Complex sentence: It is calculated for a single generation and must be calculated from absolute numder fitness is larger than 1, the number of individuals bearing that genotype increases; an absolute fitness smaller than 1 indicates an absolute fall in the number of individuals bearing the genotype.
Split into simpler sentences: It is calculated for a single generation and must be calculated from absolute numbers. When the absolute fitness is larger than 1, the number of individuals bearing that genotype increases; an absolute fitness smaller than 1 indicates an absolute fall in the number of individuals bearing the genotype.