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Modes of Natural Selection

Modes of Natural Selection. There are 3 types of natural selection that work on populations when environmental change occurs: Stabilizing Directional Disruptive This results in changes in genotype ratios, and thus cause evolution to occur.

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Modes of Natural Selection

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  1. Modes of Natural Selection There are 3 types of natural selection that work on populations when environmental change occurs: Stabilizing Directional Disruptive This results in changes in genotype ratios, and thus cause evolution to occur.

  2. Natural Selection – acts on variation within a population Aa (frequency) aa AA

  3. Stabilizing Selection Extremes are eliminated Example:  Rabbit leg length • Too short - run too slow • Too long - cannot turn sharp corners or get under cover as easily.   • Midsize – just right – most rabbits have

  4. Stabilizing Selection

  5. Directional Selection One of the extremes is selected for, the other against Example: Horse leg length • Longer legs are better to outrun predators

  6. Directional Selection

  7. Disruptive Selection Both extremes are selected for Example: Coat color of mice • Light colored mice live in light environments • Dark colored mice live in dark environments • Medium Colored Mice are Not common

  8. Disruptive Selection

  9. Few Light, Few Dark, Mostly Medium Colored The Medium is eliminated Only Medium Either Light or Dark Mostly Dark

  10. Identify the type of selection: Anteaters have not always had the extremely long tongues they now use.  Over the last 10 million years, their tongues have become impressively long.  Which type of selection probably is working here? Answer: One extreme eliminated Directional

  11. Rabbits have a temperament that allows them to sit still most of the time, but also allows them to get up and move before a predator gets too close.  Rabbits that wait too long are removed from the gene pool.  Rabbits that tend to move all the time are spotted by predators and eliminated with ease. Answer: Both extremes eliminated Stabilizing

  12. Oak trees have differentiated into species that produce many sizes of acorns.  If there is a population of oak trees that has variation in acorn size, a new squirrel population moves into the community and tends to select the medium sized acorns. Answer: Average is eliminated Disruptive

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