This butterfly was first described as a subspecies of Euchloe ausonides in 1936 from specimens collected almost exclusively from the Lake Arrowhead area, between 5000-6000' elevation. Paul Opler placed it within hyantis in a 1966 paper. Similar butterflies found at higher elevations on the east side of Big Bear Lake - esp. the north side of Sugarloaf Mountain - were assumed to be the same butterfly. But by the time the Emmels' The Butterflies of Southern California was published in 1973, lepidopterists such as Chris Henne and John Emmel had figured out that there were two distinct populations. A relatively recent paper* concluded that "[b]ecause of its phenetic distinctness and close parapatry with other E. hyantis populations, with a concomitant lack of intermediacy, we feel that E. andrewsi can be treated as a local endemic species with strong need for conservation concern and action.” Some of the visual distinctions I've learned about that distinguish andrewsi include black coloration at the dorsal wing bases; a lack of pearly luster in the ventral white areas; larger size; and a more rounded apex. The eastern Euchloe feeds mainly on Descurainia pinnata and probably Boechera pinetorum, while andrewsi mostly uses Streptanthus bernardinus.
There is a lot of information in print and on the web that probably is outdated or just incorrect concerning these butterflies, including misidentified photos and food plant information that is based on outdated assumptions. Hopefully we'll know more about these interesting populations soon.
*Werner Back, Michael A. Miller, and Paul A. Opler "Genetic, Phenetic, and Distributional Relationships of Nearctic Euchloe (Pieridae, Pierinae, Anthocharidini)," The Journal of the Lepidopterists' Society 65(1), 1-14, (1 March 2011).