ESTIMATING THE EXTENT OF NATURAL-SELECTION IN SEEDLINGS FROM DIFFERENT FAGUS-SYLVATICA (L) POPULATIONS - APPLICATION OF NEW MEASURES

Measures of the extent of viability selection are developed for the pu rpose 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 o f the realized extent of selection on the frequency distribution of ge notypes prior to selection, overall reduction in population size, and lower and upper bounds set to selection by this reduction. Using a sui table measure of distance between frequency distributions, it turned o ut that the overall selective reduction in population size equals the selection load and that this load specifies the maximum feasible amoun t of selection. Application of the measures to the data provided the m eans to discuss associated and operative effects of selection in an at tempt to explain the frequency changes observed at the isoenzyme loci. The observations suggest associated selection as a major force of fre quency dynamics of genetic types. This helps resolve the dilemma resul ting 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 acro ss breeding seasons-are discussed; these are probably typical of long- lived, iteroparous organisms. There is evidence that, across all popul ations, some loci tend to evolve stochastic independence from the resp ective operative genetic background more readily than others; this cou ld be explained by only occasional occurrence of operative selection e pisodes at these loci.