Evolutionary processes in the Tasmanian high altitude eucalypts

The rugged Tasmanian topography results in a patchy distribution of treeline habitat, which can be viewed as a mosaic of "habitat islands" varying considerably in size and degree of isolation. In recent geological time the Pleistocene glacial cycles have subjected this mosaic to dramatic perturbation involving habitat relocation, fragmentation, coalescence and, on a local scale, extinction and birth of unique adaptive modes. In this paper we examine the adaptive response of populations to a fluctuating mosaic of selective forces through a discussion of the evolution of clines in the Tasmanian high altitude eucalypts.

The eucalypts primarily associated with the treeline habitat in Tasmania (i.e. E. coccifera, E. urnigera, E. vernicosa and E. gunnii) are all endemic. Populations of these species are distributed as mosaics or disjunctions within more continuous and widespread populations of subalpine and lowland taxa (e.g. E. delegatensis, E. dalrympleana, E. pauciflora, E. nitida). In many cases, there is a geographic and genetic continuum between stunted treeline variants and surrounding subalpine forest variants resulting in complex genetic mosaics. The range of variation along these altitudinal gradients is often large, transgressing several recognized taxa. The origin of these multi-character, clinal mosaics is of particular theoretical interest. These may arise as a result of either primary or secondary intergradation and it is important from an evolutionary viewpoint to differentiate these alternatives, although this is difficult from population structure alone (Anderson 1953; Barber and Jackson 1957; Mayr 1963; Endler 1977; Potts and Reid 1985b).

Specific hypotheses accounting for the origin of clinal mosaics include:

1. Primary intergradation (parapatric differentiation)
   1. parallel evolution
   2. fragmentation of once geographically extensive clines
2. Secondary intergradation
   1. range fragmentation and introgression associated with a break down in
      1. geographic isolation
      2. local reproductive isolation
   2. long distance migration by
      1. seed dispersal
      2. pollen dispersal

Possible examples of virtually all these alternatives can be found in the Tasmanian eucalypts, and the intricate variation patterns in specific complexes probably integrate the full spectrum of these evolutionary processes.