HIV-1 vs HIV-2 evol

[Patrick O'Neil]

On 17 Mar 1995, Biology Department wrote:

> Evolution is NOT a directed process, especially the evolution of 
> viruses.  Viral evolution is dependent random genetic drift.  As Mike 
> Perdue suggests, a virus does not benefit by completely killing its 
> host--unless it can transfer rapidly from host to host.  This transfer is 
> more productive if it is laterally (to a new host) than serially (to the 

Though it may sound like it, what I have been saying concerning flu or 
HIV evolution does not imply directionality per se.  You can, however, 
set up a selection that favors one class of virus from another within the 
same strain. 
 If you have, for instance, a sexually transmitted virus that causes 
relatively mild symptoms within a host population that very rarely 
exchanges partners, then the environmental selection is for slow 
replicators that will also not kill their host, at least too quickly.  
The selection only favors those that replicate slowly, don't spark a 
massive immune response, and allows the host to live so as to exchange 
partners for spreading of progeny virus.  Those that kill the host or 
produce accute viremia automatically are selected against in this case 
because the host either dies before a partner exchange can take place or 
they are too ill for sex.  
  Now, if the situation changes - perhaps population density increases or
the social and economic structure changes so that sex partner exchange
increases, in some cases, to a very high level, then those virus strains
that previously were too virulent to make it now can.  The host will
exchange partners, perhaps several times, before the more virulent strains
can kill or lay up the host, yet they are out reproducing their slower,
benign brethren so the are now selected against.  It doesn't matter that
the host will be taken down faster because partners are also changing
faster now BEFORE he/she falls. There appears to be a directionality, and
to some extent there is in as much as the situation requires that a fast
replicator will win out over a slow one in this case.  Same host, same 
virus strains, different selective pressures.
  If you set up a situation where a plain-jane flu virus can be spread 
very easily from host to host - the sick are not, or cannot be, 
segregated from the healthy (by going home and staying in bed, for 
instance) and the environment is crowded with host, then you select for 
the most virulent variants and so on.  It doesn't matter how sick the 
host gets, he/she isn't going anywhere and they are sneezing and 
coughing, and other hosts are breathing in the virions - they aren't 
going anywhere.  This flu can get as nasty as it wants and STILL be 
successfully transmitted and this is what would happen.  There are swarms 
of strain variants in any host and some replicate faster than others.  IF 
there is no selection disfavoring extremely rapid replication and illness 
progression, then these variants win by default...they produce and spread 
the most progeny over the shortest period and the host is not limiting.  

This last is an extreme example yet it does give a definite, predictable 
direction to virulence evolution based upon selective pressure.  A 
crowded city is far more favorable to a virulent strain of airborne 
pathogen than is a rural community and if you introduce the same starter 
strain to each community, they will ultimately evolve opposite virulence 
levels. 

Patrick