Two Kinds of Selection

Darwin recognised the important differences between natural and artificial selection. We can now recognise that the complexity and adaptability of the wild plant pathosystem is the result of self-organisation, and of natural selection operating on emergents.

Many of these emergents appear, and can be discerned and selected, only at the higher systems levels.

Artificial selection of plants began with the first farmers, some nine thousand years ago. Although undoubtedly intelligent, these farmers were scientifically ignorant. They probably exerted selection pressures unconsciously, simply by keeping the best-looking plants for propagation. Some of their selections involved qualitative changes. One of the more important of these was the loss of the seed-shedding character. All crops, in which the harvestable product is the seed, retain their seed in the ear, pod, or fruit. This is also true of most crops that are merely propagated by seed, such as tobacco. Other qualitative changes included the free-threshing wheats and barleys, the adoption of entirely new allopolyploids, such as arabica coffee (Coffea arabica), and the loss of undesirable physiologic sinks, such as flower formation or seed production, as with garlic (Allium sativum) and yam (Dioscorea rotundata).

However, most of the changes in plant domestication were quantitative, including such variables as yield potential, quality of crop product, and day-length sensitivity. For example, the development of temperate cultivars of rice permitted the civilisations of both China and Japan, and changes in the day-

length characteristics of soya produced an entirely new agricultural industry in the United States. And all potatoes in industrial countries are day-neutral.

Early agriculture was close to a wild ecosystem in the sense that the human controls were minimal, and the self-organisation was maximal. It was only during the twentieth century that the human controls became excessive, and inappropriate. However, it must be clearly understood that this happened under the enormous pressure of a burgeoning human population. Food production has increased correspondingly during the same period, and this is no mean achievement. But it was made at considerable cost, including that of a lost pathosystem balance. This loss of balance must now be corrected.

Chapter Ten

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