Hybridisation and Species Boundaries in Eucalypts

ABSTRACT We review Jamie (James Barrie) Kirkpatrick's influential studies on eucalypt genecology in the 1970's and the trajectory that this research followed in subsequent decades. We focus on two themes which involve hybridisation and gene flow. The first theme is his work on phantom hybrids, which highlights the challenges in determining the origin of isolated intermediate populations and the concept that hybridisation may be an integral part of the process of species' range expansion and contraction. Phantom hybrids may represent the genetic remnants of the past distribution of a species. We argue that the usefulness of the phantom hybrid concept has now reached new levels in the genomic era, extending to cryptic hybridisation where the genomic footprint of hybridisation is detected in what visually appear to be pure species. The second theme focuses on his PhD work which extends the topic of gene flow between species through the challenges faced in delineating the four closely related species comprising the Eucalyptus globulus complex. This work demonstrated the clinal nature of the variation which may exist between the cores of many species and signalled the complexity of historical and environmental factors which interact to shape contemporary variation patterns in Eucalyptus . His and subsequent work on the E. globulus complex argue that multiple evolutionary processes are at play and that primary and secondary intergradation may have occurred at different times and places. This is supported by subsequent studies focusing on E. globulus that show haploid (pollen) dispersal is the predominant mode of gene flow, that genetic drift and ancient interspecific hybridisation have left their trace on the gene pool, but argue that natural selection by abiotic (e.g., climate) and biotic (e.g., disease) factors is still a major driver of local and regional population divergence in performance and functional traits.