| Curriculum Notebook - Grade 9 | ||||||||||||||||
Interpretive Stops
1. Along Aptos Creek
Additional Information |
Microevolution - Using the "Peppered-Moth"
Grades 9-12 Standard No. 7: Variation within a species increases the likelihood that at least some members of a species will survive under changed environmental conditions
Natural selection is the change in a population due to expressed genetic traits. These genetic traits are expressed because they have helped the species to survive in some way -- by helping the species live in a harsh abiotic environment or perhaps by helping the species respond better when competing for food or escaping predation. Over a long period of time, natural selection may result in macroevolution, where a new species is developed. Over a short period of time, within our lifetimes for instance, natural selection results in microevolution, where no new species is developed, but shifts in genotype and phenotype are evident. How does natural selection determine the survival of different groups of organisms? It's best to look at examples of natural selection to understand the concept. One of the most famous examples is the change in melanism seen in the peppered moth of the British Isles during the industrial revolution of the late 19th-century. The peppered moth (Biston betularia), expressed a color phenotype (appearance) from gray with black flecks to black with variations of color in between. Typically, these moths rested on light-colored birch trees. The dark-colored moths were preyed upon since they were easily detected against the light-colored tree bark. Since the less noticeable moths lived to reproduce, their genotype (genetic traits) which coded for light-color were passed along to their offspring and the majority of the population of peppered moths were light-colored. The phenotypically-favored light color of the moth changed, however, during the industrial revolution. Factories using fossil fuels, spewed black soot into the air which landed on streets, buildings, and natural settings, including the birch trees that the peppered moth rested on. Light-colored moths now became easy prey for birds since they were more readily seen against the darkened tree bark. The phenotype of light-color became selected against as birds now preyed upon the light-colored moth. The phenotype of dark color became selected for as dark-colored moths lived longer to reproduce and pass along their genotype of dark color. Was a new species of moth created when the darker ones were favored? No. Both the light-colored and the dark-colored peppered moth were the same species and therefore could mate with each other. Their phenotype was different because of the alternate expression of alleles. Alleles are different versions of the same gene and they occupy the same locus (location) on a set of chromosomes. Alleles give instructions for the species to have certain characteristics (physical or behavioral), like the dark coloring or light coloring of the peppered moth. In fact, it is our knowledge of alleles that allows us to explain why some species, like the domestic dog (Canis familiaris), have so many variations. Another example of natural selection is seen in the deer mouse (Peromyscus maniculatus) population of the United States. Responding to environmental conditions, certain phenotypes have become selected for in the deer mouse allowing it to survive and reproduce. For instance, the dorsal coat color of the deer mouse varies over a wide geographic range and matches the soil where the mouse is active. Matching the substrate permits some mice to escape predation. Those that survive, pass to their offspring the alleles that code for the dorsal fur color that allowed the parent to survive. In addition to a change in dorsal fur, the deer mouse has also expressed differences in hind foot and tail length. In some deer mouse populations, the hind foot is longer, probably owing to habitat where climbing is required and longer foot length is advantageous. As with the peppered moth, the deer mouse did not speciate, or diverge into another species. The slight shift in phenotype and genotype seen in both the moth and mouse populations are examples of microevolution due to natural selection.
1. In Marcel's Forest, use the Pourroy's Picnic Area, Stop No. 16 which has level ground and a picnic table. 2. Bring enough colored peppered-moth cut-outs. (At least twice the number of students). Before coming to the forest, color the cut-outs using four (4) colors from dark brown to a light gray. Color the same number of cut-outs for each color. If you color six cut-outs a dark brown, make sure you color six cut-outs a light brown, etc. Keep track of the colors used and how many peppered-moths you colored. 3. Trees. Red alders, big leave maples, redwoods, and Douglas-firs make up the majority of trees in the Pourroy Picnic area. 4. A whistle for starting and stopping the "hungry birds." 5. Medium-sized writing pad easel and at least six colored writing pens. 6. Enough graph paper and pencils for each group of students to make a graph. (Groups of three or four students are recommended).
1. While students are working elsewhere, have someone else use masking tape on the backsides of the cut-out to hang the peppered-moths on the trees at Stop No. 16. 2. Line students up and tell them that they are now hungry birds that love to eat peppered moths. 3. At the sound of the whistle or the word, "Go," tell students that they have only 15 seconds to eat (gather-up) as many peppered-moths as they can. 4. At the end of 15 seconds, tell the "birds" that they must come back with their prey. 5. Ask students how many moths they ate (captured). What colors were they? 6. Count how many of each color was "eaten," and then write these numbers on the easel pad. 7. Now ask students to form groups of three each and to prepare a graph of the numbers of each color group of peppered-moths preyed upon within the 15 seconds. 8. Simple bar graphs are the easiest to prepare; however line graphs would be appropriate, also. The "X" axis of the graph could be the colors, while the "Y" axis might represent the numbers of moths found. Remind students to label the axes and to begin think about what their graph represents. 9. After making the graph, discuss why certain colors of moths were found while other colors of peppered-moths were not. 10. Questions the teacher might ask:
11. Tell students about the actual changes in peppered-moth melanism seen in 19th-century industrial England (Read the above Concept Background). |
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Concept Background
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