The Resistance Optimum

From the fact that parasitism still occurs, it follows that there must be a resistance optimum in the host, corresponding in general terms to the parasitic optimum in the parasite. In other words, the parasite survives in spite of the resistance in the host. Obviously, the host was never able to accumulate sufficient resistance to kill off the parasite completely. There appear to be three general reasons for this limit to the level of resistance in the host.

First, we must consider a basic requirement of evolution, which is the necessity for all species to reproduce beyond the carrying capacity of their environment. This is because any species that consistently reproduces below the carrying capacity of its environment will become extinct. The carrying capacity of the environment also varies from season to season. The only way a species can guarantee survival is to reproduce with a safety margin that is well in excess of the average carrying capacity of the environment.

This means that there is always a surplus of individuals, and the least fit fail to survive. It is perhaps more accurate to think of evolution being the elimination of the least fit, rather than the survival of the most fit. Consequently, a basic principle of evolution is that every species produces an excess of biomass. Within plant ecosystems and plant pathosystems, the excess biomass of the host may occur as an excess of host individuals, or an excess of host tissue (e.g., grasses that are grazed), beyond the absolute requirements of survival.

Before the evolution of consumer species (i.e., herbivores, carnivores, and parasites), there were only producer species (i.e., green plants, and cyano-bacteria), which converted solar energy into biological energy, and reducer species (i.e., fungi and bacteria), which recycled the nutrients in dead tissues. With the evolution of consumer species, the reducers were partly replaced. That is, the consumers utilised excess individuals and excess tissues before they died of over-crowding or senescence.

Consequently, with parasitism, the loss of excess host tissue, which the host produces anyway, would not affect the survival ability of that host. In other words, a limited parasitism can occur without impairing the survival ability of the host. But both the resistance optimum and the parasitic optimum must obviously ensure that this loss of host tissue does not exceed the unwanted surplus. The loss of tissue must not impair the host's ability to compete and to survive, either micro- or macro-evolutionarily.

The second reason why the host cannot kill off the parasite entirely is that the level of resistance depends on selection pressure from the parasite. As the level of resistance increases, the level of parasitism decreases, and the selection pressure for resistance also decreases. There is little doubt that the selection pressure for resistance stops at a low level of parasitism. Once that level of parasitism is reached, the host cannot accumulate any more resistance. In other words, there is a maximum level of resistance, and this level allows some parasitism to occur. (It is worth noting that this natural maximum is not necessarily the absolute maximum, and that the natural maximum can be increased by artificial selection; that is, horizontal resistance can be domesticated to levels above those of the wild plant pathosystem; see 10.11).

The third reason why the host cannot kill off the parasite entirely is that evolution cannot anticipate. If it could, there would be a considerable survival advantage for the host to maintain enough resistance to prevent parasitism completely, until the parasite became extinct. The level of resistance, and the genetic cost of that resistance, could subsequently be reduced to zero. This would be an important evolutionary gain that would pay off handsomely in the long run. But evolution operates on the basis of current survival, not future survival, and it cannot anticipate such a future survival advantage, however great its potential may be.

There are additional arguments why this particular evolutionary anticipation could not work. First is the question of facultative parasites, which can survive saprophytically for considerable periods without parasitism. Even if the host could exhibit evolutionary anticipation, it would have to eliminate parasitism until the facultative parasite had lost parasitic ability entirely. This would presumably require geological time. Second, many parasites have a host range that embraces more than one host species. Evolutionary anticipation would have to occur simultaneously in all those other host species if the parasite were to become extinct. Third, there is the problem of geographic distribution. The evolutionary anticipation would have to occur simultaneously over the entire geographic range of the parasitism, if the parasite were to be eliminated.

It seems that the evolution of the host is quite unable to prevent a low level of parasitism. This parasitism is a level that does not impair the host's ecological and evolutionary survival ability, just as the grazing of wild grasses by wild herbivores does not normally impair their survival. The host can obviously restrict parasitism when it does threaten its ecological and evolutionary competitive ability. But, so long as the parasite is taking only host tissue that is surplus to these competitive requirements, there will be no selection pressure for resistance.

This limit to resistance in the host defines the resistance optimum. It allows parasitism to occur, and to continue at the level of the parasitic optimum, but no higher.

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