Figure 1.

Two views on the continuum of speciation. Left: Species are defined as groups of organisms resembling each other according to an arbitrary set of variables. Right: Species are defined as entities sufficiently diverged such that gene flow (arrows) is very rare or inexistent. Top: unambiguous single‐species situation. Bottom: unambiguous multiple‐species situation. Intermediate: ambiguous situation. Ambiguous situations appear when groups can be identified but intermediate individuals are common (left) and when gene flow exists but is limited to a fraction of the genome (right)