  We investigate the effect of spatial range expansions on the evolution of
fitness when beneficial and deleterious mutations co-segregate. We perform
individual-based simulations of a uniform linear habitat and complement them
with analytical approximations for the evolution of mean fitness at the edge of
the expansion. We find that deleterious mutations accumulate steadily on the
wave front during range expansions, thus creating an expansion load. Reduced
fitness due to the expansion load is not restricted to the wave front but
occurs over a large proportion of newly colonized habitats. The expansion load
can persist and represent a major fraction of the total mutation load thousands
of generations after the expansion. Our results extend qualitatively and
quantitatively to two-dimensional expansions. The phenomenon of expansion load
may explain growing evidence that populations that have recently expanded,
including humans, show an excess of deleterious mutations. To test the
predictions of our model, we analyze patterns of neutral and non-neutral
genetic diversity in humans and find an excellent fit between theory and data.
