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Evolution Rate (Was Re: Creationist Garbage)

RYBICKI, ED ED at micro.uct.ac.za
Mon Oct 18 03:13:30 EST 1993

> In article <MAILQUEUE-101.931015075712.713 at micro.uct.ac.za> 
ed at micro.uct.ac.za writes:

> >
> >And RNA viruses ARE more evolved than anything else - they CAN 
> >evolve a HELL of a lot faster than anything else (10exp5 or so), 
> >thanks to more inaccurate RNA-dep RNA pol.  Of course, that 
> >mean they DO, all the time; just that they CAN, when selected.
> >  

> I'd like to propose that we be a little more careful when we use 
the term
> "selected" in the context of the rate of evolution of 
> genetic elements (if you will forgive that term :-)
> I presume that you mean that if they replicate that they have 
enhanced fitness
> since they differentially increase their representation in genomes 
> in populations. The problem is that you'd expect selection to 
> act against many if not most of those variants created by 
> replication. 
>  I'm basing most of my reasoning on retrotransposons but I think 
that one
> can easily make similar arguments concerning real retroviruses. 
> things are made even more complicated by having to consider 
evolution during

> passage from cell to cell as well as passage from host to host, 
> So my point is that there are several levels of natural selection 
> in the evolution of reverse-transcriptase dependent replicating 
> so it's not particularly informative to issue statements that 
imply that 
> "retro-replicators" evolve faster when selected.
Now what I was talking about are RNA viruses, which replicate as RNA 
with NO DNA step in the life cycle - so mutation while in the host 
DNA does not apply.  There has been a lot written about molecular 
evolution of such viruses, including a monumental upcoming piece by 
Eugene Koonin to appear in something by CRC Press soon (got sight of 
preprint at World Virology Conf in Glasgow) - so that the concept of 
Muller's Ratchet, and of quasispecies, is not foreign to me.  I 
meant selection in the simple sense: that is, of some property in 
one of the many mutant offspring of the "master copy" giving it a 
survival advantage in a given situation, leading to its survival, 
replication, and establishment of a new quasispecies with a new 
"master sequence".  The selection could be the one most people seem 
comfortable with  - that is, non-reactivity with a neutralsing 
antibody in infections of animals - or,alternatively, better (=more 
efficient) carriage by aphids, or replication to higher 
concentration due to change(s) in sequences responsible for 
interaction with host components of the replicase....and so on.  The 
point is that RNA viruses CAN vary faster than DNA organisms, given 
some kind of selection, given that they are a quasispecies to begin 
with - but do not NECESSARILY, in the absence of obvious selection, 
as shown by a number of recent papers on variation in natural 
populations of tobamoviruses, for example.

 | Ed Rybicki, PhD             |                                      |
 | (ed at micro.uct.ac.za)        |      "Lord, won't you buy me         |
 | Dept Microbiology           |          A Mer-ce-des Benz           |
 | University of Cape Town     |     My friends all have Porsches     |
 |                             |          I must make amends..."      |                      |
 | Private Bag, Rondebosch     |                                      |
 | 7700, South Africa          |           - Janis Joplin             |
 | fax: 27-21-650 4023         |            (Pearl, 1971)             |

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