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Natural Selection: Assignment: Choose ONE animal species. Your goal is to describe the way in which an evolutionary change might occur for a particular characteristic (trait) of that species as a result of natural selection. The characteristic could be something like coloration pattern, length of the limbs, or size of the teeth or beak, or any measurable trait that is inherited. The organism you choose should be a real one but the evolutionary change you describe can be real or theoretical. In your own words, a. Explain how the change in this trait occurs in terms of the four postulates listed above and in the book. You will describe what the population starts out like at time zero and then what it will look like at a later time, as a result of natural selection. I am choosing the peppered moth (Biston betularia), and I’d like to talk about its coloration patterns and how natural selection works on the frequency of different color variations. The evolutionary change that I want to use is the example of the change in frequency of light vs dark moths in pre-industrial, industrial revolution, and current times. The population of the moths that I am discussing are in England. (There are other populations in Europe, America, and Japan.) There are several variations in the population, but the one that we’ll be talking about is the colored morphs (colored forms) of the moth. One morph is the light colored moth; the other form has dark coloration of its body and wings. There is one gene, with two alleles, that code for color – so the coloration is inheritable. (The ‘dark’ gene, that codes for increased melanin, is dominant over the lighter version.) (Postulates 1 and 2) The moths rest in trees during the day, and the males fly in search of females at night. Birds prey on the moths as they rest in the trees, and the ones that can hide better and avoid predation are more likely to reproduce. The color morph is directly related to how well they can hide, and therefore how well they can reproduce. (Postulates 3 and 4) Prior to the Industrial Revoultion in England, most trees were lighter in color – a combination of their bark and the lichens that grew on them. Most of the moth population was the “typical” or light/speckled morph. Darker (“carbonaria”) forms were far less common, making up less than 10% of the population. Call this Time Zero. During the Industrial Revolution, massive coal burning released large quantities of soot, which coated the trees, killing the lichens and making the trunks much darker. The lighter morph stood out more (could be seen better by the birds), while the dark version could hide better. During this time, the frequencies of light to dark morphs changed dramatically – with more than 90% of the population in the soot-covered areas of England being the dark morph. Call this Time One. As the Industrial Age has advanced, pollution controls have become much more stringent; and fuels other than coal have come into use. As a result, there has been less soot coating the trees, and less pollution killing the lichens. The trees have gotten lighter in color again. The light colored moths are better able to hide against these cleaner trees than the darker morph. The frequency of the light/dark morphs has changed again. At this time, call it Time Two, the percentage of light colored morphs has increased, and the percentage of dark morphs has decreased. b. Make sure to explain the process by which the changes occurred. What is the selective pressure? Explain why the change might confer more fitness to a particular environment? Since birds prey on the moths as they rest in the trees, their color is important in helping them avoid visual detection by the birds. It is important for the color of the moth to match the color of the trees in which they are hiding. If the color of the tree changes, we would expect the color of the moth to affect its fitness (its ability to survive and produce more offspring. At a genetic level, the frequency of the light / dark alleles would change, depending on the ability of the moth that carried those genes to survive and reproduce.) At Time Zero, with light colored, lichen covered trees, the light morph was better able to blend in against the tree trunks. Dark morphs stood out, and were much easier for birds to see, and so eat. Dark morphs were selected against, and the dark allele was a very low percentage of the gene pool. At Time One, the trees became very dark colored, and the situation reversed: Light morphs stood out against the dark trees, and were easily seen by the birds (and eaten.) Dark morphs were much harder to see against the dark background – so the dark morph was able to survive and reproduce more often, therefore increasing the frequency of the allele that coded for more melanin production. The gene frequency changed, with a greater proportion of the “dark allele”. At Time Two, trees have become lighter again (as soot washed off, and as older trees died and newer one grew). The darker morph does not have as strong of an advantage in hiding against these lighter colored trees, and so is being selected against by increased predation pressure. The gene frequencies are again changing, with the lighter colored allele (and so the lighter colored moth morph) increasing in frequency, and the darker colored allele decreasing in frequency. Since, at its most basic level, evolution is simply a change in gene frequency, we can see how Natural Selection (the ability to survive predation) has driven a change in the gene frequencies for moth body color in the peppered moth (Biston betularia) in England. Tips: Make sure to include the following in your answers to a and b above: Clearly state the name of the SPECIES you are going to be talking about for this assignment. (Make sure you know what is meant by a species.) Also, clearly state what population of the species you are going to talk about by specifying its geographic location. (Make sure you understand what a “population” is.) Clearly state the characteristic (trait) of the population that you are going to be talking about and explain how this trait varies among individuals. Explain what change occurs in the environment that causes a change in the above characteristic (trait)?