  In species reproducing both sexually and asexually clones are often more
common in recently established populations. Earlier studies have suggested that
this pattern arises from natural selection favouring asexual recruitment in
young populations. Alternatively, as we show here, this pattern may result from
stochastic processes during species-range expansions. We model a dioecious
species expanding into a new area in which all individuals are capable of both
sexual and asexual reproduction, and all individuals have equal survival rates
and dispersal distances. Even under conditions that eventually favour sexual
recruitment, colonisation starts with an asexual wave. Long after colonisation
is completed, a sexual wave erodes clonal dominance. If individuals reproduce
more than one season, and with only local dispersal, a few large clones
typically dominate for thousands of reproductive seasons. Adding occasional
long-distance dispersal, more dominant clones emerge, but they persist for a
shorter period of time. The general mechanism involved is simple: edge effects
at the expansion front favour asexual (uniparental) recruitment where potential
mates are rare. Specifically, our stochastic model makes detailed predictions
different from a selection model, and comparing these with empirical data from
a postglacially established seaweed species (Fucus radicans) shows that in this
case a stochastic mechanism is strongly supported.
