CONCLUSION
Traditional assumptions about the factors determining parasitic
strategy have been largely apocryphal, ignoring contradictory evidence
(Esch and Fernandez 1993). Equilibrium models synthesize the temporal
(i.e. evolutionary) factors and spatial (i.e. transmission) factors
characteristic of parasite-host systems. Time is required to modulate
virulence, while spatial factors such as host density and transmission
strategy determine the direction of the modulation.
The development of an inclusive, accurate model has significance
beyond theoretical biology, given the threat to human populations
posed by pathogens such as HIV (Gibbons 1994). Mass extinctions such
as the Cretaceous event may have resulted from parasite-host
interaction (Bakker 1986), and sexual reproduction (i.e. recombination
of genes during meiosis) may have evolved to increase resistance to
parasites (Holmes 1993). Parasitism constitutes an immense, if not
universal, influence on the evolution of life, with far-reaching
paleological and phylogenetic implications. A model which synthesizes
the key factors determining parasitic virulence and can predict the
entire range of evolutionary outcomes is crucial to our understanding
of the history and future of species interaction.