The putative hybrid zone between Eucalyptus populnea and E. brownii is examined using morphological and molecular techniques. This species complex displays continuous mor- phological variation across the study area, which has been previously interpreted as the product of hybridization between allopatric species. A microsatellite analysis indicates that there was little genetic structuring across the morphological cline and only low levels of population differentiation. The nested clade analysis of the J LA+ region of the chloroplast DNA (cpDNA) indicates that the geographical distribution of cpDNA haplotypes is unlikely to be the result of historical hybridization events, and that restricted seed-mediated gene flow with isolation by distance is responsible for the phylogeographical distribution. A more plausible explanation for the origin and persistence of the morphological cline is that the process of continuous morphological diversification has been promoted by a directional selection gradient. This study addresses species status within Eucalyptus and the belief that hybridization is widespread and is an important process in the group’s evolution.
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