Table 1 Summary statistics used in the topographic ring model, along with a brief description of biological relevance.

Size

1. Area

2. Perimeter

Larger barriers provide more opportunities for isolation by distance to promote non-adaptive divergence (that is, differentiation in neutral loci) around a ring distribution.

3. Latitudinal range

Larger latitudinal ranges span more environments and thus facilitate adaptive divergence.

Position

4. Mean distance from equator

Barriers further from the equator are larger to account for latitudinal differences in range size [31].

Permeability

5. Shape (Perimeter-to-area ratio)

Compact circular-shaped barriers (compared to elongated barriers) are uniformly wider and therefore less subject to trans-barrier dispersal and gene flow.

6. Fragmentation

More fragmented barriers (that is, barriers that split apart with changing topographic slope) offer more opportunities for trans-barrier dispersal than uniform barriers.