The Journal of Heredity 1994:85(3):183-190
© 1994 The American Genetic Association 85:183-190
research-article |
Estimating the Extent of Natural Selection in Seedlings From Different Fagus sylvatica(L.) Populations: Application of New Measures
Abteilung für Forstgenetik und Forstpflanzenzüchtung, Universität Göttingen Büsgenweg 2,37077 Göttingen, Germany
Abstract
Measures of the extent of viability selection are developed for the purpose of understanding significant frequency changes observed between the seed and the seedling stages at three isoenzyme gene loci in three populations of beech. These measures explicitly consider dependence of the realized extent of selection on the frequency distribution of genotypes prior to selection, overall reduction in population size, and lower and upper bounds set to selection by this reduction. Using a suitable measure of distance between frequency distributions, it turned out that the overall selective reduction in population size equals the selection load and that this load specifies the maximum feasible amount of selection. Application of the measures to the data provided the means to discuss associated and operative effects of selection in an attempt to explain the frequency changes observed at the isoenzyme loci. The observations suggest associated selection as a major force of frequency dynamics of genetic types. This helps resolve the dilemma resulting from the expectation of excessive selection load for selection at many loci. A likely cause of persistent stochastic associations among loci-the small reproductively effective subpopulations that vary across breeding seasons-are discussed; these are probably typical of long-lived, iteroparous organisms. There is evidence that, across all populations, some loci tend to evolve stochastic independence from the respective operative genetic background more readily than others; this could be explained by only occasional occurrence of operative selection episodes at these loci.