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Johnston'S Rockcress (Boechera johnstonii)

Description

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.

Taxonomic tree

  • Domain: Eukarya

    • Kingdom: Plantae

      • Phylum: Magnoliophyta

        • Class: Magnoliopsida

          • Order: Brassicales

            • Family: Brassicaceae

              • Genus: Boechera