[ Follow Ups ]
Posted by Pierre on October 06, 1999 at 18:39:29:
In Reply to: Re: scale of selection posted by Mark Ungrin on October 03, 1999 at 19:17:02:
: can I agree 99.5%?
: thing is - on scales of interest to the individual with cancer, the only
relevant
: selection is occurring at the cellular level. BUT, on the population biology
level,
: if this happens enough, then selection for organisms whose cells don't
transform so easily
: will eventually result in lower frequencies of cancer. ditto for viral
replication - if
: you've got ebola, then you may well be doomed - but if we all get it,
eventually
: selection among us organisms will result in a population whose cells are
capable of
: blocking viral replication.
Back to the time scale problem you raised. As a virologist, I prefer the example of viruses :)
Natural hosts (the reservoir of viruses) do not
block the virus, nor die. Host-virus coadaptation means the virus also is
adapted to its host and maximizes its fitness. Size of viral population and
variability of viruses make it highly unprobable that hosts can evolve faster
than viruses to find an ultimate solution (except for WHO and vaccinia!), or
meet by chance a low fitness/non pathogenic virus. Even in this case we should
observe in natural hosts an epidemics of death due to strongly selected
reversion to the high fitness/virulent phenotype. I do not see in viruses where
to put a stable genetic memory of a low fitness. I studied non-pathogenic
infection of sooty mangabey monkeys with Simian Immunodeficiency Virus. The
non-virulence phenotype is perfectly stable : no reversion. This is a big
surprise given the variability of such viruses. The virus replicates as
efficiently as in diseased humans. I am afraid the explanation you proposed is
not satisfactory. I do not agree this time :)
When an animal is not going to its maximal theoretical ("possible")
fitness it is because ressources are lacking, certainly not because of
selection at the ecosystem level, memorizing the surviving ecosystem including
its animals and plants. Again, I think its is the same for counterselection of
cancer or lethal viruses : a ressource problem at the cell or virus level.
Nothing to do with population biology. Back to the beginning of our discussion
: a selected unit is mainly selected by pressures defined at the same level.
What do you think ?