Generational Turnover and the Measurement of Natural Selection: A Fundamental Correction to Population Genetics

Selection coefficients are the fundamental currency of evolutionary genetics, yet their measurement in organisms with overlapping generations rests on an unexamined assumption: that each demographic generation constitutes a complete replacement of the gene pool, as specified by the Wright-Fisher model. We introduce the Selective Turnover Coefficient (d), which quantifies the fraction of the gene pool actually replaced per generation interval. Empirical estimates reveal that d varies from approximately 1.0 in bacterial serial transfer experiments to 0.47 in laboratory Drosophila melanogaster (under standard overlapping-generation protocols) to 0.015 in modern human populations. This variation has systematic consequences: selection coefficients reported in the literature are measured under different effective turnover regimes and cannot be directly compared across species, across experimental systems, or even across time periods within species, without d correction. We reanalyze five landmark studies in human and Drosophila population genetics and demonstrate that published selection coefficients require reinterpretation, with implications ranging from the tempo of human adaptation to the absence of hard selective sweeps in long-term Drosophila experiments.