California salamanders are terrestrial amphibians. An original population of California salamanders occupied an area of forest in northern California. Approximately 10 million years ago, two separate populations, A and B, started to migrate from the original population. • Population A travelled along the coast of California, to the west of the Great Central Valley. • Population B travelled east of the Great Central Valley. The two populations now live close to each other in southern California. Fig. 8.1 outlines the migratory routes of populations A and B. [Figure 8.1] Salamanders from population A rarely interbreed with salamanders of population B. If they do interbreed, the offspring are infertile.