Natural Selection

Ask students to speculate what is going on in the illustration.

Tell them that it represents a fictional case of an enclosure for rescued otters and its unintended impact on salmon reproduction.

Students will design a solution to the problem and write a persuasive letter that advocates for their design. They will use what they’ve learned about natural selection in their letter.

Read the first three paragraphs on the handout "Fish Fence Crisis" to the students.

Then pose this question:

What do you think will happen to the salmon population over several generations if only the smaller salmon are able to breed?

Ask students to write a response to the question.

Sample Response:

Because the larger salmon are blocked by the otter fence, they will not be able to breed. As a result, only the smaller salmon will be able to pass on their traits during reproduction. Over time, the river will see a reduced number of larger salmon, and if there is not a solution that allows for these larger salmon to reproduce, they will eventually disappear from the river.

Note that some students may think that larger salmon are that way just because they are older than smaller salmon. Actually, salmon live to spawn only once, and then die. So all the salmon in a particular spawning group are about the same age. Just as there are taller and shorter people, salmon have genes that influence how large they will grow.

Read the next two paragraphs at the bottom of page 1 with the class. Be sure that students understand the controversy and the need for a solution to this problem.

For page 2, place students in pairs and ask them to design a solution to the problem. Then ask students to present their solution to another pair.

Possible design solutions (students may come up with others):

• Rebuild the fence so that it encloses only one bank of the river (leaving the other side of the river open to all salmon).
• Leave the existing fence but dig a channel for the salmon that bypasses the fenced section of the river.

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Ask students to each write a letter using their solution or the solution of the pair of students that they shared with.

All students should use their knowledge of natural selection (or artificial selection) in their letter.