adaptations ("dodges")
	(2) The different adaptations are either 
	    a. inherently incompatible
	    b. reduced fitness of hybrids

Imagine the following scenario:  suppose you have an organism (A) which
generally reproduces asexually but occasionally uses sexual reproduction.
Suppose further that another species (B) begins preying on the first,
using the sexual apparatus as it's route of attack.  A situation such
as this exists in E.coli, whose F (sex) pilus is also the receptor
for filamentous phages such as M13.  There might be multiple alterations
of A's sexual apparatus which might reduce predation by B.  However,
these alterations may also hinder the ability of an altered A to mate
with other A's bearing different modifications.  Alternatively, some
B may mutate so that they can prey on particular modifications of A, but
do this by trading off their ability to prey on the wild-type A.
Here is a summary:

			mates with    attacked by
wild-type	A	   A		B
		A1	   A1		B1
		A2	   A2	 	B2


If this were the case, then it would definitely be disadvantageous
to acquire an A1-A2 phenotype, because then both B1 and B2 would be
predators.  

Again, just throwing out thoughts.  Trash away!


Keith Robison
Harvard University
Department of Cellular & Developmental Biology
Department of Genetics / HHMI

robison@biosun.harvard.edu 
Newsgroups: sci.bio