Lab 3 Natural Selection

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Feb 20, 2024

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Populations and Natural Selection The Peppered Moth ( Biston betularia ) has genetically controlled variation in their coloration and can range from nearly all black to white with black spots. Information and images above are from: http://cyberbridge.mcb.harvard.edu/evolution_2.html The main predators of the peppered moth are birds, which are highly visual predators. The moths are active at night and rest on trees during the day. Predators are more likely to see prey if they stand out from the background. Thus, moths that are well camouflaged against the tree will be more likely to survive to reproduction and pass on their genes to the next generation than individuals that stand out. Put another way, individuals that are camouflaged have higher reproductive fitness and greater chance of contributing their alleles to the future population. If this natural selective pressure is strong enough, the frequencies of the alleles controlling coloration will change in the population with each generation leading to change in the relative abundance of the phenotypes in the population. A population can have any number of individuals with different genotypes, and consequently with different phenotypes. The relative abundance of dark-colored and light- colored moths in a population will change as the result of the differential survival and reportion of moths with different phenotypes that survive to pass on their alleles to the next generation. Each parent contributes one of their alleles for coloration to each of their offspring. For a light- colored offspring to result, both parents have to contribute an ‘a’ allele to that individual. Like humans, the peppered moth is a diploid (2n) organism, meaning there are two copies of each gene locus. So, an individual in a population can have a genotype of AA, Aa, or aa. The ‘A’ allele for dark-colored wings is dominant over the ‘a’ allele for light-colored wings. Moths with a genotype of AA or Aa will have a dark wing phenotype, while moths with a genotype of aa will have a light wing phenotype. In this example, 3 different genotypes (AA, Aa and aa) result in 2 different phenotypes: dark-colored and light-colored wings.

1. Which moth is best camouflaged in this picture? (1 pt) ( https://alexhyde.photoshelter.com/image/I0000DyMGnqGwJHk The moth with the dark-colored wings is best camouflaged. 2. Given the background in this image, moths of which coloration will have higher reproductive fitness? (1 pt) https://alexhyde.photoshelter.com/image/I0000CBnr4dUjKb4 Given the background of the image, the moth with light-colored wings will have higher reproductive fitness. Selective pressures can change as environments change and individuals that had low reproductive fitness in one environment may have higher fitness in another environment. Peppered moth populations in England experienced a major change in their environment during the industrial revolution, which began in ~1835. Prior to the industrial revolution, trees were covered in light-colored lichens but as the industrial revolution progressed the lichens began to die off and expose dark-colored bark. Due to the change in background, light colored moths were selected against despite having higher fitness prior to the industrial revolution. Later in 1955, with decreased pollution, the lichens began to regrow. In this lab, we will simulate the phenotypic and genotypic changes in the moth populations over time as changes in selective pressures change which individuals have higher reproductive fitness. You will play the role of the bird. Navigate to the simulation here: https://virtualbiologylab.org/NetWebHTML_FilesJan2016/IndustrialMelanismModel.html At the start, it is 1830 and the trees are covered in lichen because it is before industrialization. There are 20 moths on the “tree” in the right panel. To run the model, set your foraging time for 5 seconds and click “go.” A bird will appear in the “tree” panel, which you control as your computer cursor. Click on the moths to eat them. Eat as many as you can in the 5 seconds. When 5 seconds is up, the bird disappears. On the left, you can see your “catch rate” which tells you how successful you were at catching moths. You can also read the phenotype and allele frequencies of the remaining moths – these moths will have the chance to pass on their genes to the next generation. Click “go” again and the model creates 20 new moths based on the genotypes of the potential parents and the years are advanced by 2. After clicking “go”, your bird appears, you forage, and repeat until you reach 2020. The lichens begin to die off in 1835 and start to grow back in 1955. You will need to record your data at various points in the simulation using the table on the next page. You will then use your data to graph changes in genotype and phenotype frequencies in the moth populations over time.

3. Fill in this data table as you advance your simulation. (Required for making the plots.) Phenotype Frequencies Allele Frequencies Year Dark Light Dark (A) Light (a) 1830 0.056 0.944 0.05 0.95 1840 0.143 0.857 0.1 0.9 1850 0.071 0.929 0.05 0.95 1860 0.286 0.714 0.12 0.88 1870 0.5 0.5 0.15 0.85 1880 0.517 0.429 0.17 0.83 1890 0.846 0.154 0.42 0.58 1900 0.769 0.213 0.583 0.417 1910 0.846 0.154 0.5 0.5 1920 0.923 0.077 0.68 0.32 1930 1 0 0.71 0.29 1940 1 0 067 0.33 1950 0.923 0.077 0.5 0.5 1960 0.923 0.077 0.5 0.5 1970 0.786 0.214 0.71 0.29 1980 0.6 0.4 0.33 0.67 1990 0 1 0.06 0.94 2000 0.176 0.824 0.07 0.93 2010 0 1 0.05 0.95 2020 0 1 0.05 0.9 Copy and paste your data into Google Sheets, or another spreadsheet program.

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