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Transcript
TYPES OF NATURAL SELECTION Pgs 324-325 EVOLUTION •Evolution is the change in a population’s genetic material (alleles) over generations •Genotype frequencies do not stay the same •A population that is not evolving is in genetic equilibrium (HardyWeinberg equilibrium) •Genotype frequencies stay the same THREE TYPES OF NATURAL SELECTION •Organisms best suited to their environment live to reproduce and pass on their genes •Acts on a phenotype •Varying types of selection 1. DIRECTIONAL SELECTION •Individuals with a more extreme form of trait have higher fitness •Shifts normal distribution to the right or left Graph Key Red: before selection event Blue: after selection event Suppose termites in an area begin to build deeper nests. Anteaters with long tongues could more effectively prey on termites than those with short or average tongue length 2. STABILIZING SELECTION •Having average form of trait has highest fitness •Normal distribution becomes narrower, see less individuals at the extremes Blue curve shows variation after a new predator is introduced. Predator can easily capture the large, visible lizards and the small slower lizards. Thus, selection against these extremes body types reduces the size range in lizards. 3. DISRUPTIVE SELECTION •Individuals with any extreme trait has a higher fitness and most likely results in two new species •Now have 2 curves in distribution •We also call this divergent evolution The white limpets blend in with barnacles on rocks. On bare rocks, dark-shelled limpets blend in. However, the tan colored limpets are easy to spot by birds. PRACTICE- ON YOUR WHITE BOARDS Oyster shells come in a variety of colors ranging from light brown to dark brown, with most being medium brown. The rocks where they live are medium brown in color. Some birds fly into the area and start eating oysters. How will the distribution curve of oyster shell color change? PRACTICE ON YOUR YOUR WHITE BOARDS Light-colored oysters in blend into the lighter colored rocks by the shore. The dark-colored oyster blend into the shadows cast by larger rocks on sunny days. A group of birds flies in and settles in this area. How will the distribution curve change? PRACTICE- ON YOUR WHITE BOARDS Oysters have shells of different strengths. Most oysters have medium strength shells, but some have thin, weak shells, and a few have thick strong shells. A population of crabs moves into the area and they are able to open weak and medium strength shells. How will the distribution of shell strength change?