How Natural Selection Works

This video introduces natural selection, preparing students to evaluate the examples in the next activity.

Project video to the whole class. You may wish to pause the video during the segments that show graphs of change over time (around 1:25 and 2:14) and explain what the graphs are showing.

In preparation for the next activity (CSI: Changing Species Investigation), revisit the list of conditions for the shift in butterfly traits your students generated following the Shape Shifters activity. Begin a discussion of what natural selection is and what it isn’t, and have students refine the lists (individually or as a class) so they reflect the key criteria for natural selection: individuals within a population with heritable trait variations, that affect their chances of (surviving and) reproducing.

• In natural selection, inherited traits that favor reproduction are passed down. Over many generations, those traits become more common while traits that do not favor reproduction become less common.
• Individuals that reproduce successfully pass their genes, and the resulting traits, to the next generation.
• Natural selection acts on many traits at once over many generations.

How Natural Selection Works (video)

Does natural selection produce perfect results every time? (no, it can only work with preexisting genetic variation)