Introduction to Classification of Organisms

Lessons

Senior 1 · Biology

Introduction to Classification of Organisms

5 lessons

Lesson 1 of 5

Why we classify: the hierarchy from kingdom to species

25 min

How do you find one thing among millions?

Walk into Kimironko market in Kigali. Fruit is in one area and clothes in another. Inside the fruit area, bananas are kept apart from avocados, and cooking bananas (ibitoke) are kept apart from sweet bananas (imineke). Because things are grouped, you can find what you want in minutes.

Biologists face the same problem on a far bigger scale. About two million kinds of organism have been named so far, and Nyungwe forest alone shelters 13 kinds of primate and more than 300 kinds of bird. Without an agreed way of grouping them, no one could find information about any of them.

Activity: group these organisms

Work in pairs. Sort these twelve organisms from around a school into groups, and write down the feature you used for each group: bean plant, maize, eucalyptus tree, mushroom, goat, chicken, housefly, spider, earthworm, frog, bread mould, human.

Compare your groups with another pair. You will almost certainly disagree: one pair may put the mushroom with the plants because it grows in soil, another may group animals by "has legs" and "has no legs". This is the reason biologists need one agreed system, based on features that really show how organisms are related.

Definition

Classification: arranging organisms into groups according to the features they share.

Taxonomy: the science of describing, naming and classifying organisms. A person who does it is a taxonomist.

Taxon (plural taxa): any one group in a classification, for example the family Hominidae or the genus Gorilla.

Definition

Hierarchical classification: a system of groups within groups. Each organism is placed in seven main ranks, from the largest to the smallest:

kingdom → phylum → class → order → family → genus → species

Each rank contains one or more groups of the rank below it. (For plants, many books call the phylum a division.)

Definition

Species: a group of organisms that are similar to one another and can breed together to produce fertile offspring (offspring that can themselves breed). Example: all domestic dogs, from the smallest breed to the largest, belong to one species, Canis familiaris.

The seven ranks

Figure 1: The classification hierarchy. Each rank sits inside the one above it. Humans and mountain gorillas share five ranks, from kingdom down to family.

Moving down the hierarchy, each group contains fewer kinds of organism, and its members share more features. The kingdom Animalia includes sponges, insects and elephants, which have little in common. The genus Gorilla contains only the two kinds of gorilla, which are alike in almost every way.

Rank	Lion	Leopard	Domestic cat	Domestic dog
Kingdom	Animalia	Animalia	Animalia	Animalia
Phylum	Chordata	Chordata	Chordata	Chordata
Class	Mammalia	Mammalia	Mammalia	Mammalia
Order	Carnivora	Carnivora	Carnivora	Carnivora
Family	Felidae	Felidae	Felidae	Canidae
Genus	Panthera	Panthera	Felis	Canis
Species	Panthera leo	Panthera pardus	Felis catus	Canis familiaris

Worked example 1: which is the closer relative?

Using the table, is a lion more closely related to a domestic cat or to a domestic dog?

Find the lowest rank the lion shares with the cat. Both are in the family Felidae (but different genera).
Find the lowest rank the lion shares with the dog. They part at family (Felidae and Canidae), so the lowest shared rank is the order Carnivora.
Compare. Family is lower in the hierarchy than order, so the lion and cat share more features.

Answer: the lion is more closely related to the domestic cat. The rule: the lower the rank two organisms share, the more closely related they are.

Worked example 2: placing a new organism

A taxonomist finds a mammal in Nyungwe that is clearly a monkey. Which ranks can she fill in straight away, and which need careful study?

All monkeys are animals with a backbone that feed their young on milk, so kingdom Animalia, phylum Chordata and class Mammalia are certain.
Monkeys, apes and humans form the order Primates, so the order is also certain.
Family, genus and species need detailed comparison with known monkeys: teeth, skull, fur pattern and DNA. If it matches no known species, she will describe and name a new one.

Answer: the four highest ranks are quick to fill; the lower the rank, the more evidence is needed.

Warning

Common mistakes. (1) Mixing up the order of the ranks, especially order and family: family is the smaller group, inside order. (2) Thinking a "higher" rank means a more advanced organism: it only means a bigger group. (3) Giving the species as one word: the species name of the lion is Panthera leo, not just "leo".

Why classification matters

It makes the variety of life manageable: learn the features of a group once, and they apply to every member.
It helps identification: an unknown organism can be matched to its group.
It shows relationships: organisms in the same genus share a recent common ancestor.
It allows prediction: a new plant in the same family as the bean (Fabaceae) will probably have pods and root nodules that enrich the soil with nitrogen.
It gives one international language: scientists in Kigali, Kampala and Paris mean the same organism by the same name.
It guides conservation: knowing that the mountain gorilla is a distinct kind of gorilla, found only in the Virunga volcanoes and Bwindi forest, is why it is protected so strictly.

Classification (Amoeba Sisters). This video adds a rank above kingdom, the domain, which you will meet in later years. Watch for the memory sentence it uses for the order of the ranks.

Tip

Memory hook: "King Philip Came Over For Good Soup" gives kingdom, phylum, class, order, family, genus, species. Or make your own in Kinyarwanda or English.

✦

Practice: the classification hierarchy

Define classification and taxonomy.
List the seven ranks from largest to smallest.
Which rank contains more kinds of organism, a class or a genus? Explain.
Give three reasons why biologists classify organisms.
Using the table, name the lowest rank shared by the leopard and the domestic cat.
Humans and gorillas share the family Hominidae; humans and lions share only the class Mammalia. Which animal is the closer relative of humans? Explain.
Two animals are in the same genus. What else must be true about their family, order and class? Explain.

Show answersHide answers

Classification is arranging organisms into groups by shared features. Taxonomy is the science of describing, naming and classifying organisms.
Kingdom, phylum, class, order, family, genus, species.
A class. It is higher in the hierarchy, so it contains many orders, each with many families and genera.
Any three: makes the variety of life easier to study; helps identify organisms; shows relationships; allows predictions about features; gives an international naming system; helps conservation.
Family (Felidae).
The gorilla, because the shared rank (family) is lower than class; organisms that share lower ranks share more features.
They must be in the same family, order and class too, because each genus sits entirely inside one family, which sits inside one order, and so on.

Key points

Classification groups organisms by shared features; taxonomy names and classifies them.
The seven ranks, largest to smallest: kingdom, phylum, class, order, family, genus, species.
The lower the rank two organisms share, the more closely related they are.

Sources and credits

12.1: Organizing Life on Earth, from Concepts of Biology by OpenStax (Samantha Fowler, Rebecca Roush, James Wise), via LibreTexts Biology, licensed under CC BY 4.0. Adapted by Ganzaa.
Photo: A lion and a lioness in Akagera National Park, Gilson1111, CC BY-SA 4.0, via Wikimedia Commons.
Video: Classification, Amoeba Sisters, embedded from YouTube.

Figure 1, the table, the activity, the worked examples and the practice questions are written by Ganzaa.

1How do you find one thing among millions?

2Activity: group these organisms

3The seven ranks

4Why classification matters

How do you find one thing among millions?

Activity: group these organisms

The seven ranks

Why classification matters