Topic 1.2 · Core
Species
A single word in this definition decides the whole thing, and it’s worth learning the definition with that word intact rather than paraphrasing it away.
A species is a group of organisms that can reproduce to produce fertile offspring. The word fertileis doing the real work here. Fertile offspring are themselves capable of reproducing and producing another generation — so the definition isn’t just about whether two organisms can mate and produce young, it’s about whether that lineage can actually continue.
Members of the same species don’t have to look identical to each other. Variation between individuals is completely normal within a single species — humans differ enormously in visible characteristics such as height, skin tone and hair colour, and still belong to exactly one species. That gives you a rule worth keeping in your back pocket for exam questions that show you strikingly different-looking organisms: variation between individuals does not automatically mean that they belong to different species. The test that actually decides the question is whether they can produce fertile offspring together, not whether they resemble each other.
This is the biological foundation everything else in classification sits on top of. Once organisms are grouped into species on this basis, the binomial system gives each species a name that scientists everywhere can use consistently, and a dichotomous key can help you work out which species an unfamiliar organism actually belongs to.