L. S. Penrose

Chromosomes and Natural Selection

Gene mutations can be lethal in their effects, they can be mildly disruptive and diminish individual fitness, they can be neutral or even slightly favourable. The same is true for chromosomal aberrations. Consequently, natural selection in populations can be set up by alterations in chromosomal structure just as it is by gene changes which affect the hereditary material of a species. There are, however, important differences in the manner by which selective effects are produced following the various kinds of spontaneous change. The present paper deals primarily with chromosomal changes but there are two ways in which selection may act on them. First, there is the immediate effect of an aberration upon the fitness of an individual who carries it and which leads to persistence or extinction of that particular aberration. Secondly, there is the less direct effect of an aberration upon the frequency of any gene which may be responsible for inducing it. Study of the immediate effects is comparable to the analysis of the behaviour of mutant genes in populations by classical methods (Fisher 1930, Wright 1931, Haldane 1932). Study of the second, less direct, process is comparable to the analysis of the behaviour of mutagenic genes (mutators).