Résumé
Range expansions and biological invasions are prime examples of transient
processes that are likely impacted by rapid evolutionary changes. As a
spatial process, range expansions are driven by dispersal and movement
behaviour. While it is widely accepted that dispersal and movement may be
context-dependent, for instance density-dependent, and best represented by
reaction norms, the evolution of density-dependent movement during range
expansions has received little experimental attention. We therefore tested
current theory predicting the evolution of increased movement at low
densities at range margins using highly replicated and controlled range
expansion experiments across multiple genotypes of the protist model
system Tetrahymena thermophila. Although rare, we found evolutionary
changes during range expansions even in the absence of initial standing
genetic variation. Range expansions led to the evolution of negatively
density-dependent movement at range margins. In addition, we report the
evolution of increased intra-strain competitive ability and concurrently
decreased population growth rates in range cores. Our findings highlight
the importance of understanding movement and dispersal as evolving
reaction norms and plastic life-history traits of central relevance for
range expansions, biological invasions and the dynamics of spatially
structured systems in general.