6983Population genetic structure and the mating system were investigated in the endangered plant Lambertia orbifolia. This species is geographically restricted with two disjunct groups of populations. Twelve out of 19 allozyme loci were polymorphic and four were suitable for mating system studies. Levels of genetic variation within populations were comparable to other long-lived woody shrub endemics. Genetic divergence between population groups was very high (D 0.252) and the F ST over all populations was 0.441. Gene¯ow estimates within population groups were low even though the maximum geographical distance between any pair of populations is 15 km and could be attributed to the localized movement of bird pollinators. Mating system studies on four populations showed consistently low levels of outcrossing, compared with other Proteaceae. Correlations of outcrossed paternity were moderately high and all were signi®cantly greater than zero. Values ranged from r p 0.33, in the two largest and very dense populations, to the highest value of r p 0.54 in a smaller low-density population. The current population genetic structure in L. orbifolia is probably the result of local extinction of intervening populations because of Pleistocene climatic change and increased aridity, and extended isolation of the two remnants. It is proposed that the phylogenetically distinct Narrikup population group be recognized as a separate conservation unit and be given high priority for conservation action.Keywords: conservation units, endangered species, gene¯ow, genetic structure, habitat fragmentation, mating system. IntroductionNaturally fragmented and disjunct population systems are a feature of the rich and diverse¯ora in south-west Australia. A signi®cant proportion of the plant species is likely to be relictual and probably had wider, more continuous distributions during favourable climatic regimes up to the early Pleistocene. Increased aridity and climatic instability during the Pleistocene are considered to be major factors in the localized extinction and fragmentation of many plant species in this region (Hopper et al., 1996).The level of geographical separation between population groups within these fragmented species suggests that recent gene¯ow, either by long-distance seed dispersal or pollen movement, has been limited or absent for a long period. Reduction or absence of genē ow, and¯uctuations in population size caused by climatic instability, are likely to lead to signi®cant interpopulation divergence. Studies to date on rare and geographically restricted species from south-west Australia support this view. Signi®cant genetic divergence among populations associated with clear geographical separation is a feature of a range of species in a number of¯owering plant families such as Mimosaceae , Myrtaceae (Moran & Hopper, 1983, 1987Sampson et al., 1988), Proteaceae (Coates & Sokolowski, 1992) and Stylidiaceae (Coates, 1992).A major confounding factor in assessing the evolutionary patterns observed in the south-west¯ora are th...

The Stylidium caricifolium (Stylidiaceae) complex consists of seven currently recognized species and a taxon of putative hybrid origin endemic to southwest Western Australia. These taxa vary in geographical distribution from widespread, extending over a range of 500 km, to extremely localized, covering a range of only 0.5 km. Patterns of allozyme variation were investigated in 61 populations covering all taxa and two closely related species. Measures of genetic diversity were consistently lower and in some cases significantly lower in four rare and geographically restricted taxa compared with their widespread relatives. In contrast, genetic diversity in two other localized taxa was comparable or higher than in the widespread taxa. The level of divergence among populations was moderate to high, with a significant trend of higher F(ST) values for the widespread species to lower values for the geographically restricted and rare taxa. Phylogenetic relationships and levels of divergence indicate that most taxa are probably relictual rather than recently evolved. Geographical localization and rarity in this complex can be attributed to a range of factors associated with habitat specificity, historical and ecological processes that characterize the southwest region, and mode of origin.