Читать книгу Parasitology - Alan Gunn - Страница 30

Evolution can be defined as a change in gene frequency between generations, but for this to occur three criteria need to be met. First, there must be genetic variation within the population. If the population is genetically homogeneous, then variation can only occur sporadically through random mutation. The second criterion is that the variation must be heritable: if the variation cannot be passed on to offspring, then it will be lost regardless of the benefits it imparts. The third and final criterion is that the variation must influence the probability of leaving reproductively viable offspring. If the variation is beneficial, then the organism possessing it will leave more offspring; however, unless these are reproductively viable, the variation would be quickly lost from the gene pool. Parasites live in close association with their hosts and the two organisms will co‐evolve. The nature of the host: parasite relationship may therefore change with time. For example, provided the three criteria are met, the host will evolve resistance/susceptibility factors depending upon the pressure exerted by the parasite. Although ever greater resistance to infection may appear to be ‘ideal’, this is unlikely to arise if the energetic cost impacts on the ability to leave viable offspring. At the same time, the parasite will evolve virulence/avirulence factors that promote its own survival.

It is often stated that long‐standing parasite: host relationships are less pathogenic than those that have established more recently. This is based on the reasoning that if the parasite kills its host, then it will effectively ‘commit suicide’ because it will have destroyed its food supply. Consequently, over time, it is to be expected that the parasite will become less harmful to its host – that is, it becomes less virulent. However, this assumption is questionable because a pathogen’s virulence often reflects its reproductive success. For example, let us consider two hypothetical strains, A and B, of the same nematode species that lives in the gut of sheep. Strain A is highly virulent and causes the death of the sheep whilst strain B is relatively benign and seldom causes any mortality. At first glance, one might expect that strain B would leave more offspring because its host lives for longer. However, if virulence was linked to the nematode’s reproductive output and the eggs were released at a time when they were likely to infect new hosts, then strain A would bequeath more of its genes to subsequent generations. Consequently, the proportion of strain A in the nematode population would increase with time and there would be constant selection for increasing virulence. The sheep and the parasites may eventually be driven to extinction by these changes, but individual animals (and humans) are almost always driven by their own immediate self‐interest rather than hypothetical future prospects.