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virulence and selection

Thu Dec 29 15:17:49 EST 1994

zelmeda4 at wfu.edu ("derek a. zelmer") writes:

-If virulence increases to the point of high host mortality, the
-host resource will be depleted, and selection will be again for
-less virulent forms.

My understanding of natural selection is that it lacks foresight.
Thus, if producing a virulent infection increases an organism's
genetic representation in the next generation, virulence will be
selected for whether or not this ultimately leads to host number
depletion. Likewise, virulence *can* only be selected for when
it has a positive effect on representation. As Derek indicates,
this will become a balance. Remember K and r selection?

Virulence is not always linked to reproductive output however,
in fact it may well be negatively correlated in some organisms.
In Entamoeba histolytica, virulent infections are ones
in which tissue invasion takes place. Amebae that invade
tissue do not encyst and thus do not contribute to further
infections.  In causing dysentery they also flush out many
amebae before they are able to encyst and they cannot mature into
infectious forms. Presumably virulence is being selected against in
this organism, unless virulent organisms have other characteristics
that increase their representation in the population.

Whatever increases survival/representation will be selected for.
When virulence increases survival/representation, it will be
selected for. If this is what Ewald is saying then I believe he
is right, but why is this a big deal? Am I missing something?

C. Graham Clark, Ph.D.
Laboratory of Parasitic Diseases,
National Institutes of Health,
Bethesda, MD 20892

Ph.: 301-496-4740
FAX: 301-402-4941
e-mail: cge at cu.nih.gov

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