Why we classify, and the basis of classification

So many living things

Look around a pond, a forest or even a garden. Insects, birds, worms, grass, trees, mushrooms. Scientists think that there may be about $9$ million kinds of living things on Earth. Only about $2$ million of them have been named. The variety of living things is called biodiversity.

How can anyone study so many kinds? We sort them into groups. Sorting living things into groups by their features is called classification.

Why classify?

• It makes the study easier. We can study one group, and learn about all its members.
• It shows how living things are related to each other.
• It lets us give every living thing a name that scientists everywhere can use.
• It tells us about the history of life. The groups with the most in common came from a common ancestor not long ago.

Think of a big library. If the books are in a heap, it takes a long time to find one. If they are on shelves by subject, you find a book in a minute.

What do we use to sort them?

We cannot sort by size, colour or one habit alone. A bat and a bird both fly, but they belong to different groups. We look at features that tell us about the basic plan of the body. We ask questions in an order, and the biggest differences come first.

1. Does the cell have a true nucleus? Cells without a true nucleus are prokaryotic (bacteria). Cells with a true nucleus are eukaryotic.
2. Is the body one cell, or many cells?
3. Does the cell have a cell wall?
4. How does it get food? Does it make its own food by photosynthesis (an autotroph), or get it from others (a heterotroph)?
5. How is the body built? Does it have organs and organ systems? Is there a backbone?

The features that came first in the history of life are more basic. A living thing with a nucleus, for example, came after one with no nucleus.

Measuring biodiversity

A forest with many kinds of living things in similar numbers is more diverse than a field where one kind is common. Simpson's diversity index puts a number to this.

Here $n$ is the number of individuals of one species, and $N$ is the total number of individuals. A value near $1$ means high diversity. A value near $0$ means that one species dominates.

You should find that the index is largest when all four species have the same number. With $15$ of each, $D = 0.75$. If one species has all the individuals, $D = 0$.

Try it in Python

This program finds the diversity index of a forest and of a farm field. Each has $100$ individuals of $4$ species.

Practise with new numbers

Common mistakes

• Sorting by one habit. Flying, or living in water, does not decide the group. The basic plan of the body does.
• Thinking a higher index means more individuals. It means the individuals are spread more evenly over the species.
• Forgetting the "1 minus" in Simpson's index. The sum of the squares alone is large when diversity is low.
• Thinking the groups never change. New evidence can move a living thing to another group.

Remember these

• Classification is sorting living things into groups by their features.
• We start with the basic features: nucleus, number of cells, cell wall, way of getting food, and body plan.
• Biodiversity is the variety of living things. Simpson's index $D = 1 - \sum (n/N)^2$ measures it.
• $D$ is near $0$ when one species dominates, and larger when the numbers are even.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. Give two reasons for classifying living things.

2. Is a bacterium prokaryotic or eukaryotic?

3. A habitat has only one species. What is its diversity index?

4. A bat and a sparrow both fly. Why are they not put in the same group?