Genetic Flexibility

Genetic flexibility means that a population can respond to selection pressures, and this response depends on diversity within the population. Suppose, for example, that a population is susceptible to a particular parasite. There will be variation within that population, and the most susceptible individuals will be parasitised most. They will then reproduce least, while the least susceptible individuals will reproduce most. With each successive generation, there will be a gain in resistance in the population as a whole. This gain results from a higher proportion of resistant individuals in the population, and a higher level of resistance in those resistant individuals, because of transgressive segregation. With increasing resistance, there will be decreasing selection pressure for resistance, until a balance is reached. This balance is normally maintained as a dynamic equilibrium. (Transgressive segregation means that the progeny have a higher level of a quantitative variable than either parent).

Wild plant populations are genetically mixed and genetically flexible, and they can respond to selection pressures. Most crops on subsistence farms are also genetically mixed and flexible. They can respond to selection pressures during cultivation. This was dramatically demonstrated by the maizes of tropical Africa, following the arrival of Pucciniapolysora (see 7.2). But genetically uniform crops cannot respond to selection pressures during cultivation, because no individual has a reproductive advantage. These crops can respond to selection pressures only during the breeding process, and only if the correct breeding method is employed. That is, they will respond only if the breeding technique ensures that the correct selection pressures are applied. Unfortunately, for the whole of the twentieth century, the wrong selection pressures have usually been applied, and we have been gradually losing horizontal resistance to crop parasites for all of this time (see 6.6).

Flexibility is also necessary at a higher systems level. The modern tendency is to extend genetic uniformity to an entire region, and this too can lead to a dangerous instability. The failure of a single resistance that is employed over a very wide area can obviously lead to major losses. In complete contrast, a perfect example of diversity at this systems level is seen with the self-organising system of crop improvement that would be achieved by a multiplicity of plant breeding clubs (see 11.9).

0 0

Post a comment