  Species introductions promote secondary contacts between taxa with long
histories of allopatric divergence. Anthropogenic contact zones thus offer
valuable contrasts to speciation studies in natural systems where past spatial
isolations may have been brief or intermittent. Investigations of anthropogenic
hybridization are rare for marine animals, which have high fecundity and high
dispersal ability, characteristics that contrast to most terrestrial animals.
Genomic studies indicate that gene flow can still occur after millions of years
of divergence, as illustrated by invasive mussels and tunicates. In this
context, we highlight three issues: 1) the effects of high propagule pressure
and demographic asymmetries on introgression directionality, 2) the role of
hybridization in preventing introduced species spread, and 3) the importance of
postzygotic barriers in maintaining reproductive isolation. Anthropogenic
contact zones offer evolutionary biologists unprecedented large scale
hybridization experiments. In addition to breaking the highly effective
reproductive isolating barrier of spatial segregation, they allow researchers
to explore unusual demographic contexts with strong asymmetries. The outcomes
are diverse from introgression swamping to strong barriers to gene flow, and
lead to local containment or widespread invasion. These outcomes should not be
neglected in management policies of marine invasive species.
