• adaptive radiation;
  • AFLPs;
  • Asteraceae;
  • geographical isolation;
  • hybridization;
  • Microseris;
  • selection

The disjunct allotetraploid lineage of the North American genus Microseris in New Zealand and Australia originated from one or a few diaspores after a single introduction via long-distance dispersal. The plants have evolved into four morphologically distinct ecotypes: ‘fine-pappus’, ‘coastal’, ‘murnong’, and ‘alpine’, from which the first two are grouped as Microseris scapigera, mainly from New Zealand and Tasmania, and the latter two as M. lanceolata, endemic to the Australian mainland. Three chloroplast (cp) DNA types were distinguished in each of the species, but their distribution, especially in M. lanceolata, showed discrepancies with ecotype differentiation. Here, we analyse the genetic structure of the nuclear (n) DNA among two plants of each of 55 New Zealand, Tasmanian, and Australian Microseris populations for amplified fragment length polymorphisms (AFLPs). The nuclear genetic structure is compared to geographical, ecotype, and cpDNA distribution, in order to resolve and illustrate the early process of adaptive radiation. The strongest signal in the AFLP pattern was related to geographical separation, especially between New Zealand and Australian accessions, and suggested an initial range expansion after establishment. The ecotypic differentiation was less-well reflected in the AFLP pattern, and evidence was found for the occurrence of hybridization among plants at the same geographical region, or after dispersal, irrespective of the cpDNA- and ecotypes. This indicated that the ecotype characteristics were maintained or re-established by selection. It also showed that genetic differentiation is not an irreversible and progressive process in the early stage of adaptive radiation. Our results illustrate the precarious balance between geographical isolation and selection as factors that favour differentiation, and hybridization as factor that reduces differentiation.