  The Wright-Fisher model and the Moran model are both widely used in
population genetics. They describe the time evolution of the frequency of an
allele in a well-mixed population with fixed size. We propose a simple and
tractable model which bridges the Wright-Fisher and the Moran descriptions. We
assume that a fixed fraction of the population is updated at each discrete time
step. In this model, we determine the fixation probability of a mutant in the
diffusion approximation, as well as the effective population size. We
generalize our model, first by taking into account fluctuating updated
fractions or individual lifetimes, and then by incorporating selection on the
lifetime as well as on the reproductive fitness.
