Two kinds of population flexibility

Being genetically diverse, wild plant populations can respond to selection pressures. Such populations are described as being genetically flexible. The mechanism of this flexibility is reproductive fitness. For example, if a population has too little horizontal resistance, there will be selection pressure for resistance. The most resistant individuals will have a reproductive advantage, and the least resistant will have a reproductive disadvantage. Within a few generations, the population as a whole will gain resistance.

Cultivated plant populations, on the other hand, are usually genetically uniform, being grown as pure lines, clones, or hybrid varieties. Consequently, they are genetically inflexible because they cannot respond to selection pressures. This is a considerable agricultural advantage because it means that valuable characters, such as yield and quality of crop product, are not lost during the cultivation process. Furthermore, there is uniformity of crop product, and this too has commercial advantages. The majority of cultivars can respond to selection pressures during the breeding process only. This means that we can increase levels of horizontal resistance only during the breeding process, and, if we are to do this, our breeding methods must be designed accordingly.

Ecologists are well familiar with the concept of genetic flexibility, and the production of different ecotypes, because of different selection pressures in different parts of the wild ecosystem. An identical process produces different wild pathodemes, because of differing levels of epidemiological competence, in various parts of the wild ecosystem. Crop scientists are much less familiar with these concepts because of the genetic inflexibility in the agro-ecosystem. Plant breeders should perhaps begin to think ecologically.

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