parental clones if immunity is clone-specific, cotransmission may
be advantageous in itself. There is some evidence that coinoculated
clones have a greater success rate in initiating an infection in humans
(by antagonizing the host immune response (Gilbertet al., 1998)) and
that there is greater than expected cotransmission in low transmission-
intensity human malaria foci (Paul et al., 1999b). Plasticity in sex
determination may therefore be additonally maintained by the weak
advantages of cross-fertilization. The uncertain relevance of such ‘down-
stream’ effects on sex allocation and the widely variable nature of the
host–parasite interaction present novel challenges for sex-allocation
theory, with potentially important significance for the fight against the
malaria parasite.
Until recently, research concerning parasites of medical and veteri-
nary importance has been largely the domain of specialists in the field
and there has been an absence of an evolutionary approach to essentially
practical issues. The dawn of Darwinian medicine (Williams and Nesse,
1991), whose premise is the application of adaptationist argumentation
to infectious diseases, has encouraged evolutionary biologists to take
an active role in infectious-disease research. The apparently successful
application of sex-allocation theory to malaria and related parasites (Read
et al., 1992, 1995; Westet al., 2000) provides optimistic grounds for
expanding adaptationist reasoning to more complex and perhaps more
pertinent medical phenotypes, such as virulence (Pickeringet al., 2000).
As well as evolutionary biology making a significant contribution to
infectious-disease research, model systems such as malaria provide an
increasing opportunity for developing our understanding of behavioural
ecology. The medical importance of malaria has resulted in one of
the most well-developed and best-documented systems, which is now
amenable to laboratory manipulation, as well as field experimentation.
The variability in the host environment, whether as a result of host
immunology or of parasite epidemiology, presents a challenging range
of circumstances for parasite evolution. The rapid expansion in our
understanding of the mechanisms involved in parasite–host interactions,
whether host immunological responses to the parasite or the molecular
subtleties of the parasite life cycle, will enable a more detailed investi-
gation of the adaptive nature of observed parasite behaviours. Application
of such adaptationist approaches to life-history traits must, however,
proceed with caution. At present, such methodology assumes equilibrium
states that are not at all certain in host–parasite systems, which can
display quite variable and even chaotic dynamics. In such systems, there
is the danger of stretching adaptive theories too far and generating ‘just-so’
stories. On the other hand, application of adaptationist ideas to more
complex systems does offer the possibility of elaborating evolutionary
theories, as well as making a significant contribution to serious
public-health issues.Parasite Sex Determination 217