Читать книгу Parasitology - Alan Gunn - Страница 31

The scenario described above naturally begs the question of, if this is true, why does life still exist today. This is because, on this basis, parasites and other pathogens should have killed everything off many millions of years ago. The answer is that the scenario is too simplistic and all host: parasite/pathogen relationships involve a complex array of competing factors. Consequently, the evolutionary endpoint of any relationship is case‐dependent. Sometimes the parasite becomes more virulent, and sometimes its virulence attenuates to an intermediate level, but one cannot assume that the natural endpoint is a mutually beneficial form of mutualism. Indeed, the relationship between a parasite and its host is often likened to a ‘co‐evolutionary arms race’ in which the parasites attempt to acquire more resources from the host to produce their offspring whilst the host evolves mechanisms for reducing its losses and eliminating the parasite. This has given rise to the ecological theory known as the Red Queen’s Race. The name derives the Red Queen in Lewis Carroll’s Alice Through the Looking Glass who says, “Now, here, you see, it takes all the running you can do, to keep in the same place” (Ladle 1992). One should also bear in mind that a parasite and its host are not co‐evolving in isolation. Hosts usually harbour various parasites and other pathogens, and these may influence its response to an infection. Similarly, the parasite may be competing with other infectious agents for the host’s resources. For example, experiments using bacteria infected with phage viruses suggest that the presence of numerous pathogens can speed up host evolution (Betts et al. 2018).

Parasites and other pathogens are generally smaller than their hosts are and reproduce faster. Consequently, one might expect them to win any arms race because, potentially, they could evolve adaptations to overcome their host’s defences faster than the host could generate new ones. However, hosts that are comparatively long‐lived usually have sophisticated immune systems that identify and kill or neutralize new parasite variants. The host is therefore not a constant environment for the parasite. Parasite virulence is also affected by the mode of transmission. Horizontally transmitted parasites, especially those with a wide host range, can ‘afford’ to be highly virulent because there are lots of potential hosts and if one or more of them dies it has no direct consequences. However, when the parasite is vertically transmitted (e.g., via the eggs of its host or across the placenta) there is a direct link between the effect of the parasite on its host and its own reproductive success. For example, a virulent parasite’s genes will not be transmitted; if the parasite is so pathogenic, it kills the host before it can reproduce. Similarly, if it kills the host’s eggs while they are in utero or reduces the number of host eggs that are produced or survive to become adults and reproduce themselves, then the parasite is compromising its own reproduction. It is therefore to be expected that, as a rule (there will always be exceptions), vertically transmitted parasites should be less pathogenic than those that are transmitted horizontally. There is some support for this hypothesis. For example, two ectoparasites of swifts – a louse and fly – that are vertically transmitted have no effect on nestling growth or fledgling success even when the numbers of these parasites are artificially increased or the birds are stressed (Tompkins et al. 1996). Similarly, in feral pigeons, a vertically transmitted louse has little impact on the birds’ health but horizontally transmitted ectoparasitic mites cause so much distress that the birds’ reproductive success drops to zero (Clayton and Tompkins 1995).