The genus Arctostaphylos, which encompasses the manzanita, appeared roughly 15 million years ago, a time when western North America was warm and moist. But as inland conditions grew colder and drier, "a regionally endemic flora, including Arctostaphylos, got pushed towards the California coast or down into the highlands of Mexico. Today several species along the California coast are probably relicts that have persisted over time because coastal conditions are more mild," biologist Mike Vasey says.

Several circumstances combined to make the coast a haven for manzanita. The moderating effect of the ocean prevented glaciers from gaining a foothold along the coast. Together with the emergence of a wildfire-prone Mediterranean climate, these conditions permitted a long run of rapid diversification among coastal manzanita. The sailing wasn't entirely smooth. Fast-growing trees shut out these shrubs from more favorable land. Instead, the most marginal soils — nutrient-starved sand dunes, shale outcroppings, and toxic serpentine — became manzanita strongholds. As Vasey describes the process: "They were moving across the landscape over time, going from south slopes to north slopes and up and down, getting established on all these crazy soil types. They hybridized and created new forms and, under more mild coastal climate conditions, these new forms survived. So there was both a trend of relicts surviving along the coast and a great diversification event taking place simultaneously."

Species or subspecies of Arctostaphylos found within maritime chaparral stands in the Monterey Bay area. Ten occur within the eight maritime chaparral clusters (or archipelagos) mapped by the Elkhorn Slough Foundation. The Fort Ord cluster contains the most diversity, with eight different manzanita taxa; soil type affects the dominant species at each location. Asterisks denote endemics restricted to this geographic area.
Figure modified by M. Vasey from http://www.elkhornsloughctp.org/training by Eric Van Dyke.