Movie S1.

Development of railroad lines in the United States from 1830-1972. Railroad line data were obtained from (2) and plotted by their date of operation. Note the completion of railroad lines connecting eastern and western US in 1872, a few years prior (1879) to when a small population originating from North America (east) was believed to be introduced to that exact region—North America (west) (i.e., central California).