Negative heritability of fitness model of advantage of sexuality
The negative heritability of fitness model is basically a sort of application of the Red Queen evolutionary principle (van Valen 1973; Bell 1982). This principle, named after the Red Queen’s Race in Lewis Carroll’s “Through the Looking Glass”, states that, in some situations, it is necessary to run as fast as you can to stay in the same place. In order to move forward, it is not enough to just run, but it is necessary to run faster than the others. The hypothesis takes into consideration the fact that, in an environment in which biotic factors, especially parasites and predators, are responsible for most of the selection pressure, it is frequently advantageous to differ from one’s parents and from most of the other individuals of the same species. Experimentally, it has been repeatedly confirmed that genetically more diverse sexually reproducing organism are much more resistant to parasites than their asexually reproducing competitors (Fig. XIII.10 and Fig. XIII.11).
Extremely high pressure of this kind is exerted by parasitic organisms, bacteria, viruses and eukaryotic parasites (Hamilton, Axelrod, & Tanese 1990). It has been documented that a population of host organisms can be decimated by its parasite and only a few resistant individuals can survive the epidemic. In the following generation, they lead to the establishment of a new population of individuals that are resistant to the original strain of the parasite but, because of their uniformity (they come from only a few ancestors) can easily become the victims of another epidemic wave of a mutated parasite. Simultaneously, compared to their hosts, parasites are exposed to much stronger selection pressure on a change in their properties, for example, a change in the antigen properties of the proteins that are the target of the immune response of the host. In addition, they almost always have a shorter generation time than the host, so that their microevolution usually proceeds faster than evolution of the host. Thus, the parasite constantly maintains an advantage over its host in the co-evolutionary battle. This is manifested, for example, in that a host can be most readily infected by parasites derived from the same location (Fig. XIII.12). The only effective counter-strategy of the host consists in the production of diverse progeny, as this is the only way to ensure that at least some individuals survive the succession of waves of the epidemic. Simultaneously, resistance to epidemics exhibits marked negative heritability. Parasites in the new wave of the epidemic are generally better adapted to the most common variant of the host, i.e. the one that was most resistant in the last epidemic (Fig. XIII.13).