  Biological populations are subject to two types of noise: demographic
stochasticity due to fluctuations in the reproductive success of individuals,
and environmental variations that affect coherently the relative fitness of
entire populations. The rate in which the average fitness of a community
increases has been considered so far using models with pure demographic
stochasticity; here we present some theoretical considerations and numerical
results for the general case where environmental variations are taken into
account. When the competition is pairwise, fitness fluctuations are shown to
reduce the speed of evolution, while under global competition the speed
increases due to environmental stochasticity.
