Classification of living organisms

Biology · Inheritance, variation and evolution. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

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The idea

Classification puts living things into groups. Linnaeus grouped organisms by their structure and characteristics. His system has seven levels, from large groups to small ones: kingdom, phylum, class, order, family, genus, species.

Every organism has a two-part Latin name, called its binomial name. The first word is the genus and the second is the species.

Better microscopes and the study of biochemical processes inside cells gave new evidence. Using chemical analysis, Carl Woese proposed the three-domain system: Archaea, Bacteria and Eukaryota. Evolutionary trees show how scientists think organisms are related, using classification data for living species and fossil data for extinct ones.

Rules to remember

Order of groups: kingdom → phylum → class → order → family → genus → species.
Binomial name = genus then species, e.g. Homo sapiens: genus Homo, species sapiens.
Archaea: primitive bacteria, usually living in extreme environments.
Bacteria: true bacteria.
Eukaryota: protists, fungi, plants and animals (cells with a nucleus).
On an evolutionary tree, each branch point is a common ancestor. The more recent the shared branch point, the more closely related the two species are.

Common mistake

On an evolutionary tree, species drawn next to each other are not always the closest relatives. Trace the lines back and find the most recent common ancestor.

Worked example

A tiger is in the family Felidae, the genus Panthera and the species tigris. What is its binomial name, and which of these three groups contains the most kinds of organism?

A binomial name is the genus followed by the species: Panthera tigris.
The family is not part of the name.
The order of groups is family → genus → species, from larger to smaller.
A family is made of several genera, so it holds more kinds of organism than one genus or one species.

Answer: Panthera tigris; the family Felidae is the largest group.

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