Natural Selection in Protected and Unprotected Populations

Author: Crissy Huffard

Overview: In this lesson, students will learn how two elephant seal populations fare during successive generations. One population is protected from mortality by human interference, while the other is not. The unprotected population loses valuable variation when a few key individuals are killed.

Lesson concepts:

  1. Variation is good! If a population loses some of this variation, it is more vulnerable to environmental changes than a population with more variation.

  2. There is no perfect set of traits. What is good in one year (one environment) may not be good in the next.

  3. Traits are passed on to younger generations.

Grade span: 6-8 or 9-12

Advanced preparation:
Prepare two (equal) sets of Seal ID cards (see appendix) allowing for one card per student, equal numbers of males and females if possible, and spare cards for “making babies”

Time: 50 minutes

Grouping: Divide the class in two (each group gets the same set of cards). If the class is small (less than 30), then you may want to run through each simulation together as one group.

Teacher background:
The Theory of Natural Selection has some basic ideas:

  1. Living things produce more offspring than the environment can support.

  2. Living things face a constant struggle for existence.

  3. Individuals in a population vary in their physical characteristics (phenotypes).

  4. Some of this variation is inherited from their parents (and passed on to their offspring), that is, it is a reflection of variations in the genes (genotype).

  5. Those variants with the combinations of traits that suits the conditions of their life better than others are most likely to survive and reproduce themselves ("survival of the fittest").

To the extent that their combination of traits is heritable, those traits will be passed on to their offspring. Bigger populations tend to have more variation, and thus are better buffered against environmental change than smaller (less diverse) populations.

Teaching tips:
Have one instructor/helper with each group to double check mating pairs, dead animals. Have groups arrange their chairs so they can talk easily.

Procedure:

  1. Divide the class into two groups of equal size.

  2. Give each group a set of cards. The sets should start off the same, and have an equal number of males and females if possible (see example set).

  3. Make a table on the chalkboard with the columns: protected population, unprotected population; sub-columns in each column: adults, juveniles, babies; and the rows: year 1, 2, etc.

  4. Have students look over their cards. Make sure they know their traits, sex.

  5. For each year, describe the environment and which traits will lead to the death of an individual (see examples below).

  6. Move dead students to another part of the room (such as behind the group they worked with). They will be in charge of the babies/juveniles for their group.

  7. Remaining males and females (per group) will pair up and produce one offspring. Make a card for this offspring by letting students play “scissors, paper, rock” to decide which parent’s trait makes it into the offspring (once per trait- don’t forget sex). Mark the offspring year in the upper right hand corner.

  8. Tally the adults and babies.

  9. Give the baby cards to the dead to keep track of.

  10. Kill someone from the unprotected group. I tried to kill someone who I knew was not going to die according to the subsequent environments.

  11. Repeat steps 5-9, describing the environment, pairing mates, making baby cards, and tallying adults, babies, and now juveniles (babies of previous years). Juveniles are subject to the same risks as adults, they just don’t get to mate (yet).

  12. Kill someone in the unprotected population every few years.

  13. After four or five generations (you decide based on class size- small class, shorter generation time), juveniles get to join the adult population.

  14. Repeat until ten minutes before class is over.

  15. Count how many seals are in each population. Compare.

  16. After the simulation, have a class discussion. Ask questions like “Was there one perfect seal?” “What happened to the unprotected population?” Try to lead the students to come up with the ideas such as 1) variation is good; 2) variation is inherited; and 3) the environment changes. Discuss why bigger populations are better, and why it’s important to protect natural populations.

Example environments/results:

  1. Cold winter. Any seal with thin blubber and thin fur dies.

  2. Sea urchin disease. Any seal that eats only urchins dies. (this might be a seal that’s weak and slow and can’t catch fish).

  3. Not much food this year either. If a small female is paired with an extra large male, then the female dies. Male survives, no young produced by that pair. (you will now have a skewed sex ratio, and some males will not be able to mate during the next generations- they are still alive, but make no babies).

  4. Hot summer. Seals with thick fur and thick blubber die.

  5. Anchovy population crashes (favorite food of young- keep in mind I’m making this up), and young must rely entirely on breast milk. Young nursed for 8 months or less die.

Optional: If you want to get fancy and introduce the idea of dominant and recessive traits, then put D and r next to each trait on the card (be consistent), and explain how to figure out offspring traits from the D’s and r’s the parents have.

Example Cards:

Male
Thin blubber
Thick fur
Small as adult
Eats urchins only


Female
Thin blubber
Thin fur
Nurses young for 1 year
Medium size
Eats fish and urchins
Male
Thin blubber
Thin fur
Medium-sized as adult
Eats fish and urchins


Female
Thin blubber
Thick fur
Nurses young for 6 months
Small size
Eats fish and urchins
Male
Thick blubber
Thin fur
Medium-sized as adult
Eats fish and urchins


Female
Thick blubber
Thin fur
Nurses young for 9 months
Eats urchins only
Male
Thick blubber
Thick fur
Extra-Large as an adult
Eats fish and urchins


Female
Thin blubber
Thick fur
Nurses young for 8 months
Medium size
Eats fish and urchins
**Male
Thin blubber
Thick fur
Large as adult
Eats fish and urchins


Female
Thick blubber
Thin fur
Nurses young for 8 months
Small size
Eats fish and urchins
Male
Thick blubber
Thin fur
Small as adult
Eats fish and urchins


Female
Thin blubber
Thick fur
Nurses young for 1 year
Large size
Eats fish and urchins
Male
Thick blubber
Thin fur
Large as an adult
Eats fish and urchins


Female
Thick blubber
Thick fur
Nurses young for 10 months
Large size
Eats fish and urchins
 
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