e of parasite-independent host mortality and v is the rate of recovery of infected hosts. Parasite populations grow when transmission or host density increase, when host mortality decreases or when hosts recover slowly. Studies have established a positive correlation between transmissibility (B) and host mortality (a) (Ebert 1994, Antia et al. 1993, Lenski and May 1994). Parasite populations which exhibit high transmissibility (i.e. virulence) within a host population are simultaneously lowering host density. When host density is low, parasites which exhibit high virulence may kill their hosts before contact with new hosts occurs. Thus, transmissibility is a spatial factor which describes the likelihood of contact between hosts and, ultimately, between a parasite and its host. Lenski and May (1994) propose an evolutionary sequence in which parasite populations adapt to the changes they cause in host density (Fig. 1). A parasite suprapopulation is likely to include a range of genotypes which are expressed in different potential levels of virulence (Lenski and May 1994). When host density is high, more virulent parasites are successful and host density is reduced. At a lower density of hosts, less virulent strains of the parasite are at a selective advantage as they increase host survival during infection and allow more time for transmission to occur. Also, more virulent strains of the parasite are prone to induce mortality in entire subsets of the host population, driving themselves to extinction along with their hosts. This pattern repeats over time, lowering virulence with each adjustment to declining host population size. Extinction of the host population is avoided when sufficient variation is present in the parasite population (Lenski and May 1994). The evolutionary sequence may be reversed to explain evolution toward higher virulence when parasitic virulence is below the equilibrium level. More...
