Oenothera californica ssp. eurekensis

This is the very rare Eureka Dunes evening primrose, Oenothera californica ssp. eurekensis, that is only known from three locations in the southern Eureka Valley, which is part of Death Valley National Park. Eureka Dunes is the 2nd highest sand dune system in the United States at about 700' tall. This evening primrose grows on the lower slopes of the sand dunes and blooms from April to June with large showy white flowers that age to red. These pictures were taken in 2005 (our last good rainfall year) on a field trip with the Jepson Herbarium.

 

What differentiates it from related species?

 

According to the Jepson Manual, the differentiating characters between ssp. eurekensis and the two other California ssp. (californica and avita) are as follows:
eurekensis: stem hairs dense, short, appressed and long, spreading; roots fleshy; new rosettes at stem tips
californica and avita: stems hairs usually absent; roots not fleshy; new rosettes not forming at stem tips
If you're asking about the differences between the species Oenothera californica in general and other California Oenothera species (as opposed to ssp.), I'd have to know which ones you're curious about.