Biology · Classification. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.
There are millions of kinds of organisms, so biologists sort them into groups using the features they share. Organisms in the same group have more features in common, and are more closely related, than organisms in different groups.
The smallest group is the species. Members of one species can breed together and their offspring can also breed. Each species has a two-part scientific name used by scientists in every country. This is the binomial system.
A dichotomous key identifies an unknown organism. Each step gives two opposite statements. You choose the one that fits and follow it to the next step until you reach a name.
Saying two animals are the same species because they can mate and have young. The young must be fertile too. A horse and a donkey produce a mule, but mules cannot breed, so horses and donkeys are different species.
Answer: Salmo trutta and Salmo salar are most closely related; two genera are shown.
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