Identification and Classification Of Living Things

Section 1 of 7Why classify?

Making sense of diversity

• Making sense of diversity Millions of species exist, so scientists sort them into groups based on shared features.
• A common system lets scientists in every country communicate clearly.

Shared features and ancestry

• Shared features and ancestry Organisms in the same group share features and are more closely related, descending from a recent common ancestor.

Changing methods

• Changing methods Early classification used visible features only;
  • DNA sequencing now shows relationships more reliably and has led to changes in some groupings.

Section 2 of 7Levels of the hierarchy

1Kingdom

• Kingdom The largest group, such as animals or plants.

2Phylum, class, order

• Phylum, class, order Groups become smaller and share more features at each step.

3Family and genus

• Family and genus Closely related organisms are grouped here.

4Species

• Species Organisms that can interbreed to produce fertile offspring.

Section 3 of 7Binomial naming

1

Every species has a two-part Latin name: the genus first, then the species.

2

The genus begins with a capital letter and the species name with a lower-case letter, for example Homo sapiens.

3

In print the name is in italics; handwritten, it is underlined.

4

The system gives one internationally agreed name, avoiding confusion caused by different common names.

5

Two species in the same genus, such as the lion and the tiger, are more closely related than two in different genera.

Section 4 of 7Three-domain system

Domain	Cell type	Examples
Archaea	Prokaryotes living in extreme conditions such as hot springs	Methane-producing microbes
Bacteria	True prokaryotes	Salmonella, Escherichia coli
Eukarya	Cells with a nucleus	Animals, plants, fungi, protists

Section 5 of 7Using a key

Dichotomous key

Paired statements

• Paired statements Each step offers two choices about an observable feature, such as legs or wings.

Follow the path

• Follow the path Choose the statement that fits the specimen and go to the number or name it points to.

Check carefully

• Check carefully Use clear, measurable features and verify the final identification against its description.

Worked example

• Worked example 1a Has six legs: go to 2.
  • 1b Has eight legs: spider.
  • At 2, wings present leads to a fly and no wings to an ant.

Section 6 of 7Key terms

Term	Meaning
Species	A group of organisms that can interbreed to produce fertile offspring.
Genus	A group of closely related species.
Binomial system	Naming organisms with a genus name followed by a species name.
Dichotomous key	A series of paired statements used to identify organisms.
Domain	The highest level of classification, above kingdom.
Prokaryote	An organism whose cells have no nucleus.

Section 7 of 7Exam technique

1

Writing a binomial name with a capital letter on the species word or without italics or underline.

2

Saying a species is a group that looks the same; the key test is fertile offspring.

3

Assuming classification never changes, when new DNA evidence has revised many groups.

4

Using vague features such as 'big' or 'nice colour' in a key instead of measurable ones.

5

Thinking a horse and a donkey are one species. Their offspring, the mule, is sterile.