18.1.1—Species concepts

A species concept is a rule for deciding which organisms should be grouped as one species. Different concepts answer different questions, so classification must match the evidence available.

Biological species concept — asks whether organisms with similar morphological and physiological features can interbreed and produce fertile offspring. Its boundary is reproductive evidence, but testing fertility can be difficult and time-consuming; it is also limited when organisms cannot be tested directly.

Morphological species concept — asks whether organisms share physical features that distinguish them from other groups. It is practical when morphology is observable, but appearance alone may not reveal reproductive or ecological differences.

Ecological species concept — groups similar organisms living in the same area at the same time. Its boundary is ecological context, so it should not be treated as a fertility test or as a purely visual definition.

Use the evidence in this order:

1. Identify whether the question gives reproductive, morphological or ecological evidence.
2. Match that evidence to the corresponding concept.
3. State the conclusion with its boundary: a concept supports a classification decision, but no single concept works universally.
4. Do not infer a new species from one visible difference alone when the relevant reproductive or ecological evidence is missing.

Phylogenetic definitions are not added here because the matched SME page supports biological, morphological and ecological concepts, not a separate phylogenetic treatment. Do not move from this card into domains, kingdoms or viruses.