Natural populations and the breakdown of classical genetics

Abstract

Those who first applied genetics to the study of natural populations—and it was, we must remember, fifty years ago—applied it from what we may now call the classical point of view. This is the point of view which assumes that the properties of heredity and also of variation can be deduced from breeding experiments using the methods of Mendel, Bateson and Morgan. It is the point of view expressed by Morgan in 1926 under the title of the Theory of the Gene. The fact that Morgan believed in the chromosomes while Bateson did not, failed to produce the cleavage in this classical view that might have been expected. It failed to do so because, for Morgan and also for those who followed him, his theory did not raise questions: it answered them. The chromosomes did not make the law: they obeyed it. It is thus not the chromosome theory but the Mendelian situation which is crucial for classical genetics. The inbred lines of close relationship, the regulated succession of selfing or sibbing and crossing, the chosen and standard environment, the individual as the unit of observation and selection: these were necessary ingredients and premises for the first phase of getting to know heredity. Generalizations were reached in this way which proved to be valid. They did so because they rested on the properties of cell structures, nuclei and chromosomes, at mitosis and meiosis, which are found to be universal.