MARINE BIOLOGICAL INVASIONS AS MODELS OF DISPERSAL - TRACKING SECONDARY SPREAD AND INTROGRESSIVE GENE FLOW

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.