Ask students: How is inheritance involved in natural selection?
You might also want to use the following scenario as a formative assessment: Many years ago, on an island off the coast of Florida, there was a population of primarily brown mice that lived in brown grass by the beach. Slowly the light-colored beach sand expanded into the grassy areas. Now there are two colors of mice on the island. Some mice live on sand dunes and are light-colored, while brown mice still live in the grass next to the beach. Ask students to give a scientific explanation of how the light-colored beach mice population developed. Encourage them not to use the word ‘adapt’. For example, a low level answer is “The mice adapted by becoming brown.” A higher level answer is, “There must have been some light-colored mice among the brown mice, and the light-colored ones would be less likely to be eaten by sand dune predators, so they could pass their genes to their offspring. This kept repeating for many generations until they were all light-colored.”
Tell students that they will take a short side trip from the evolution of mud worms to look at other examples of evolution, starting with the rock pocket mice that live in the deserts of the U.S. southwest. Students are often unclear about the meaning of the term “trait.” As a review, you may want to ask students to define the word and ask for examples of traits.
- Show slides #1-16 of the PowerPoint presentation, “Natural Selection at Foundry Cove.”
- Pause at slide #14, which asks: “How did natural selection change the population of pocket mice, beetles, or drug-resistant bacteria?” Encourage students to be specific and descriptive as they explain how natural selection works.
- Introduce the next activity with slides #15-16 while distributing student worksheets.
- Have students complete questions #1-5. Discuss their answers as a class.
- Students complete the diagram: Pass out a colored pencil to each student, and instruct them to color-code the resistant worm shown in the key and in the “First Generation as Young.” They should then complete the diagram according to the events written in the arrows. (See diagram below for an example).
- Depending on the level of your students, you may want to use slides #17-20 to slowly walk through the steps of the Natural Selection Process Diagram. We recommend asking the students to try working through the diagram alone or in groups first and then using the PowerPoint to provide support.
- The diagram is intended to provide students with a visual model for the evolution of an invisible trait (e.g. cadmium resistance). Three generations of worms are shown.
- Students complete their worksheets.
- Lead a group discussion of how natural selection led to the evolution of cadmium resistance. This should give you insight about gaps in students’ understanding of evolution.
- Students may use imprecise language, such as “the worms became ‘immune’ to cadmium or ‘wanted/needed’ to become resistant, alerting you to their misunderstandings or misconceptions.
- See the explain section or the Understanding Evolution website listed in the Materials section.
The PowerPoint includes four terms (in bold below) each of which is an important component of natural selection
- existing genetic variation in the species;
- heritable (here, of the fur color trait);
- differential success (differential survival and reproductive success in dark versus light mice);
- occurring through time—i.e. across multiple generations.
About 1000 years ago, a volcanic eruption in the American southwest spread lava over a wide area. The population of rock pocket mice living on the lava evolved from a mostly light-colored to a mostly-dark colored population in less than 1000 years. Though not as rapid as the evolution in Foundry Cove worms, this is still relatively fast evolution.
**Be explicit about the initial existing variation in the population to help students understand that the genetic color variation is already present before dark color becomes an asset. Students may think that evolution occurs because the mice “need” or “want” dark fur. Ask students to explain how the population of mice (in locations with lava) changed from mostly light to mostly dark. Help students to understand that it is the population of mice that changed—not the individual mice that changed. The first two examples shown in the PowerPoint illustrate natural selection of visible traits (color). The following two examples involve the evolution of invisible traits: drug-resistance in bacteria and cadmium-resistance in mud worms.
Some of the Limnodrilus hoffmeisteri mud worms in Foundry Cove were resistant to cadmium even before the Marathon Battery factory began to dump their waste into the cove. The cadmium-resistance trait was already present, meaning that there was existing genetic variation in the worms’ tolerance to cadmium. Once cadmium was discharged into the cove, cadmium-resistant worms were more likely than others to survive, reproduce, and pass their resistance genes to their offspring. Cadmium was first dumped into Foundry Cove in 1952, and the resistant worms were discovered in the 1980’s, so the evolution of cadmium-resistance occurred quickly, in about 30 years or about 30 generations.
Though students may understand the concept of multiple alleles for one gene, it is difficult for many to grasp the concept of existing variation. As mentioned above in relationship to the pocket mice, organisms don’t acquire a mutation because they want or need it. Though some forms of pollution can themselves cause mutations to occur, it is highly unlikely that damage to DNA caused by a pollutant would result in a mutation thatprotect the organism.
- Extend student knowledge by asking them to apply what they learned to make hypotheses about the evolution (or lack thereof) of another Foundry Cove animal—muskrats. See slide 21 for prompt and discussion questions.
- Completed diagrams, questions #6-7 on the worksheet, and student answers from discussions can be used to assess student understanding.
- Students can show their knowledge of natural selection by creating a representation or model. This might be a labeled sketch, their own version of a process diagram, a PowerPoint, or other means of communicating the concept of natural selection.