Group Selection

As we have just seen, were the vertical subsystem to operate on a basis of unfettered individual selection, the entire system will move rapidly to uniformity. Every host individual would possess all the resistance genes, and every parasite individual would possess all the parasitism genes. The moment this happens, the vertical subsystem ceases to function because every allo-infection is a matching infection. Furthermore, in terms of genetic homeostasis, this loss of functionality would be irreversible, because genetic changes could involve only a reduction in the maximum number of vertical genes, and this would not contribute to survival value.

It follows that the vertical subsystem can have evolved only by group selection. And the group must have included the two taxonomically remote species of the host and the parasite. It is impossible to explain this evolution on the basis of natural selection operating on random mutations (see 2.10). That mechanism can allow selection at the level of the individual only. It would invariably lead the vertical subsystem towards homogeneity and non-functionality.

This topic requires a major emphasising of complexity theory, and of complex, adaptive, non-linear systems. Group selection would be impossible if the only mechanism of evolution were natural selection operating on random mutations. Group selection becomes possible when the mechanism of evolution is natural selection operating on the emergents that originate at every systems level. The system of locking that emerges from the vertical subsystem (see 4.15) is possibly the most important emergent to occur in discontinuous (see 4.11) plant pathosystems. This group selection cannot occur at any systems level lower than that of the pathosystem.

It is now postulated that the chief mechanism of evolution is natural selection operating on emergents. Group selection is then possible by natural selection operating on emergents at the higher systems levels.

Individual selection is totally inimical to both the functioning and the evolution of the vertical subsystem. Nevertheless, vertical subsystems have evolved, repeatedly, in many different hostparasite associations. These associations involve species as evolutionarily remote from each other as host plants that are Angiosperms and Gymnosperms, and species of parasite that are as evolutionarily remote from each other as Angiosperms, insects, mites, nematodes, fungi, bacteria, and viruses. Plant pathosystem evidence compels us to conclude that natural selection operating on random mutations is not the sole, or even the main mechanism of evolution.

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