Whether interspecific hybridization is important as a mechanism that generates biological
diversity is a matter of controversy. Whereas some authors focus on the potential of
hybridization as a source of genetic variation, functional novelty and new species, others
argue against any important role, because reduced fitness would typically render hybrids
an evolutionary dead end. By drawing on recent developments in the genetics and ecology of
hybridization and on principles of ecological speciation theory, I develop a concept that
reconciles these views and adds a new twist to this debate. Because hybridization is common
when populations invade new environments and potentially elevates rates of response to
selection, it predisposes colonizing populations to rapid adaptive diversification under
disruptive or divergent selection. I discuss predictions and suggest tests of this hybrid
swarm theory of adaptive radiation and review published molecular phylogenies of adaptive
radiations in light of the theory.