0610 · 1.2Concept and uses of classification systems

Classification and the species concept

0:00Imagine walking into a library with millions of books, and none of them are on shelves. They are just piled on the floor in no order at all. Finding the one book you need would be almost impossible. Biologists face the same problem with living things. Scientists have already described well over a million different kinds of organism, and new ones are found every year. To make sense of them, we need a way to sort them into groups and give each one a name that everyone understands.

0:24By the end of this video you will understand how organisms are classified into groups, what a species is, how the binomial system names every species, and how to build and use a dichotomous key to identify an organism. For the extended syllabus, you will also see why modern classification aims to show evolutionary relationships, and how scientists use DNA to work those relationships out. The basic idea is simple.

0:49The syllabus says that organisms can be classified into groups by the features that they share. A feature is something about an organism that you can observe or measure, such as whether it has feathers, how many legs it has, or whether it makes its own food. Every bird has feathers, so feathers are a feature that birds share, and we can use it to put all the birds into one group.

1:08Biologists use features like these to sort organisms into big groups, and then split each big group into smaller and smaller groups. The biggest groups are called kingdoms, such as the animal kingdom and the plant kingdom. The smallest group is the species. You will look at the kingdoms and the features that define them in the next video. For now, let us focus on that smallest group, the species, because it is the one every name is built on.

1:31The syllabus describes a species as a group of organisms that can reproduce to produce fertile offspring. The key word there is fertile. Fertile offspring are young that can grow up and have young of their own. So two lions can mate, and their cubs can grow up and have cubs too. Lions are all one species. Here is why that word fertile matters. A horse and a donkey can mate, and they produce a mule.

1:55So they can reproduce together. But a mule is almost always infertile. It cannot have young of its own. Because horses and donkeys cannot produce fertile offspring together, they are two different species. This also shows why you cannot sort species by looks alone. A tiny chihuahua and a huge great dane look very different, but they can breed and produce fertile puppies, so they are the same species. The test is whether they can produce fertile offspring, not whether they look alike.

Binomial species names

2:22Now, how do we name a species? In everyday life we use common names, like robin or daisy. The trouble is that common names cause confusion. The bird called a robin in Britain is a small bird with an orange chest. The bird called a robin in North America is a completely different, larger bird. And every language has its own word for each animal, so a scientist in Brazil and a scientist in Japan would use different names for the same organism.

2:48To solve this, biologists use the binomial system. The syllabus describes the binomial system of naming species as an internationally agreed system in which the scientific name of an organism is made up of two parts showing the genus and species. Binomial simply means two names. The first part is the genus. A genus is a group of closely related species. The second part is the species. Together, the two parts name exactly one species, and every scientist in the world uses that same name.

3:19The lion is Panthera leo, and the tiger is Panthera tigris. They share the genus name Panthera, which tells you straight away that they are closely related big cats. But the second part is different, so they are different species. Humans are Homo sapiens. Homo is our genus, and sapiens is our species. There are some rules for writing these names, and examiners do check them. The genus starts with a capital letter.

3:45The species name starts with a small letter, even if it comes from a place or a person's name. When the name is typed, it is written in italics. When you handwrite it in an exam, you cannot write in italics, so you underline it instead. A common mistake is writing both words with capital letters. Homo sapiens has a capital H and a small s. So we have a way of naming organisms.

4:07But suppose you find an organism you do not recognise. How do you find out what it is?

Building a dichotomous key

4:12This is where a dichotomous key comes in. The syllabus asks you to construct and use dichotomous keys based on identifiable features. Dichotomous means dividing into two. A dichotomous key is a series of steps, and at each step you are given two choices about one feature of the organism. You pick the choice that matches, and that tells you which step to go to next. Step by step, you narrow the possibilities down until only one organism is left, and the key gives you its name.

4:40Let us build one ourselves. Here are five small animals you might find in a garden: a snail, an earthworm, a spider, a butterfly and an ant. Our job is to write a key that could identify any one of them. Start by looking for a feature that splits the whole group into two smaller groups. A good first choice is whether the animal has legs. The snail and the earthworm have no legs.

5:01The spider, the butterfly and the ant all have legs. So the first step of our key gives two statements. Has legs, go to step three. Has no legs, go to step two. Now split each smaller group again. For the animals with no legs, a feature that separates them is a shell. So step two says, has a shell, it is the snail. Has no shell, it is the earthworm.

5:24That side of the key is finished, because every animal on it now has its own name. For the three animals with legs, we can count the legs. The spider has eight legs, while the butterfly and the ant have six. So step three says, has eight legs, it is the spider. Has six legs, go to step four. Finally, step four separates the butterfly and the ant using wings.

