Grevillea rhizomatosa is a spreading shrub which exhibits multiple breeding strategies within a narrow area in the fire-prone heathlands of eastern Australia. Reproductive strategies include self-compatibility, self-incompatibility and clonality (with and without sterility). The close proximity of contrasting breeding systems provides an opportunity to explore the evolution of sterility and to compare and contrast the origins of genotypic diversity (recombinant or somatic) against degrees of sexual expression. ISSR markers for 120 band positions (putative loci) were used to compare genetic diversity among five populations at a macro-scale of 5 m between samples (n = 244 shrubs), and at a micro-scale of nearest neighbours for all plants in five 25-m² quadrats with contrasting fertilities (n = 162 shrubs). Nearest-neighbour sampling included several clusters of connected ramets. Matrix incompatibility (MIC) analyses were used to evaluate the relative contribution of recombination and somatic mutation to genotype diversity. High levels of genotypic diversity were found in all populations regardless of fertilities (fertile populations, G/N ≥ 0·94; sterile populations, G/N ≥ 0·97) and most sterile populations had a unique genetic profile. Somatic mutations were detected along connected ramets in ten out of 42 ramet clusters. MIC analyses showed that somatic mutations have contributed to diversity in all populations and particularly so in sterile populations. Somatic mutations contribute significantly to gene diversity in sterile populations of Grevillea rhizomatosa, the accumulation of which is the likely cause of male and female sterility. High levels of genetic diversity therefore may not always be synonymous with sexual fitness and genetic health. We hypothesize that frequent fires drive selection for clonal reproduction, at the cost of flowering such that sexual functions are not maintained through selection, and the build-up of somatic mutations in meristems results in high genotype diversity at the cost of pollen and ovule fertilities.

Population bottlenecks during founder events tend to constrict the genetic diversity in introduced populations, thereby limiting their evolutionary potential and subsequent ability to adapt to their new environment. Paradoxically, rapid evolutionary changes have been recently found to be widespread in invasive species and have been proposed as a precursor to successful invasions. Information about population structure, introduction history and genetic diversity is essential for addressing this paradox and testing evolutionary hypotheses for any specific invasive species. In this study, we used microsatellite markers to investigate the genetic properties that may underpin the evolutionary potential of the invasive herb Phyla canescens, for which rapid, selection‐driven evolution has already been demonstrated. Population structure and genetic diversity were compared between the native (South America) and two introduced ranges (eastern Australia and southern France). South American populations included all the 64 alleles found in the study and most individuals belonged to two distinct genetic clusters originating from northwest Argentina and central Argentina respectively. Invasive populations in eastern Australia and southern France were most closely linked to the central Argentine cluster. Microsatellite results also showed both a reduced genetic diversity at the population level, and the occurrence of a significant genetic bottleneck in many introduced populations. Our results suggest that P. canescens can undergo rapid, selection‐driven evolution despite significant population bottlenecks and reductions in diversity. The question about the ‘genetic paradox' for P. canescens therefore is how much genetic diversity is enough to underpin rapid evolution, or whether it is the type of diversity rather than the amount that is important.