tions which exhibit high transmissibility (i.e. virulence) within a hostpopulation are simultaneously lowering host density. When host density islow, parasites which exhibit high virulence may kill their hosts before contactwith new hosts occurs. Thus, transmissibility is a spatial factor whichdescribes the likelihood of contact between hosts and, ultimately, between aparasite and its host. Lenski and May (1994) propose an evolutionarysequence in which parasite populations adapt to the changes they cause inhost density (Fig. 1). A parasite suprapopulation is likely to include a range ofgenotypes which are expressed in different potential levels of virulence(Lenski and May 1994). When host density is high, more virulent parasitesare successful and host density is reduced. At a lower density of hosts, lessvirulent strains of the parasite are at a selective advantage as they increasehost survival during infection and allow more time for transmission to occur.Also, more virulent strains of the parasite are prone to induce mortality inentire subsets of the host population, driving themselves to extinction alongwith their hosts. This pattern repeats over time, lowering virulence with eachadjustment to declining host population size. Extinction of the host populationis avoided when sufficient variation is present in the parasite population(Lenski and May 1994). The evolutionary sequence may be reversed toexplain evolution toward higher virulence when parasitic virulence is belowthe equilibrium level. More 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 besuccessfully transmitted, while in c...
