Frozen Evolution. Or, that’s not the way it is, Mr. Darwin. A Farewell to Selfish Gene.

The probability of superinfection and thus the optimal rate of multiplication of the infrapopulation and optimum parasite virulence do not depend only on the size of the parasite population at the given site, but also on the biodemographic parameters (life history) of the host population. If the host population is short-lived (for example small rodents) or if, for example, the host population at the given time exhibits a higher death rate for reasons not associated with the actual parasitosis, it is advantageous for the parasite to multiply more rapidly in the hosts and thus to damage them more than if the hosts were long-lived (Ebert & Herre 1996; Restif, Hochberg, & Koella 2002). This again contributes to an increase in the virulence of all parasites at times of famine, epidemics or military battles.

Similarly, greater mobility of individuals in the host population leads to an increase in parasite virulence. If the members of the given population exhibit low mobility and motility, infection is transferred especially between close neighbors. From the viewpoint of the rate of spreading, this is advantageous for those parasites that do not damage their hosts much (Haraguchi & Sasaki 2000). In contrast, if individuals of the host population move over large distances in their area of occurrence, low virulence does not constitute any advantage for the parasites.

More virulent parasite populations survive in more numerous host populations compared to small populations that are present, for example, at the edges of the areas of occurrence of a particular species,. Similarly, more virulent strains of parasites are usually at an advantage in growing host populations; while parasites with lower virulence are at an advantage in populations with constant or even decreasing size. This is a result of the fact that, in a growing population, there is a high probability that the infectious stage of the parasite will encounter an uninfected and unimmunized host, while this probability is lower in a stagnant or decreasing population and thus it is preferable for the parasites in a shrinking population to harm their hosts less (Knolle 1989; Combes 2001; Ebert 2000). In tropical areas, mosquitoes, acting as vectors for malaria (i.e. hosts whose role in the life cycle of the parasite is to provide mobility for immobile parasites), are active throughout the year and thus infection is transmitted throughout the year. This is perhaps why Plasmodium species causing malaria (Plasmodium falciparum) are more virulent in these areas than in subtropical or temperate regions. In areas where the frequency of transmission of a disease is decreased in some seasons of the year, for example because of the absence of insect vectors, species that are capable of forming latent stages in the host or species that have a long incubation time (Plasmodium ovale, P. vivax) are at an advantage. A similar phenomenon can be observed for Leishmania (Combes 2001).