Naming species: the binomial system and DNA

Last time

Before we start: last time, in Ferns, flowering plants, viruses and keys, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.

One animal, many names

In English it is a lion. In Urdu it is a sher, and in Spanish a león. Scientists everywhere need one name that means exactly one kind of organism. That is the scientific name: for the lion, Panthera leo.

What is a species?

A species is a group of organisms that can reproduce to produce fertile offspring. A horse and a donkey can breed, but their offspring, a mule, is infertile. So horses and donkeys are different species. Two dogs of very different breeds can have fertile puppies, so all dogs are one species.

The binomial system

The binomial system is an internationally agreed way of naming organisms using two names. The first name is the genus. It starts with a capital letter. The second name is the species. It starts with a small letter. The whole name is printed in italics, or underlined when you write it by hand.

Reading a scientific name

Look at the lion and the tiger. The lion is Panthera leo, and the tiger is Panthera tigris. They share the first name, Panthera, so they are in the same genus. Their second names differ, so they are different species.

Why classify at all?

Classification sorts organisms into groups using the features they share. The groups go from large to small: kingdom, then down to genus and species. Organisms in the same small group share more features, so they are more closely related.

Features or DNA?

Traditionally, organisms were classified by what they look like: their morphology and anatomy. Now scientists also compare the sequence of bases in DNA, and the sequence of amino acids in proteins. The more similar the base sequences, the more closely related the organisms are. DNA sometimes shows that animals that look alike are not close relatives at all.

Why DNA works

Organisms that share a recent common ancestor have had less time for their DNA to change. So their base sequences are more alike. It is the SEQUENCE, the order of the bases, that is compared. All organisms use the same four bases.

Comparing base sequences

Here are short pieces of DNA from three species. Compare them one base at a time. A and B differ at only one place. A and C differ at four places. So A is more closely related to B.