Abstract:
Understanding the genetic background of invading species can be crucial information clarifying
why they become invasive. Intraspecific genetic admixture among lineages separated in the
native ranges may promote the rate and extent of an invasion by substantially increasing standing
genetic variation. Here we examine the genetic relationships among threespine stickleback that
recently colonized Switzerland. This invasion results from several distinct genetic lineages that
colonized multiple locations and have since undergone range expansions, where they coexist and
admix in parts of their range. Using 17 microsatellites genotyped for 634 individuals collected
from 17 Swiss and two non-Swiss European sites, we reconstruct the invasion of stickleback and
investigate the potential and extent of admixture and hybridization among the colonizing
lineages from a population genetic perspective. Specifically we test for an increase in standing
genetic variation in populations where multiple lineages coexist. We find strong evidence of
massive hybridization early on, followed by what appears to be recent increased genetic isolation
and the formation of several new genetically distinguishable populations, consistent with a
hybrid ‘superswarm’. This massive hybridization and population formation event(s) occurred
over approximately 140 years and likely fuelled the successful invasion of a diverse range of
habitats. The implications are that multiple colonizations coupled with hybridization can lead to
the formation of new stable genetic populations potentially kick-starting speciation and adaptive
radiation over a very short time.