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N is for Natural Selection


To investigate the biological adaptations that produced the modern-day whale.


Life Science, Grades 58, Diversity and Adaptations of Organisms: Biological adaptations include changes in structures, behaviors, or physiology that enhance survival and reproductive success in a particular environment.


120 minutes (including research time)


  • paper

  • pencils

  • research materials

(including access to the Internet)


Have students research the difference between toothed and baleen whales. Which evolved first? (Toothed.) What pressure might have led to the evolution of baleen whales? (Abundance of plankton as a food source.)

The Mesonychid


  • Review with students what they learned in N is for Natural Selectionabout how natural selection favors some characteristics over others and how it can affect populations of organisms over time.

  • Show students a picture of a whale, toothed (including dolphins) or baleen. Explain that, like most animals, whales evolved from an ancestor that bore little resemblance to the organisms we see today.

  • Have students research what evidence suggests is the ancestor of todays whales, both toothed and baleen. Students might check out Enchanted Learning.com (www.enchantedlearning.com) and click Zoom Whalesand then Whale Evolution.Or go to Darwin Day Web (fp.bio.utk.edu/darwin/) and search for whale.Then click Whale Transitional Fossil Evidence.

  • Students should encounter the animal Mesonychid in their research. Have students list the physical, behavioral, and physiological characteristics of Mesonychid. Then have them create a list of the same characteristics for their favorite modern whale.

  • Once students have completed their research, tack pictures of both the Mesonychid and examples of modern whales (toothed and baleen) on the board as a visual reference for students as they begin step 6.

  • Challenge students to describe a scenario whereby the Mesonychid could have evolved step by step over millions of years to become a modern whale. Which structures and behaviors would have to change and how would they change? (As the species began spending more time in the water and less on land, their bodies became more streamlined. Eventually the species developed fleshy appendages, including a dorsal fin and horizontal fluke, to help them swim better. Rear appendages were no longer needed, and arm and hand bones shortened and thickened into paddles. The vertebral column became less stiff so it could undulate. Two nostrils became one, and moved to the top of the head so the animal could breathe while swimming along the waters surface. The animal developed blubber to keep it warm and a sleek skin as hair was no longer needed for warmth. Physiological adaptations included an increased blood volume and a suspension of the automatic breathing response experienced by most mammals. Both adaptations allowed the animal to make deeper, longer dives.) Students may want to create labeled diagrams of modern whales showing each of these adaptations.

    • Finally, have students speculate as to why

the Mesonychid might have returned to the sea after having evolved from it originally. (Mostly likely, the animal returned to the sea for the abundance of food it provided.)

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