Aust. J. Bot., 1988, 36, 273-86 Genetic Diversity and Population Genetic Structure in the Rare Chittering Grass Wattle, Acacia anomala Court David J. Coates Department of Conservation and Land Management, Western Australian Wildlife Research Centre, P.O. Box 5 1, Wanneroo, W .A. 6065. Abstract There are 10 known populations of Acacia anomala occurring in two small disjunct groups some 30 km apart. The Chittering populations reproduce sexually whereas the Kalamunda populations appear to reproduce almost exclusively by vegetative multiplication. The level and distribution of genetic variation were studied at 15 allozyme loci. Two loci were monomorphic in all populations. In the Chittering populations the mean number of alleles per locus was 2.0 and the expected panmictic heterozygosity (genetic diversity) 0.209. In the Kalamunda populations the mean number of alleles per locus was 1.15 and the expected panmictic heterozygosity 0.079, although the observed heterozygosity of 0.150 was only marginally less than the Chittering populations (0.177). These data support the contention that the Chittering populations are primarily outcrossing whereas the Kalamunda populations are clonal, with each population consisting of individuals with identical and, in three of the four populations, heterozygous, multilocus genotypes. The level of genetic diversity within the Chittering populations is high for plants in general even though most populations are relatively smsll and isolated. It is proposed that either the length of time these populations have been reduced in size and isolated is insufficient for genetic diversity to be reduced or the genetic system of this species is adapted to small population conditions. Strategies for the adequate conservation of the genetic resources of Acacia anomala are discussed. Introduction The level and partitioning of genetic variation within and among plant populations are known to be influenced by a wide range of factors such as mode of reproduction, mating system and geographic distribution. Species which have wide geographic ranges, long generation times, are wind-pollinated and outcrossing, such as many of the coni- fers, tend to have very high levels of variation, most of which occur within populations. In contrast, annual herbaceous species which are primarily selfing have lower levels of genetic variation which occur mostly between populations (Brown 1979; Hamrick et al. 1979; Hamrick 1983; Loveless and Hamrick 1984). The mating system has a major influence on the structuring of genetic variation within a species (Gottlieb 1973, 1975; Brown et al. 1975; Levin 1975, 1978; Schaal 1975; Phillips and Brown 1977; Brown and Jain 1979; Moran and Brown 1980; and others). However, much less is known about the effects of other factors, in particular population size, the degree of population isolation and geographic distribution. Popu- lation genetic theory predicts that large populations maintain higher levels of genetic variability than small populations and that the greater the isolation between populations the greater the level of between-population variation. Recent studies provide contradic- tory evidence. Moran and Hopper (1987), in a study on localised mallee eucalypts, found that small populations have the same level of genetic diversity as larger~popu- lations within the same species. However, localised species were found to have generally lower levels of genetic variability than widespread species (see also Hamrick 1983). The 0067-1924/ 88/030273$02.00