How Biologists Classify Species: Similarities and Differences

To create a new classification group, characteristics must be identified which are found in every member of that group. The size or colour of an animal may vary quite a lot but the skeleton or the teeth always have the same characteristics. As a result it would not be a good idea to group together all animals which are of a certain colour.

Cats, for example, can be grey, black, brown, or even striped. They can have long hair or short hair. Their skulls, however, will always have the same characteristics as the one shown in the photographs.

The badger's skull on the right looks very similar to the cat's skull but upon close examination it is possible to see that there are slight differences. As a result, it can be concluded that the badger and the cat have a common family tree but that they are different enough to be considered two separate species.

There are, of course, problems. Some organisms do not have the same physical characteristics all through their lives. A seed, for example, will grow and develop into a plant or a tree. If a biologist examines the physical structure of the seed of a species and then examines the physical structure of the adult of that species, the conclusion could easily be that they are not the same. The only way to find out for sure would be to plant the seed and observe how it develops.

This type of difficulty exists not only in plants but also in insects. The young form of an insect, called a larva, is often very different in structure and physical appearance from its adult form. A good example is a caterpillar and a butterfly. Both are from the same species but they look very different. Some species have similar characteristics but they are not closely related. The zebra has a physical characteristic similar to the tiger. Such a coincidence does not necessarily mean that they are related.

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