Testing a long-standing biogeographic theory with genetic data: The case of the alpina eucalypt clade in western Victoria, Australia

Background/Question/Methods   How do specious genera radiate and maintain genetic and morphological integrity in confined spaces? Speciation is a key process in evolution yet its ecology is still relatively poorly known. The ‘alpina' clade of eucalypts speciated on an isolated mountain range in Victoria, Australia. Species within this clade (E. serraensis, E. verrucata, and E. victoriana) are restricted to elevated rocky outcrops on this range. Previous studies of morphology and volatile leaf oils suggest this clade diverged via ecological speciation from an ancestor of the more widepsread E. baxteri and then subsequently speciated allopatrically. More recent gene flow between E. baxteri and neighboring ‘alpina' populations is thought to have created some morphologically intermediate populations. We investigated genetic differentiation of morphotypes and species. We also determined which functional traits correspond to morphological and genetic variation.

Results/Conclusions   We found that populations identified as extreme morphotypes were genetically differentiated from all other populations. Extreme morphotypes had the lowest specific leaf area and the most robust capsules and seeds. Putative hybrid populations were genetically variable with some individuals more related to extreme alpina morphotypes and some to E. baxteri supporting the hybridization hypothesis. Additionally, incongruent variation in nuclear and chloroplast markers indicates widespread hybridization and introgression. Seed migration within the alpina clade is most likely limited by seed size and short stature of the trees. There is at least one example of an isolated alpina population colonizing a neighboring peak via pollen flow instead of seed migration.