Immunity hypothesis
The nature of a host’s antiparasite tactics and the ability of the parasite
to respond should influence virulence. As noted above, behavioural
defences that reduce the opportunities for transmission should select for
decreased virulence. However, a potent immune attack once transmission
is successful could produce an opposite selective force (van Baalen, 1998;
Gandon and Michalakis, 2000). An effective immune response would
limit the duration of the infection, and selection would favour more rapid
replication of the parasite (lowDfavours highPt, just as presented for the
host-demography hypothesis) (Anitaet al., 1994).
Host-specificity hypothesis
Parasites that specialize on a single (or very few) host species should
evolve highly specific ways of dealing with the host immune response
and are thus more likely to exploit their hosts efficiently and produce
severe pathology (Ewald, 1983). Frank and Jeffrey (2001) turn this view on
its head. They propose that newly established parasite–host associations
(the equivalent of a parasite with a very large range in host species) will be
among the most virulent because of the lack of an efficient, specialized
host defence.
Transmission-mode hypothesis
Parasites typically move from host to host in the environment (infectious
or horizontal transmission). Some parasites, though, are transmitted via
the host’s offspring (congenital or vertical transmission). In such cases,
the fitness of the host translates to fitness of the parasite, and the fitness
‘desiderata lists’ (Dawkins, 1990) of both species in the association
coincide. Thus, vertically transmitted parasites should be less virulent
than those using horizontal transmission (Messengeret al., 1999). If the
death of the host is required for transmission (one host must eat the other,
for example), the desiderata lists diverge completely, and the parasite may
actually manipulate its host to increase its chance of being killed (Poulin,
1998).
Small worlds – diminishing-returns hypothesis
Parasites with low dispersal could have a small world of potential hosts
(Herre, 1993; Lipsitchet al., 1995; Boots and Sasaki, 1999). Over time,
there would be diminishing returns on rapid transmission, because the
opportunity for new hosts would decline as a parasite genotype fully
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