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Natural Selection One of the most important contributions made to the science of evolution by Charles Darwin is the concept of natural selection. The idea that members of a species compete with each other for resources and that individuals that are better adapted to their lifestyle have a better chance of surviving to reproduce revolutionized the field of evolution, though it was not accepted until several decades after Darwin first proposed it. Today, natural selection forms the foundation for our understanding of how species change over time. Natural selection may act to change a trait in many ways. When environmental pressures favor the average form of the trait, selection is said to be stabilizing. Directional selection occurs when selection pressures favor one extreme of the trait distribution. Selection is disruptive when the average form of the trait is selected against while either extreme is unaffected. In addition to natural selection, there are two other types of selection. Sexual selection, which Darwin believed was distinct from natural selection, involves the selection of traits based on their role in courtship and mating. Artificial selection is the selective breeding of species by humans to increase desirable traits, though the traits do not necessarily have to confer greater fitness. Summary of Types of Natural Selection Natural selection can take many forms. To make talking about this easier, we will consider the distribution of traits across a population in using a graph. To the right is a normal curve. For example, if we were talking about height as a trait, we would see that without any selection pressure on this trait, the heights of individuals in a population would vary, with most individuals being of an average height and fewer being extremely short or extremely tall. However, when selection pressures act on a trait, this distribution can be altered. Stabilizing Selection When selective pressures select against the two extremes of a trait, the population experiences stabilizing selection. For example, plant height might be acted on by stabilizing selection. A plant that is too short may not be able to compete with other plants for sunlight. However, extremely tall plants may be more susceptible to wind damage. Combined, these two selection pressures select to maintain plants of medium height. The number of plants of medium height will increase while the numbers of short and tall plants will decrease. Directional Selection In directional selection, one extreme of the trait distribution experiences selection against it. The result is that the population's trait distribution shifts toward the other extreme. In the case of such selection, the mean of the population graph shifts. Using the familiar example of giraffe necks, there was a selection pressure against short necks, since individuals with short necks could not reach as many leaves on which to feed. As a result, the distribution of neck length shifted to favor individuals with long necks. Disruptive Selection In disruptive selection, selection pressures act against individuals in the middle of the trait distribution. The result is a two-peaked, curve in which the two extremes of the curve create their own smaller curves. For example, imagine a plant of extremely variable height that is pollinated by three different pollinators, one that was attracted to short plants, another that preferred plants of medium height and a third that visited only the tallest plants. If the pollinator that preferred plants of medium height disappeared from an area, medium height plants would be selected against and the population would tend toward both short and tall, but not medium height plants. Such a population, in which multiple distinct forms exist can lead to speciation or the creation of two new species. Name: _______________________________ Types of Natural Selection 1. Who came up with the conecpt of Natural selection _______________ 2. Natural selection forms the foundation for understand how species ____________ over time. 3. What are the 5 types of natural selection mentioned in paragraph 2 and 3? _____________________________________ _____________________________________ _____________________________________ _____________________________________ _____________________________________ 4. Without selective pressure on a trait the graph would have a _______________ curve. where most individuals will be average, or in the middle of the graph. 5. When pressure selects against the extremes the graph becomes ______________ 6. When the average trait beomes more common and selective pressures select against the two extremes of a trait we have ___________________________ selection 7. During _________________________ the graph shits to one side or the other 8. When there is selection pressures acting against individuals who are average or in the middle of the trait distribution then the two ___________________ have peaks. 9. What three types will we focus on using graphs? _____________________________________ _____________________________________ _____________________________________ 10. In a population, in which multiple distinct forms exist, ______________________ or the creation of two new species can occur. .
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