The Parasitism Arms Race

Parasitism is often regarded as an 'arms race', and as a competitive situation in which mutual antagonism between the host and the parasite leads to an unending struggle for supremacy. This argument postulates that the parasite constantly increases its parasitic ability, while the host constantly increases its resistance, each in competition with the other. This competition is often referred to as an 'arms race', with a clear derivation from the nuclear arms race during the cold war between the U.S.A. and the Soviet Union. However, there are cogent arguments that refute this ridiculous idea.

The case for an arms race postulates that the host species would evolve a gene for resistance, and the parasite species would then evolve a gene that overcame that resistance, and that this process was repeated ad infinitum, during periods of geological time. While the term 'arms race' was obviously borrowed from the competing industrial-military complexes of the two super-powers, the concept itself had its roots in both the single-gene vertical resistances of modern plant breeding, and the mammalian system of antigens and antibodies.

The first refuting argument concerns the sheer weight of genetic code that would accumulate in the course of extended periods of evolution. There is no evidence of such an accumulation. It must be remembered also that an arms race does not allow the host to discard a single gene for resistance, at any time, without a loss of fitness. And a corresponding observation applies to the parasite.

The second argument concerns the evolutionary instability of such an arms race. If we assume that the host has no resistance at all, following the appearance of a new gene in the parasite, we must also assume a very real danger of extinction of the host before it is able to evolve a new gene for resistance. Equally, if that new host gene confers a complete resistance, there is then a very real danger of extinction of the parasite before it too is able to evolve a new gene for parasitism. Such an evolutionary yo-yo would be an unstable system and precarious to the point of self-destruction.

Alternatively, it can be argued that stability could be achieved if the resistance and parasitism were quantitative, by virtue of being controlled by polygenes, but the arms race itself would then disappear. Suppose that the resistance (or parasitism) was controlled by one hundred genes of equal effect. The evolution of a new gene would alter that resistance (or parasitism) by only one percent. The effect of each new gene that evolved would be progressively less, until any suggestion of an arms race would disappear entirely. Such an absence of an arms race apparently exists with every known example of a polygenically inherited, horizontal subsystem.

The third argument stems from the remarkable stability, and economy, of the vertical subsystem, when it operates as a system of locking in accordance with the n/2 model (see 4.15). This system is the very antithesis of an arms race. A similar case can be made for the Person model (see 4.18), which could be mistaken for an arms race. In fact, this model provides remarkable stability without any suggestion of a true arms race.

Finally, the classic Lotka-Volterra cycle describes fluctuations in the relative population sizes of predator and prey, and it is tempting to assume that a similar fluctuation occurs in the Darwinian evolution of parasitism in plants. However, this fluctuation is concerned with population numbers, in which each prey individual is liable to be killed, and each predator individual is liable to die from starvation. The fluctuation is not concerned with parasitism and resistance, in which neither the host nor the parasite threatens the survival of the other. Furthermore, the Lotka-Volterra cycle is an ecological fluctuation, not an evolutionary fluctuation. It is a micro-evolutionary rather than a macro-evolutionary (see 10.5) fluctuation. . The primary function of the system of locking, the emergent of the vertical subsystem, appears to be a stabilisation of the pathosystem and, as such, it provides the converse of Lotka-Volterra fluctuations.

It is probably safe to conclude that parasitism is not competition between the host and the parasite. However, it is clearly not co-operation either. It should perhaps be regarded as controlled exploitation. The limitation and control of this exploitation is the autonomous control of a wild pathosystem, and this control is an emergent from self-organisation. The control leads to stability. It is biological 'order'. Pathosystems are self-organising, complex, adaptive systems. Their evolution has involved natural selection operating on emergents. This natural selection must have occurred at all systems levels, but most particularly at the higher levels of the pathosystem and ecosystem.

We must view the evolution of parasitism in terms of the pathosystem itself, rather than in terms of the separate evolution of either the host or the parasite, as independent species. Within an ecosystem, each pathosystem is competing with other pathosystems, both ecologically and evolutionarily. This competition depends on an effective pathosystem balance. If the pathosystem balance is inadequate, the pathosystem survival is impaired.

On this basis alone, it follows that each pathosystem must have both a parasitic optimum, and a resistance optimum, each with negative feedback mechanisms to ensure homeostasis of these optima. Any failure of homeostasis will lead to instability, and a reduction in ecological and evolutionary competitive ability.

It follows also that pathosystem evolution involves group selection, operating at the systems level of the pathosystem. That is, the group selection operates on the interaction between a population of the plant host and a population of the taxonomically distant parasite. The selection cannot in any sense be regarded as competition between host and parasite. Still less can it be regarded as an arms race. It is a beautiful example of biological self-organisation, and biological 'order'.

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