5:46Has wings, it is the butterfly. Has no wings, it is the ant. That is the finished key. Notice something about it. We had five animals, and we needed four steps. A key always needs one step fewer than the number of organisms, because each step separates out one more organism. That gives you a quick way to check your own key in an exam. Now let us use the key.

6:07Say you find a small creature with eight legs. Start at step one, always at step one. Does it have legs? Yes, so go to step three. Does it have eight legs? Yes, so the key tells you it is the spider. You only followed two steps, and you never had to look at the parts of the key about shells or wings.

Rules for effective keys

6:25When you construct a key, there are some rules that make it work. First, use features that you can actually see on the organism, such as legs, wings, shells or the shape of a leaf. Do not use things like where the animal lives, what it eats or how it behaves, because you cannot tell those by looking at one organism in front of you. Second, avoid vague words like big, small, long or short.

6:47Big compared to what? Two people could look at the same animal and disagree. If size really is the best feature, give a measurement, such as longer than 5 centimetres. Third, the two statements at each step should be a matched pair, about the same feature, like has wings and has no wings. Then every organism fits exactly one of them.

Evolution, relationships and DNA

7:06Now for the extended part of the syllabus. Why do biologists classify organisms in the way they do? The syllabus says that classification systems aim to reflect evolutionary relationships. Every species alive today has evolved from earlier species over millions of years. Two species that evolved from the same ancestor are related, a bit like cousins in a family. Biologists try to put organisms that are closely related into the same group, so that the groups are like branches of one huge family tree.

7:34The trouble is that looks can be misleading. A whale and a shark have a similar body shape, with fins and a streamlined body, because both live in water. But a whale is a mammal. It breathes air and feeds its young on milk. Its closest living relatives are actually animals such as the hippopotamus. So similar features do not always mean a close relationship. To find out how closely organisms are really related, scientists look at something deeper than their appearance, their DNA.

8:02DNA is the genetic material found in every living cell. It is a long chain, and along that chain is a sequence of four chemicals called bases, which are given the letters A, T, C and G. The order of these bases is called the base sequence, and it carries the instructions for building the organism. The syllabus says that the sequences of bases in DNA are used as a means of classification.

8:23Here is the reason it works. When an organism reproduces, it copies its DNA and passes it on. Every so often, a small change happens in the base sequence, and that change is passed on to later generations. When one ancestor species splits into two new species, each new species starts collecting its own changes, separately from the other. The longer ago the two species split apart, the more changes each has collected, and the more different their DNA becomes.

8:49That leads to the final syllabus point. Groups of organisms which share a more recent ancestor, which means they are more closely related, have base sequences in DNA that are more similar than those that share only a distant ancestor. Humans and chimpanzees shared an ancestor only a few million years ago, so their DNA is very nearly the same. Humans and mice share a much more distant ancestor, so there are many more differences.

DNA comparison exam example

9:14Let us work through an exam-style example. Scientists compared a section of DNA, 20 bases long, from species A with the same section from three other species. Species B differs from species A in 2 places. Species C differs in 7 places, and species D differs in 12 places. Which species is most closely related to species A, and why? Start by looking for the smallest number of differences. Species B has only 2, so its base sequence is the most similar to species A's.

9:43A more similar base sequence means fewer changes have built up since the two species split, so they share a more recent common ancestor. That means species B is the most closely related to species A. Species D, with 12 differences, is the most distantly related, because it shares only a distant ancestor with A. In the exam, the full answer has two parts. First name the species, species B.

10:08Then explain, it has the fewest differences in its base sequence, so it has the most similar DNA, so it shares the most recent common ancestor with species A. A common mistake is to say that species B looks the most similar. The question is about DNA, so your answer must be about the base sequence. Watch out for one more mistake. Students sometimes say that species with similar DNA are the same species.

10:32They are not. Lions and tigers have very similar DNA, but they are still two species. Similar DNA tells you how closely related two species are, not that they are one species. Let us recap. Organisms can be classified into groups by the features that they share. A species is a group of organisms that can reproduce to produce fertile offspring, which is why the horse and the donkey, whose mule cannot breed, are different species.

10:57The binomial system is an internationally agreed system that names each species with two parts, the genus and the species, with a capital letter only on the genus, and underlined when handwritten. A dichotomous key identifies an organism through a series of steps, each with two choices about one identifiable feature. Build it from features you can see, avoid vague words like big and small, and expect one step fewer than the number of organisms.

11:23For the extended syllabus, classification systems aim to reflect evolutionary relationships. Scientists compare the sequences of bases in DNA, and the more recently two groups shared an ancestor, the more similar their base sequences are. In the next video, we will look at the features used to place organisms into kingdoms and into the main groups of animals and plants.