Variation in the Eucalyptus gunnii-archeri Complex. II. The origin of variation

Previous studies of the Eucalyptus gunnii-archeri complex indicate marked differentiation of the adult phenotype. In this study, genetically based variation in the seedling phenotype is demonstrated. The five main phenetic groups in the complex are shown to be genetically defined and account for most of the genetic variation between populations. Further, the two major phenetic clines in the adult phenotype are paralleled by genetically based clines in the seedling phenotype, with greater genetic differentiation occurring along the cline between subspp. archeri and gunnii than along the cline between altitudinal extremes within subsp. gunnii. Detailed analyses of the structure of populations along the former continuum indicate no evidence for recent secondary intergradation since intermediate phenotypes are widespread and genetically stable and the various character clines appear to be independent. However, historical and biogeographical evidence is presented which suggests that a suture zone occurred between southern and northern populations during the last Pleistocene glacial. The area occupied by intermediate populations would have been amongst the last to be colonized following deglaciation. It is argued that extreme morphs differentiated in allopatry and that the continuum between the subspecies is a result of selective stabilization of a zone of secondary intergradation as opposed to primary intergradation. In contrast, the altitudinal continuum within subsp. gunnii appears to be mainly a result of primary differentiation within a population migrating up slope from a southern glacial refuge. Progeny trials indicate that the high variability and differentiation of several low-altitude, relic populations may be due to hybridization with surrounding species, whereas hybridization has little effect at the boundary of large stands. It is argued that hybridization may be important in the process of range contraction and extinction, and may be a significant evolutionary stimulus in small peripheral isolates.