Quasispecies

michael coulthart mcoulthart at cyberus.ca
Thu Oct 30 18:29:50 EST 1997


Dear Allen Rodrigo and other quasispecies discussants,

I was interested to read your comment on chemical and biological species.

Ever since I first heard him explain the concept in a 1980 lecture, I have
suspected that Eigen's invention and use of the term "quasispecies" was a
result of a conflation of chemical with biological species. I don't believe
that the original definition had anything to do with suggesting that
viruses, being largely clonally reproducing, were not as susceptible to
decisive speciation or clear species demarcation as sexually reproducing
organisms are.

To this day, I have been unable to discern, in any publication that uses the
term "quasispecies", why the term "population" could not have been directly
substituted, with a major improvement in clarity. For an interesting recent
commentary on the concept of quasispecies, its operational usage, and the
way in which application of the term has taken on a kind of
pseudo-explanatory life of its own, see D.B. Smith et al. (1997) Virus
'quasispecies': making a mountain out of a molehill? Journal of General
Virology 78:1511-1519.

Cheers,

Mike Coulthart

At 10:33 AM 10/30/97 -0800, you wrote:
>Hi Joe
>
>Joe Felsenstein wrote in message <63acpl$ad5 at net.bio.net>...
>>In article <637ntb$s0t at net.bio.net>, toni  <toniv at iname.com> wrote:
>>>Nobody talks about quasispecies?
>>
>>The flip answer is "no, they don't".  The more interesting answer is
>>"should they?"  It is important to talk about genetic variation in
>>species.  When someone working on humans or Drosophila does this, they
>>say they are talking about genetic variation in natural populations.
>>When a virologist talks about it, they say they are talking about
>>quasispecies, which sounds infinitely more mysterious and novel.
>>
>>So the question is, is anything gained by considering it as a
>>discussion of "quasispecies" rather than "genetic variation"?
>
>
>It turns out that when Eigen first introduced the term "quasispecies" he was
>actually likening species to chemical species.  He has a very precise
>mathematical characterization of the quasispecies as the equilibrium
>frequencies of a pool of closely related (read "similar") variants that
>jointly act as an entity.  These entities differ from host to host, so that
>they behave as chemical species might.  However, since each entity cannot be
>characterized by a certain molecular form but rather the equilibrium
>frequencies of different forms (equivalent, I think, to the largest
>eigenvalue of the transition matrix), he used the term "quasispecies".
>
>I must admit that slip-sliding between chemical and biological species
>concepts is confusing.  When I was first introduced to the term
>"quasispecies" as it is used in HIV, I thought that it was "quasi-" because
>viruses cannot truly be classified as species, but behave genetically as
>though they are (at least in the case of HIV).
>
>As to whether anything is gained by using the term "quasispecies", I guess
>it depends on whether you are doing the kind of modelling that Eigen and his
>co-workers are doing. Even if you are not, it may be suitable to adopt the
>term simply to distinguish the swarm of viral variants that has no Latin
>binomial from other biological entities that do.
>
>-- Allen Rodrigo
>
>
>
>
>
>

============================================================

Michael B. Coulthart, Ph.D.
Laboratory Centre for Disease Control
Room 240, HPB Building #7, PL 0700F
Tunney's Pasture
Ottawa, Ontario
Canada K1A 0L2

Telephone:  (613) 952-7312
Facsimile:  (613) 941-2408
e-mail:     mcoulthart at cyberus.ca
            mike_coulthart at inet.hwc.ca




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