Figure 4.

Relationship between environmental gradients and isolation in rates of group-boundary formation (i.e. formation of diversity in cultural units). Environments may have high or low probabilities of forcing populations to become isolated, and they may also have either large or small habitat differences in resource availability and richness. The former will determine the rate of boundary formation due to drift and isolation; the latter the rate of boundary formation due to competitive fissioning. Modified from Foley [46].