The importance of dispersal and recent speciation in the flora of New Zealand

The origins of the New Zealand flora have been the subject of considerable debate, particularly amongst botanists discussing the relative importance of plant dispersal in the Southern Hemisphere. Wardle (1963, 1968, 1978), although explaining some plant distributions by dispersal, has suggested that New Zealand’s flora arose largely through Figure 1 Scattergraph for a Principal Components Analysis (PCA) involving seventeen diversification of taxa present on southern morphological characters which could be scored for sixteen representative taxa. Data points are ocean lands following the break-up of the coded according to the subgeneric groupings of Grau & Schwab, 1982. Section Myosotis Gondwanan supercontinent. In contrast, (species are from Eurasia, North America and Africa) is indicated by black circles, Section Raven (1973) has proposed a main route of Exarrhena discolor group (species are from Eurasia and Africa) are open and grey circles dispersal from the Northern Hemisphere via denote Section Exarrhena Austral group (species are from Australasia and South America). Australia for the (sub)alpine flora of New Data taken from Grau & Leins, 1968; Grau & Schwab, 1982, various regional floras not cited Zealand. Pole (1994) and Macphail (1997) and from the authors’ personal observations. have also argued for the general importance of Australia as a source for the New Zealand flora. However, their view is more extreme. fluctuations during the Pleistocene resulting that the austral taxa display greater variation They assume that New Zealand was comin the glacial and interglacial cycles of that than the other two groups (Grau & Leins, pletely submerged during the Oligocene and period. Such changes were unlike the more 1968; Grau & Schwab, 1982). Members of that all extant plant lineages in New Zealand stable conditions of the preceding 60 MY the austral group of section Exarrhena exhibit (both alpine and lowland) have arrived from (Cooper & Millener, 1993). Emerging mogrowth forms, floral and pollen charAustralia by long distance dispersal since lecular studies on the New Zealand flora acteristics not apparent in the remaining subgthe Miocene. Their conclusion follows from indicate that these phases of climatic and eneric groups. This greater morphological observations that in New Zealand there is a geological change were also times of indiversity has led to the suggestion that the poor match between extant species and those tensified speciation in the New Zealand flora, austral group of Myosotis holds an ancestral from the palynological and/or macrofossil and periods when long-distance dispersal had position within the genus (Grau & Leins, record of the Tertiary period. Pole (1994) an increased importance in establishing 1968). However, we also report comparative and Macphail (1997) argue that, if the current trans-Tasman distributions between AusDNA sequences from several chloroplast loci flora is truly of ancient origin, one must tralia and New Zealand. This is well iland a nuclear locus (Fig. 2). Analysis of these expect to find extant or morphologically lustrated by our recent studies on Myosotis data show that, in comparison to the situation similar forms in the Tertiary fossil record of L. (Boraginaceae). in representative northern hemisphere taxa, New Zealand. Myosotis is a genus with approximately little genetic diversity exists within and beIt is generally agreed that from the late 100 species and a worldwide distribution (Altween members of the austral group. Further, Pliocene onward, New Zealand has underShehbaz, 1991). Centres of diversity have these molecular data, taken together with gone periods of great change, both geodeveloped in Western Eurasia & New ZeaITS and matK sequences from more widely logically and climatically. Increased land. Grau & Schwab (1982) recognized three sampled taxa and outgroups (Winkworth interactions between the Pacific and Ausmorphologically defined groups within the et al. in prep) suggest that the Myosotis spetralian tectonic plates led to the rapid uplift genus, two occurring in the northern hemicies studied from Australia, New Zealand, of the axial mountain ranges, possibly as sphere and Africa (section Myosotis and secand South America are closely related and early as 5 million years ago (Raven, 1973; tion Exarrhena, discolor group), the third of northern origin, that their morphological Cooper & Millener, 1993). Profound climatic being restricted to Australasia and South diversification is recent and not of great anchanges were initiated by a pronounced drop America (section Exarrhena, austral group). tiquity, and that they have reached their in the average temperature during the Late We report here a morphometric analysis widely disjunct distribution by long-distance dispersal. Pliocene, followed by marked temperature (Fig. 1) which supports earlier observations