Download You must email your answers to Mr. Widmer to complete the

You must email your answers to Mr. Widmer to complete the assessment tasks for this “Natural Selection” elective
Chapter 3
Natural selection and evolution
Name: ___________________________
In his book On the Origin of Species
Charles Darwin drew a diagram he called a
‘Great Tree of Life’. The tree diagram
showed how species should be related to
each other if his theory of evolution was
correct. Each branching of the tree showed
where new species evolved because new
structures appeared in the fossil record of
the descendents.
Phylogeny
Darwin’s technique is still used
today. Often hundreds of features are used
to create the tree. Most of these are based
on anatomy. However, now biologists also
have molecular evidence such as DNA
structure to help them. Consider Figure 1
on the following page which shows how
tetrapods may have evolved. (Some of these
species are also discussed in the Activity
Book Activity 2.4.) This diagram shows the
branching tree structure originally
developed by Darwin. This type of diagram is now called a phylogeny.
Phylogeny of tetrapods.
To read the diagram, start at the far left-hand side where the label says ‘Bony skeleton, lungs’.
You proceed through the diagram like you would use a biological key. The first line branches into two
choices: ‘ray-finned fish’ and another branch that is labelled (with a small vertical line) ‘lobe fins’.
What the diagram is saying here is that the ray-finned fish and the lobe-finned fish had an ancestor in
common. That ancestor had a bony skeleton and also had structures similar to lungs. The ancestor split
into fish that evolved into both ray finned fish and into lobe-finned fish. The lobe-finned fish
eventually split into the Coelacanths and another group that then further split. This group evolved into
lungfish and another group which evolved limb bones having a humerus, radius and ulna.
Using a phylogeny like this, biologists are able to deduce which species should be the most
closely related to each other. For example, consider which two of the following are the most closely
related – ray-finned fish, Coelacanth or lungfish. The answer is the Coelacanth and the lungfish. This is
because at one point in time (about 425 million years ago based on the timeline at the top) there were
no Coelacanths or lungfish, only a common ancestor of both. This common ancestor was a lobe-finned
fish that lived at the same time as ray-finned fish.
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You must email your answers to Mr. Widmer to complete the assessment tasks for this “Natural Selection” elective
Questions
1
Explain what Darwin’s Great Tree of Life diagram showed.
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2
Identify a new type of evidence used by modern biologists to create phylogenies.
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3
a Deduce which two of the following are most closely related: Eusthenopteron, Panderichthys
and Tiktaalik.
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b Justify your answer.
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4
a Deduce which two of the following are most closely related: lungfish, Tiktaalik, Ichthyostega
and Tulerpeton.
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b Justify your answer.
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5
Describe some features of the most recent ancestor of Acanthostega and Ichthyostega.
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6
Describe some features that were different between Acanthostega and Ichthyostega.
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