People and urban sprawl have a significantly negative effect on the landscape and the animals and plants that live there. Urbanization can cause habitat fragmentation, which leads to populations of organisms essentially being stranded on “islands” of suitable habitat. We studied how four common species, three lizards and a bird, in Southern California have been affected by this isolation, which has occurred within the last 40 years. We found very large genetic distances across very small geographic distances, and found lower genetic diversity within smaller or more isolated patches. Urbanization in southern California represents a severe form of habitat fragmentation, as shown by the striking pattern of similar and rapid effects across four common and widespread species, including a volant bird.