  Temporal environmental variations are ubiquitous in nature, yet most of the
theoretical works in population genetics and evolution assume fixed
environment. Here we analyze the effect of variations in carrying capacity on
the fate of a mutant type. We consider a two-state Moran model, where selection
intensity at equilibrium may differ (in amplitude and in sign) from selection
during periods of sharp growth and sharp decline. Using Kimura's diffusion
approximation we present simple formulae for effective population size and
effective selection, and use it to calculate the chance of ultimate fixation,
the time to fixation and the time to absorption (either fixation or loss). Our
analysis shows perfect agreement with numerical solutions for neutral,
beneficial and deleterious mutant. The contributions of different processes to
the mean and the variance of abundance variations are additive and commutative.
As a result, when selection intensity $s$ is weak such that ${\cal O}(s^2)$
terms are negligible, periodic or stochastic environmental variations yield
identical results.
