A species concept states what should make organisms members of the same species; an operational criterion states what observations will test that claim. Reproductive isolation, distinguishable morphology and ancestry inferred through phylogenetics answer different questions. In a lineage-based interpretation, they are evidence for separation of evolving biological populations, and need not appear together. [ref]
Species Concepts and Species Delimitation by Kevin de Queiroz. pp. 880–883, Table 1, Figure 1 and “Conceptualization versus Delimitation”: distinguish defining properties from operational evidence
The biological species concept groups naturally interbreeding biological populations with viable, fertile offspring, distinguished from other such groups by reproductive isolation. An operational test examines mating, fertilization, offspring survival and offspring reproduction, rather than only whether two adults can be paired. Sexual reproduction is its scope: fossils and organisms studied only through asexual reproduction cannot supply the same mating evidence. [ref]
Biology 2e by Mary Ann Clark and others. §18.2, “Species and the Ability to Reproduce”, “Speciation” and “Reproductive Isolation”
For an original hypothetical example, imagine two separated biological populations that reproduce successfully after contact. The separation alone did not demonstrate reproductive isolation. Conversely, failure to mate in one unfamiliar chamber could reflect the assay conditions. Field observations of gene flow and tests under suitable conditions address the biological question more directly. A successful laboratory pairing does not establish frequent interbreeding in nature.
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. [ref]
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
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.
Phylogenetic species concepts use ancestry and diagnostic inherited differences; versions differ over requiring a clade, an exclusive ancestry pattern at genetic loci, or a distinguishable character combination. State which criterion is intended. [ref]
Species Concepts and Species Delimitation by Kevin de Queiroz. p. 880, Table 1: distinct monophyletic, genealogical and diagnosable versions of phylogenetic criteria
For an original operational example, suppose a phylogenetic tree inferred from one genetic locus separates two sampled biological populations. Compare further genetic loci and additional local samples before interpreting that branch as a species boundary. Otherwise the test may merely identify the particular ancestry of the sampled alleles. Adding more sites from the same genetic locus improves that locus's evidence without creating an independent ancestry test.
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