The onset of progressive summer drought and intense fire regimes in California's Mediterranean-type climate have been implicated as drivers in the evolutionary diversification of Arctostaphylos (manzanita), the most speciose woody genus in the California Floristic Province (CFP).  Recent treatments of Arctostaphylos recognize 105 taxa and all but one subspecies occur in the CFP.  We analyze the distributions, environmental settings, ploidy levels, deep cladistic relationships, and morphological traits of 95 manzanita taxa in California.  Out of 62 species, 43 (69%) are found along the immediate coast within the summer fog zone or on fog-influenced uplands.  Of 66 local endemics (species and subspecies), 50 (73%) are restricted to this narrow coastal zone.  Most coastal taxa inhabit maritime chaparral, a vegetation-type that covers less than 10% of the area occupied by chaparral in foothills and mountains of interior California.  Morphological traits, e.g., bifacial distribution of leaf stomata, are restricted to coastal taxa despite occurring in different clades, ploidy levels, and morphological groups. These macroscale patterns suggest an alternative hypothesis in which mild coastal climate and summer fog, rather than extreme drought and more intense fires, have stimulated evolutionary diversification in Arctostaphylos.  We describe research designed to test this hypothesis by using stable isotopes and comparative ecophysiological studies of several manzanita species along a heavy-fog to fog-free gradient in the Monterey Bay region.  Given future climate change uncertainties, possible disruption of summer fog regimes, and conservation concerns about Arctostaphylos and maritime chaparral, we argue that it is important to evaluate these competing climate-based hypotheses.