Hypothetical scenario of two mitochondrial candidate species distributed along a geographic and ecological gradient. A observed patterns: 1) parapatrically distributed mitochondrial candidate species CS1 and CS2; 2) clinal phenotypic variation along geographical gradient; 3) genomic signature of isolation by distance along geographical gradient as revealed by software such as STRUCTURE (Pritchard et al. 2000); 4) continuous distribution through ecological gradient. Patterns 2–4 reject the candidate species hypothesis derived from 1. B consequences of analyses treating the candidate species CS1 and CS2 as a priori units of comparison: 1) misleading impression of confirmation of candidate species from phenotypic differences; 2) misleading impression of confirmation of candidate species from ecological niche differences.