Geographic Barrier
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GEOGRAPHIC BARRIER. Quercus agrifolia, Coast live oak.
West side of Page Mill Road.
Speciation often occurs because organisms of two incipient groups are separated by a geographic barrier that prevents migration between groups. In evolutionary trees, a geographic barrier is often depicted as a line that lies between two emerging species lineages, and that members of those lineages cannot cross.
Photo: Noah Rosenberg, January 21, 2021