Genetic variability within the infrapopulation of parasites and virulence
As was mentioned above, it is frequently advantageous for the subpopulation of parasites to reduce its growth rate or to terminate its growth after attaining a certain value. From the standpoint of the individual parasite, it is, however, more advantageous if it multiplies more rapidly or if it continues to reproduce for a longer time than the other members of the infrapopulation, as it thus increases the probability that its progeny will colonize the new host (Bonhoeffer & Nowak 1994). Thus, individual natural selection acts in the direction increasing the growth rate and it is well known that the effectiveness of individual selection is generally greater than the effectiveness of group selection.
However, the functioning of individual selection within an infrapopulation requires the existence of genetic variability within this infrapopulation. Genetic variability can be formed directly within the infrapopulation in two ways, either through mutations or through genetic recombination accompanying sexual reproduction. It is known, for example, that RNA-viruses, with their lower accuracy of replication and thus greater frequency of mutations, have greater virulence and damage their hosts more than viruses whose genome is formed of DNA (Ewald 1997). Viruses must reproduce as fast as possible in an organism in order to produce as many infectious particles before a mutant appears in the infrapopulation that would reproduce so quickly that it would “unscrupulously” exterminate the host. It is also known that bacteria that occur in humans as innocuous commensals, contain approx. 3% mutators, i.e. bacterial clones that most frequently exhibit an order of magnitude more mutations because of a defect in some component of a DNA reparation system. It is indicative that pathogenic bacteria contain more (2 – 20%) of these clones (Pennisi 2000b).
Amongst sexually reproducing organisms, genetic variability can occur in the population even faster than through mutations by recombination processes occurring during sexual reproduction. The danger of the formation of this generic variability is reduced in parasitic organisms in that the parasite infrapopulation generally reproduces asexually rather than sexually.