A large amount of genetic diversity in ecological traits exists both within and between the Tasmanian eucalypt species. This variation is generally associated with spatial variation in habitat. Major changes in the distribution of eucalypt species have no doubt occurred in response to past environmental changes and even today, populations are continually reacting to short- or long-term changes in the environment. The processes by which populations respond to environmental changes are addressed, with particular emphasis on gene flow and the role of hybridisation in the expansion and contraction of species ranges.