  Several groups have recently modeled evolutionary transitions from an
ancestral allele to a beneficial allele separated by one or more intervening
mutants. The beneficial allele can become fixed if a succession of intermediate
mutants are fixed or alternatively if successive mutants arise while the
previous intermediate mutant is still segregating. This latter process has been
termed stochastic tunneling. Previous work has focused on the Moran model of
population genetics. I use elementary methods of analyzing stochastic processes
to derive the probability of tunneling in the limit of large population size
for both Moran and Wright-Fisher populations. I also show how to efficiently
obtain numerical results for finite populations. These results show that the
probability of stochastic tunneling is twice as large under the Wright-Fisher
model as it is under the Moran model.
