Parallel evolution of dwarf ecotypes in the forest tree Eucalyptus globulus

Abstract

Three small populations of a dwarf ecotype of the forest tree Eucalyptus globulus are found on exposed granite headlands in south-eastern Australia. These populationsare separated by at least 100 km.• Here, we used 12 nuclear microsatellites and a chloroplast DNA marker to investigatethe genetic affinities of the dwarf populations to one another and to their nearest populations of tall E. globulus. Cape Tourville was studied in greater detailto assess the processes enabling the maintenance of distinct ecotypes in close geographical proximity.• The three dwarf populations were not related to one another and were more closely related to adjacent tall trees than to one another. At Cape Tourville the dwarfand tall ecotypes were significantly differentiated in microsatellites and in chloroplast DNA. The dwarf and tall populations differed in flowering time and no evidence ofpollen dispersal from the more extensive tall to the dwarf population was found.• The three dwarf populations have evolved in parallel from the local tall ecotypes.This study shows that small marginal populations of eucalypts are capable of developing reproductive isolation from nearby larger populations through differencesin flowering time and/or minor spatial separation, making parapatric speciation possible