  We propose a new definition for the error threshold of a population evolving
through mutation and selection. We compute the correction term due to the
finiteness of the population by estimating the lifetime of master sequences.
Our technique consists in bounding from above and below the number of master
sequences in the Moran model, by birth and death chains. The expectation of
this lifetime is then computed with the help of explicit formulas which are in
turn expanded with Laplace method. The first term after $\ln(\sigma)/\ell$ is
computed, it scales as $1/\sqrt(\ell m)$, where $\ell$ is the genome size and
$m$ is the number of individuals in the population.
