virulent strains of the parasite outcompete less virulent strains when host density is above equilibrium. Conservation of virulence over time occurs when a stable equilibrium is maintained. Conserved virulence may be high (Lenski and May 1994), but it reflects stability within a system dictated by a unique set of transmission factors. Many parasites must reach a certain population size within the host to be successfully transmitted, while in certain systems, sacrifice of one host facilitates transmission to the next host (i.e. interspecies transmission). The inclusiveness of the equilibrium model gives it great potential for accurate predictability of a broad range of parasite-host interactions. 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 h...
