Classifying Living Things: Choosing the Basis

A brief note on Classifying Living Things: Choosing the Basis. Try each question at the end before you open its answer.

Why we classify

• It makes the study of millions of kinds easier: study one group, learn about all its members.
• It shows how living things are related, and tells us about the history of life.
• It gives every living thing one scientific name, used all over the world.

A good basis and a poor basis

A good basis is a deep feature, which goes with many other features and does not change much. Ask in order:

1. A true nucleus or not?
2. One cell or many?
3. A cell wall or not?
4. Makes its own food, or gets it from others?
5. How is the body built?

Flying is a poor basis: insects, birds and bats all fly, but they are not related. Habitat is poor too: a whale lives in water, but it is a mammal.

The levels and the names

• From the largest: kingdom, phylum, class, order, family, genus, species.
• Living things that share more levels are more closely related.
• A scientific name has two parts, Genus species: the genus with a capital letter, the species with a small one. Example: Mangifera indica, the mango.

The five kingdoms (Whittaker, 1969)

Measuring diversity

Simpson's index: $D = 1 - \sum \left(\dfrac{n}{N}\right)^2$. Near $0$: one species dominates. Larger: the numbers are even.

A worked example

A school bird table has $6$ mynas, $3$ bulbuls and $1$ sunbird.

1. The total. $N = 6 + 3 + 1 = 10$.
2. The shares. $0.6$, $0.3$ and $0.1$.
3. The squares. $0.36 + 0.09 + 0.01 = 0.46$.
4. The index. $D = 1 - 0.46 = 0.54$.

Practice with new numbers

A program to try

The program works out the worked example.

Mistakes to avoid

• Using one feature only, or the habitat. Look at the deep features of the body.
• "Mushrooms are plants." A mushroom has no chlorophyll and feeds on dead matter. It is a fungus.
• Writing the species with a capital letter. Only the genus starts with a capital.
• Forgetting the "1 minus" in Simpson's index.

Check yourself

1. A bat flies, as a bird does. Which features show that it is a mammal, and why do they matter more than flying?

2. Two organisms share a genus. Two others share only a class. Which pair is more closely related?

3. You sort six animals by where they live, then by whether they have a backbone. Are the groups the same?

4. Two meadows each have $4$ species and $40$ insects. In one, the four counts are equal. Which meadow has the larger index?