I would like first to clear up an apparent misunderstanding. I understand
that, as C. Graham Clark puts it, "natural selection lacks foresight",
and that is not what I meant when I said that evolution towards increased
pathogenicity will eventually use up host resources resulting in
selection for more benign strains. In terms of evolution and natural
selection we are most often left with an end product that has us
guessing about selective forces. Because selection for increased
virulence will rapidly use up the parasites resource, i.e. susceptible
hosts, the conditions favoring this type of selection will be short
lived. In other words, selection for host mortality is not evolutionarily
stable, and the endpoint we are most likely to see is a more benign
relationship. In a recent post C.T. Faulkner makes a few points that I
feel could be misleading. In stating that "virulence is a side effect or
cost of high reproduction rates" the association between reproductive
ratios and virulence is again brought up. As I posted earlier, in studies
that have looked at host mortality and parasite reproductive ratios
simultaneously no correlation other than a negative one has been found
between the two. This means that host pathology is not merely the by
product of a parasite maximizing its fitness, but is the result of some
other selective force(s), such as, perhaps, competition. Also mentioned
by C.T. Faulkner was Ewald's view that parasite induced behavioral
changes equate with pathology, and that in these instances host death by
predation is the equivalent of parasite induced host mortality. This is
not the case. Normally parasite induced host death prevents further
transmission, whereas with behavior modification the death of the host
*ensures* transmission. In the case of behavior modification, it is also
desirable for the host to stay alive until eaten, so parasite induce
death is still not favored. Take for example the classical example of
Dicrocelium dendriticum, where infected ants are induced to take hold of
a blade of grass, increasing their probability of being eaten by sheep.
This behavior only occurs when it is cool enough outside for the ant to
survive. During the heat of the day the ant is "allowed" by the parasite
to let go of the grass, and return to the cool depths of its burrow. The
evolutionary pathway leading to such associations must surely differ from
that which leads to host death due to disease. High "virulence" is only
favored under conditions where transmission rates are high. Mechanisms of
host manipulation would seem to have evolved under conditions where
transmission needed a little help. - Derek
