= Morphological species concept
A <morphological species concept> distinguishes <species> using <morphology>: measurable differences in form or structure. It can be applied to preserved specimens and <fossils> when mating cannot be observed. Similar appearance need not imply the same <species>, and different appearance need not imply different <species>. <Biology 2e by Mary Ann Clark and others>[§18.2, “Species and the Ability to Reproduce”, paragraphs accompanying Figures 18.9–18.10: appearance can mislead]{ref}
An original operational example is to compare a chosen structure in specimens of comparable age, sex and rearing conditions. Suppose two apparent groups differ in size, but one group contains only juveniles. The observed difference does not yet diagnose two <species>. Specify the measured <phenotype>, variation within each sample, and whether the distinction persists across comparable specimens. A single striking specimen is a poor substitute for that comparison.
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