Two organisms belong to the same species if they are able to produce fertile offspring
Two organisms belong to the same species if they can mate and produce offspring that are themselves able to reproduce. Producing offspring is not enough — that offspring must also be fertile.
Real World
A mule — the offspring of a horse and a donkey — is strong and healthy but sterile because it inherits an unmatched set of chromosomes (63 instead of an even 64 or 62), so horses and donkeys are classified as separate species despite being able to mate.
Exam Focus
Never write 'produce offspring' without the word 'fertile'; dropping that word loses the definition mark every time.
Evaluation Scaffold
A four-step framework for high-quality evaluation. Use this for 'assess', 'evaluate', and 'to what extent' questions.
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