Running tally
What you just did has a name
"Species" is a prediction: the hypothesis that two populations will not influence each other's evolutionary trajectory in the future. All five rounds were grey-zone cases. For each, there is usually a defensible call — but always an observation that would flip it. The ring species and Rhagoletis rounds were the hardest; both are real ongoing arguments in the literature.
Three common resolutions recur across the five rounds:
- Complete reproductive isolation on secondary contact flips a "one species" call to "two." F1 sterility, behavioral isolation, gamete incompatibility — any one is decisive.
- Ongoing gene flow at the biologically relevant loci flips a "two species" call to "one." Shared alleles at functional loci, not just neutral markers, mean the populations have not yet separated in the way that matters.
- Collapse of the intermediate populations (ring species, host races) flips "intermediate" to "two" because you lose the continuous gene-flow chain that makes the species call fuzzy.
The ~120 species definitions in the literature are arguments about method, not about the underlying phenomenon. The phenomenon is evolutionary independence. Pick the method that fits the data you have, make a specific hypothesis, and name the observation that would refute it.