School activities: Evolutionary tree of mammals

Introduction

This course provides an introduction to the evolution of mammals.

We will be considering Darwin's observations on a great many mammals, and how he noticed that species fell into natural groups. We take as an example the evolution of one particularly interesting mammal, the whale, and look at evidence both from fossils and from DNA to see which other mammals are most closely related to whales. We see how the evidence from these two very different sources points to the same relationship and that this therefore provides overwhelming evidence that the conclusions must be correct.

This OpenLearn course provides a sample of Level 1 study in Arts and Humanities.

Learning outcomes

After studying this course, you should be able to:

• develop an appreciation of the huge variety of different mammals that exist on Earth today

• see how fossil evidence can help us to understand evolutionary history

• understand how the structure of DNA can help us to detect differences between different species

• apply the techniques of DNA analysis to work out which mammals are most closely related to each other

• appreciate the importance of bringing together evidence from different sources.

1 Natural groups

Darwin made extensive observations on a great many creatures, including mammals, and noticed that species fell into natural groups, e.g. lions, tigers and leopards have many similarities, and resemble cats. On the basis of his observations, he was able to place mammals in distinct groups.

His work has continued, and we now recognise that mammals have evolved from a common ancestor, and have branched into many different groups, or ‘Orders’. Figure 1 shows the different Orders of mammals.

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Figure 1

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Evolutionary tree showing the origin of the main mammalian groups, each of the different Orders of mammals, and the approximate number of species in each Order.

Figure 1

What does an evolutionary tree like this really show?

To understand what the tree shows, we need to look at the different branches on the tree, and how long ago they had their origins.

It shows that Monotremes are on a separate branch of the tree, and that they branched off from the branch which led to other mammals more than 100 million years ago.

Marsupials are on a separate branch which originated between 60 and 100 million years ago.

All other mammals fall into four main groups, shown in Figure 1 in purple, red, blue and green, and all the main Orders of mammals were already present by 60 million years ago.

2 Odd one out

The image below shows models of four mammals:

• Rhinoceros

• Whale

• Elephant

• Hippopotamus

Figure 2

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Models of four mammals, rhinoceros, whale, elephant and hippopotamus.

Figure 2

Which of these four do you think is the ‘odd one out’? To help you to think about the differences, try answering the following two questions.

You may alternatively suggest that the rhino is the ‘odd one out’ as it is the most endangered, or the elephant as it has a trunk. There can be a good argument for any answer. However, to find out which one is the most different from the others in evolutionary terms, look again at the evolutionary tree. See which branch each is on, and which one branched off first.

Now we will look at some fossil evidence.

Figure 3 provides a summary of the evolutionary origin of whales and dolphins.

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Figure 3

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Some representative mammals in the evolutionary history of modern whales. These are not drawn to scale (measurements refer to body length) and the dashed lines do not reflect direct descent. Shown here are imaginative reconstructions of the fossil species, suggesting how they might have looked in life.

Figure 3

Click on the link for a printable version of Figure 3

If you look closely at the skeletons of modern whales shown in Figure 3, you can see that the residual ankle bone is just still visible there.

Here we have seen that animals which look most similar are not necessarily the most closely related and that fossil evidence can give us clues about an animal's ancestry.

3 DNA: Spot the difference

Here we look at DNA, the molecule which contains the instructions for making each living creature. It is contained within the genes of every individual living thing on Earth. Closely related creatures have DNA that is very similar, and distantly related creatures have DNA that is very different. By looking at how similar or different their DNA molecules are, we can see how closely related two species are.

Activity 1

We'll start by looking at the models of two short DNA molecules shown below. Each model resembles a twisted ladder or ‘double helix’ and the rungs of the ladder are made of pairs of bases, here shown in colours green, yellow, red and blue, and you will see that the green one always pairs with the yellow one and that the red one always pairs with the blue one. It is the order, or ‘sequence’ of these coloured bases which is crucial. There is one difference between the molecules represented here. Can you spot it?

Figure 4

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Two models representing the same short stretch of double stranded DNA, but with one base difference between the two.

Figure 4

Click on the link below for a printable version of the sequence of coloured bases in the model.

Spot the difference

In the diagram it is as if the model had been untwisted, so that the rungs of the ladder can be seen more clearly. Here the bases are shown not only in colour but also using the letters by which they are commonly known.

You will see that (adenine) always pairs with (thymine) and that (cytosine) always pairs with (guanine).

A difference like this represents a mutation. Such differences accumulate over time, so creatures which are on different branches of an evolutionary tree have more differences in their DNA than creatures on the same branch, which are more closely related.

This means that if we look at the similarities and differences between the DNA of different creatures, we can tell which ones are most closely related.

Now look back at the evolutionary tree of mammals. You will see that the whale, the mammals in the Order Artiodactyla (hippo, camel, cow, giraffe, etc.) and the zebra and tapir are all in the ‘blue’ group.

The mouse is a rodent, in the Order Rodentia, the rabbit is in the Order Lagomorpha, and the human is in the Order of Primates. All these are in the green group, and have a more distant evolutionary origin from those in the blue group. Therefore it is not surprising that their DNA is more different from the whale than the others.

Here we have seen that whales are closely related to four-legged land mammals such as the hippo, camel, cow and giraffe. The evidence for this comes from both DNA sequences and from fossils. These are two very different sources of evidence, and both point to the same conclusions, which means that the evidence for these conclusions is overwhelmingly strong.

Conclusion

This free course provided an introduction to studying the arts and humanities. It took you through a series of exercises designed to develop your approach to study and learning at a distance and helped to improve your confidence as an independent learner.

Acknowledgements

Except for third party materials and otherwise stated (see terms and conditions), this content is made available under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 Licence.

The material acknowledged below is Proprietary and used under licence (not subject to Creative Commons Licence). Grateful acknowledgement is made to the following sources for permission to reproduce material in this free course:

• The Evolutionary Tree was adapted by Janet Haresnape from a figure from: What is a Mammal? in The New Encyclopedia of Mammals, edited by David MacDonald which forms part of the S182 course ‘Studying Mammals’, and the Graphic version of this was created by Martin Scott and Linda Hurst.
• Haresnape J. M. Darwin and evolution: a set of activities based on the evolution of mammals, School Science Review 91 (337), 2010, 75-86. Published by the Association for Science Education (ASE).
• DNA Sequences: from Gatesy, J., Hayashi, C., Cronin., M.A. and Arctander, P. (1996) Evidence from milk casein genes that cetaceans are close relatives of hippopotamid actiodactyls. Molecular Biology and Evolution, 13, 954-963.

Every effort has been made to contact copyright owners. If any have been inadvertently overlooked, the publishers will be pleased to make the necessary arrangements at the first opportunity.

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