the humboldt current system of northern and central chile - figema

MARTIN THIEL ET AL.Population connectivityConnectivity can be defined as the extent to which populations in different parts of a species’ rangeare linked by exchange of larvae, recruits, juveniles or adults (Palumbi 2003) and determines thedegree of cohesion of its genetic pool and the geographic structure of its genetic diversity. Theintensity and geographical scale of connectivity within a species is given by the realised dispersalthrough active and passive mechanisms, which depend on species life-history traits and environmentalcharacteristics. Among the numerous dispersal mechanisms reported in marine organisms,active swimming/crawling and planktonic larval transport, together with rafting and anthropogenicdispersal, are considered as the most relevant to achieve connectivity among local populations(Thiel & Haye 2006). Of particular relevance for connectivity of marine species are the temporaland spatial oceanographic characteristics such as currents, upwelling, water masses and gyres. Forexample, even though a species may have long-lived planktonic larvae, in a particular bay the larvaemay not effectively disperse due to local larval retention (e.g., Swearer et al. 1999, Poulin et al.2002a, Baums et al. 2005). In contrast, benthic species lacking dispersive larval stages can achievelong-distance dispersal by rafting or anthropogenic transport (Thiel & Haye 2006). Indeed, a recentstudy demonstrated that biogeographic patterns along the coast of South Africa are reflected in thegenetic population structure of littoral organisms regardless of their dispersal stages (i.e., with orwithout planktonic larvae) (Teske et al. 2006). It seems particularly interesting to pursue this avenuein the HCS of northern-central Chile where no distinct biogeographic barriers but rather taxondependentbreaks exist (see section on biogeography). Moreover, the unique characteristics of theHCS make it an interesting system to study the genetic connectivity of marine populations. Importantcharacteristics of the system for genetic connectivity are its wide geographic extent and theoceanographic cyclic variations that lead to temporal and spatial changes in population size anddistribution. It is expected that both life-history traits and oceanography play crucial roles indetermining the realised dispersal of marine populations and thus their connectivity and the extentof their geographic ranges. Few population genetic studies have been published on marine speciesof the HCS, although there are several currently being developed on pelagic fishes, marine invertebratesand algae. Nevertheless, some predictions may be formulated and, where possible, validatedthrough existing examples.The pattern of genetic connectivity among local populations of a species determines thegeographic structure of its genetic diversity (Figure 16). The frequency, intensity and geographicalscale of dispersal within a species shape the resulting gene flow that counteracts the action ofgenetic drift and local selection. In this context, the intensity of genetic drift is principally determinedby population dynamic processes such as population size variation, local extinction, recolonisationand founder effects, all intimately related to connectivity among populations. Therefore,acting both on gene flow and genetic drift, the different patterns of connectivity should result indifferent geographic structuring of the genetic diversity. Very high gene flow (at the scale of thegeographic distribution of the species) leads to genetic homogeneity among local populationsindependently of the geographic distance. With lower levels of gene flow, different patterns ofpopulation genetic structure may result depending on the association between gene flow andgeographic distance. If the magnitude of the gene flow is associated with geographic distance,which may be the case for many organisms with planktonic larvae, a genetic cline may form throughthe range of distribution, characterised by genetic differentiation being proportional to geographicdistance, a pattern known as isolation by distance (IBD). This pattern will also be influenced bythe direction and strength of the currents. If the magnitude of the gene flow is not strongly associatedwith geographic distance, which may be the case for organisms that disperse through passivemechanisms, the resulting pattern may be chaotic patchiness. So far two parameters have been252