[Truncated abstract] The species-rich Southwest Australian Floristic Region (SAFR) is a global biodiversity hotspot. Characterised by a Mediterranean-type climate and nutrient deficient landscape, this region is endowed with 7380 native vascular plant species/sub species, of which 49% are endemic and 2500 are of conservation concern. Despite the global significance of this region, there is still only a poor understanding of the factors influencing high diversity and endemism, and especially the population genetic consequences of narrow endemism and naturally fragmented species distribution. Holly leaved banksia (Banksia ilicifolia R. Br.), although widespread through Southwest Western Australia (SWWA), has a naturally fragmented distribution, with generally small populations restricted to swales and wetland fringes with depth to groundwater less than 10 m. As such, it provides an excellent model to better understand the ecological genetic consequences of local endemism, population size and natural population fragmentation . . . Products of wide outcrossing (over 30 km) showed a heterosis effect over local outcrossing, indicating increased ecological amplitude of offspring following interpopulation mating. These results suggest that the breeding and mating biology of B. ilicifolia counters the negative genetic erosion effects of narrow ecological amplitude and small population size. Recent habitat fragmentation, and reductions in population size and increased isolation, is impacting on these processes, but further research is required to assess the ultimate consequences of these genetic effects for population viability.