A major limitation of genetic approaches to the study of ongoing dispe
rsal is that site- or population-specific markers are rarely available
. Over spatial scales that encompass transport of propagules, newly ge
nerated neutral polymorphisms will have been spread by prior dispersal
. Marine biological invasions provide an alternative approach to the s
tudy of larval dispersal in two ways. First, primary invasions general
ly occur in one or few sites. The sources of recruits in subsequent ra
nge extension can therefore be traced. Second, invading species can in
ject novel genetic markers into native populations when hybridization
is possible. Similarly, when the distribution of native and invading s
pecies overlaps narrowly, dispersal can be traced by the movement of h
ybrid genotypes from the hybrid zone. Dispersal of the invading barnac
le Elminius modestus in Great Britain, the crab Carcinus maenas in Cal
ifornia, and the green alga Codium fragile in New England are examples
of the first approach. The unidirectional introgression of the mitoch
ondrial genome of the native northern mussel Mytilus trossulus into in
vading populations of Mytilus galloprovincialis in southern California
is an example of the second approach. Marine biological invasions are
increasing in frequency and in these ways provide population biologis
ts and geneticists with useful model systems.