  The concept of fitness as a measure for a species's success in natural
selection is central to the theory of evolution. We here investigate how
reproduction rates which are not constant but vary in response to environmental
fluctuations, influence a species' prosperity and thereby its fitness.
Interestingly, we find that not only larger growth rates but also reduced
sensitivities to environmental changes substantially increase the fitness.
Thereby, depending on the noise level of the environment, it might be an
evolutionary successful strategy to minimize this sensitivity rather than to
optimize the reproduction speed. Also for neutral evolution, where species with
exactly the same properties compete, variability in the growth rates plays a
crucial role. The time for one species to fixate is strongly reduced in the
presence of environmental noise. Hence, environmental fluctuations constitute a
possible explanation for effective population sizes inferred from genetic data
that often are much smaller than the census population size.
