Ocbil theory was developed to better understand the origins, ecology and conservation of biodiversity and cultural diversity on the Earth's oldest, climatically buffered, most infertile landscapes (Ocbils), which are especially prominent in the Southwest Australian and Greater Cape Floristic Regions. Natural hybridization involves the mating of individuals from distinguishable populations, usually of distinct taxa, and often involves the production of later-generation derivatives from such matings. Drawing upon Edgar Anderson's Hybridized Habitat hypothesis and Arnold's modern development of Anderson's and Stebbins’ ‘genetic exchange in fluctuating environments’ thesis, it is predicted that natural hybridization, introgression and hybrid speciation will be reduced in Ocbils and more common in Yodfels (young, often-disturbed, fertile landscapes) globally – the Reduced Hybridization Hypothesis for Ocbils. Evidence supporting this hypothesis is explored in herbarium and field data on hybridization rates and hybrid speciation in southwest Australia, contrasted with reviews of hybridization rates in other global regions. Further research is recommended and independent tests in South Africa and other countries with Ocbils would be instructive. The solitary Ocbil dwelling Eucalyptus × graniticola Hopper is described in supplementary material to exemplify the Reduced Hybridization Hypothesis.