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

If a parasite begins to be transmitted only vertically, i.e. from parents to offspring, there will be a gradual tendency towards a decrease in its virulence (Clayton & Tompkins 1994; Bouma & Lenski 1988; Bull, Molineux, & Rice 1991).Microevolution in the direction towards a decrease or an increase in virulence in dependence on the manner of transmission of the infectious stages has been experimentally demonstrated, for example, on the model of the mosquito Aedes aegypti – Edhazardia aedis (Microsporidae) (Agnew & Koella 1997).

If a parasite spreads exclusively through the gametes of its host, it can gradually completely “evolutionarily dissolve” its host (Law & Dieckmann 1998).For example, this fate encountered some parasitic bacteria – the predecessors of modern mitochondria and plastids.A the present time, a number of examples can be found amongst protozoa and especially amongst unicellular algae, where an endosymbiont (originally a parasite) acting as a cellular organelle in the host cell  can even be a eukaryotic organism (McFadden & Gilson 1995;McFadden et al. 1994).The cause of evolutionary dissolution consists in the fact that the genes of the parasite and the host are transferred to the next generation by the same pathway, through the same gametes.Thus, both groups of genes necessarily begin to cooperate in controlling and optimizing the properties of the organism producing the gametes, i.e. the organism of the host.Those genes of the parasite that would damage the host organism in some way are progressively inactivated during evolution and the functions of a great many other genes can gradually be taken over by the genes in the chromosomes of the host.Finally, of the original set of parasite genes, only those that provide some advantage for the host organism remain.

The transfer of extra-nuclear genetic information is not as even as transfer of nuclear genetic information, which occurs through mitosis and meiosis.The relevant information can enter some daughter cells in a great many copies, while it need not enter some others at all.Consequently, from the standpoint of the parasite genes, it is more advantageous to gradually “move” to the host chromosomes.For chloroplasts and mitochondria, a large portion of the genes of the original endosymbionts has already moved to the nuclei.At the present time, it is not possible to determine in how many endosymbionts / parasites this process is complete, i.e. to the phase where all the genes of the endosymbiont have ended up on the host chromosomes.However, a great many biologists assume that the nucleus of a eukaryotic cell is a multiple chimera formed by the combination of genes derived from a series of bacterial and archaebacterial organisms (Golding & Gupta 1995; Gupta & Golding 1996).