Parallel Evolution

As a general rule, evolution can be expected to find the best solution within the existing possibilities. Some possibilities may not exist. For example, the evolution of a wheel was clearly outside the range of possibilities. But electricity (electric eels), navigation (homing pigeons), sound radar (bats), sonar (fish), schooling (fish, insects, and birds), light production (fireflies), to say nothing of human intelligence, indicate how wide the range of possibilities really is. These phenomena are all emergents (see 1.9), and they become apparent only at their own systems level.

When we consider alternating plant pathosystems, it is obvious that alternation is within the range of evolutionary possibilities. It is also obvious that the survival advantage of alternation must be considerable, because it has evolved so many times, in such a wide range of plant hosts, and in plant parasites as disparate as fungi and insects. Clearly, this alternation in both fungi and insects is analogous, not homologous evolution. That is, it corresponds to wings having evolved analogously in birds, insects, and bats, without common descent, in contrast to the bone structure of a bat wing, which is homologous to the mammalian forelimb, having evolved directly from it.

There seems to be little doubt that the survival advantage of an alternating pathosystem lies in the obligatory allo-infection that occurs twice in each life cycle of the parasite. Two separate gene-for-gene relationships are then possible, and the whole pathosystem is doubly stabilised. There can be little doubt also that the evolution of this stabilisation is the result of group selection, occurring at the systems level of the pathosystem. That is, at the level of the two interacting populations of host and parasite. There is also little doubt that the stabilisation can evolve only by natural selection operating on emergents at this high systems level (see 2.10).

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