Читать книгу Principles of Plant Genetics and Breeding - George Acquaah - Страница 121

Natural habitats are generally not homogeneous but consist of a number of “ecological niches” that are distinguishable in time (seasonal or long‐term cycles), space (microniches), or function. These diverse ecological conditions favor diverse phenotypic optima in form and function. Disruptive selection is a mode of selection in which extreme variants have higher adaptive value than those around the average mean value. Hence, it promotes diversity (polymorphism). The question then is how the different optima relate (dependent or independent) for maintenance and functioning. Also, at what rate does gene exchange occur between the differentially selected genotypes? These two factors (functional relationship and rate of gene exchange) determine the effect of genetic structure of a population. In humans, for example, a polymorphism that occurs is sex (female and male). The two sexes are 100% interdependent in reproduction (gene exchange is 100%). In plants, self‐incompatibility is an example of such genetically controlled polymorphism. The rarer the self‐incompatibility allele at a locus, the higher the chance of compatible mating (and vice versa). Such frequency‐dependent selection is capable of building up a large number of self‐incompatibility alleles in a population. As previously indicated, several hundreds of alleles have been found in some species.