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Polygenic traits traits controlled by more than one gene that show continuous variation are susceptible to environmental influences, as a shift in the environment can lead to a corresponding shift in the phenotypes of organisms. Natural selection can affect these phenotypes in one of three ways… Let’s imagine Darwin’s finches on the Galapagos Islands as an example. There is slight variation in beak length among the finches, ranging from… -small beaks (for obtaining small seeds) -medium beaks (for obtaining medium seeds) -large beaks (for harder, thick-shelled seeds) Imagine that the environment changes and the supply of small and medium-sized seeds runs low. Which beak size will be favored? Those birds with the larger beak will be able to survive on the large seeds! In “directional selection,” natural selection favors a characteristic on the extreme of the phenotypic variation. How does this affect variation? Variation of beak size shifts in ONE direction. Original distribution of beak sizes How distribution of beak sizes change due to a changing environment Imagine that the environment changes and the supply of small and large-sized seeds runs low. Which beak size will be favored now? Those birds with the medium beak will be able to survive on the medium seeds! In “stabilizing selection,” natural selection favors characteristics near the middle of the phenotypic distribution. How does this affect variation? Variation of beak size shifts in towards the middle, decreasing genetic variation at the extremes. Imagine that the environment changes and only the supply of medium-sized seeds runs low. Which beak size will be favored now? Those birds with BOTH the smaller and larger beak will be able to survive! In “disruptive selection,” natural selection favors characteristics at BOTH extremes of the phenotypic variation. How does this affect variation? Variation shifts to favor two subgroups of birds specialized in eating different sized seeds. Original distribution of beak sizes How distribution of beak sizes change due to a changing environment In a population of plants, flowers with the brightest color might be selected for in order to attract the most pollinators whereas plants of moderate color or dull color are selected against. There are two types of seeds to eat for a population of birds. Either of two different beak shapes (sharp or blunt) might be selected for, but a beak that's the average of the two shapes might not be particularly good at eating either seed, so it would be selected against. Bright coloration of many species of birds evolved over time to help males find a mate Human height varies, however most humans tend to be of average height. Abnormally tall or short humans are rare The African butterfly, Pseudacraea eurytus, has colorations ranging from a reddish yellow to blue. Different ends of the color spectrum look like (mimic) other species of butterflies that are not normally the prey of other local predators. Those butterflies that are moderate in coloration are eaten in far greater numbers than those at the extreme ends of the color spectrum Antlers on deer evolved to help males acquire a mate