Genetic and ecological analysis of the putative natural hybrid zone formed Banksia robur Cav. and Banksia oblongifolia Cav.

Natural hybridization provides the opportunity to study the processes involved in speciation, since hybridizing populations are thought to represent some intermediate stage between a common ancestral population and the complete divergence of two species. The presence of hybrids within a population indicates that reproductive isolation has at some stage been incomplete. Therefore, the determination of the current level of gene flow can indicate the degree to which speciation is complete. Further, the fitness of the hybrids in comparison to that of the parent species can indicate the mode of formation and maintenance of the hybrid zone: either (i) a genetic/morphological dine can form in association with an environmental dine if the species are suited to conditions on different extremes of the dine or if the hybrid are as fit as the parents in the ecotone (environmental dine) and (ii) a dine may be formed as a result of an equilibrium formed between the constant gene flow into, and the selection against hybrids within, the hybrid zone (tension zone). Banksia robur Cav. and Banksia oblongifolia Cav. form the most commonly reported hybrid zone within the genus. The two species are morphologically different and are also closely associated with very different soil water regimes. Morphologically intermediate individuals, present in regions where the two species coexist, are thought to be hybrids.