What is a node?
Biogeographic nodes can be characterized as sites of biological endemism, high diversity, distribution boundaries, anomalous absences, disjunct populations, taxonomic incongruence, parallelism and altitudinal anomalies. Their interpretation has depended on the evolutionary model used, in particular the mode of speciation: Croizat's vicariance or Mayr's ‘peripatric’ or ‘founder dispersal’ (=Darwin's ‘chance dispersal’, Hennig's ‘speciation by colonization’). All authors agree that the first process, together with movement of individual organisms and diaspores, occurs, but the second is much more controversial, with panbiogeographers and many geneticists denying its importance. Although nodes have often been interpreted as centres of origin – as in refugium theory – this is not accepted here as it fails to account for their constituting both centres and margins of distribution as well as zones of absence. Instead they are interpreted as sites of vicariance related to different kinds of tectonic activity which have been shown to occur in the same locality, such as terrane accretion, subduction, regional metamorphism, granitization, volcanism, faulting, folding, uplift, subsidence and regression of epicontinental seas. It is concluded that the identification of nodes is a more productive approach to biogeographic analysis than dividing a study area into ‘biogeographic regions’, which are usually based firmly on current geography and represent geological and biogeographic composites.