Figure 1.

(a) Phylogeny for five biological species (A–E) and two geographically separated populations (C1 and C2) of C. Branch widths are proportional to the populations’ or species’ sizes and also indicate a geographic orientation. Thus, A is a peripheral isolate from C1, and B arose within the range of C2. The sundering agents are intrinsic RIBs (black areas), extrinsic barriers to gene flow (gray areas), or both in temporal order of appearance (gray then black). (b) Simplified “stick” representation of the phylogeny in a.