(Boechera johnstonii)
Arabis johnstonii-and-A. parishii-are an interesting species pair. They are very similar, but distinct, taxa, and each species is endemic to a very small area, with those two areas separated by just 40 miles.-Arabis johnstonii-is found only at the south end of the San Jacinto Mountains between 4450 and 6800 feet, and-A. parishii-is found only at 6500-9800 feet in the San Bernardino Mountains. Presumably these two species are very closely related. One possible scenario is that they differentiated from a common ancestor after populations of that ancestor became reproductively isolated. The San Bernardino Mountains and San Jacinto Mountains formed only geologically recently as part of the San Jacinto / San Andreas Fault System. Prior to ~five million years ago, both those areas were part of the same flattish contiguous area, without significant elevation ranges and with no barrier like the Banning Pass separating them.