  We show that the Eigen model and the asexual Wright-Fisher model can be
obtained as different limit cases of a unique stochastic model. This derivation
makes clear which are the exact differences between these two models.
  The two key concepts introduced with the Eigen model, the error threshold and
the quasispecies, are not affected by these differences, so that they are
naturally present also in population genetics models. According to this fact,
in the last part of the paper, we use the classical diploid mutation-selection
equation and the single peak fitness approximation to obtain the error
threshold for sexual diploids. Finally, we compare the results with the asexual
case.
