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Natural Selection

Natural Selection. The Mechanism of Evolution. Artificial Selection. 3 Types of Natural Selection: Directional Selection. Selects for one of the extreme phenotypes - directional selection occurs in response to a change in the environment

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Natural Selection

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  1. Natural Selection The Mechanism of Evolution

  2. Artificial Selection

  3. 3 Types of Natural Selection: Directional Selection • Selects for one of the extreme phenotypes - directional selection occurs in response to a change in the environment that gives a competitive advantage to a particular phenotype

  4. Examples of Directional Selection Bacteria developing resistance to antibiotics The beak sizes of ground finches on Daphne Island

  5. 3 Types of Natural Selection: Stabilizing Selection • Selects for the average phenotype and against the extreme phenotypes • Occurs when the environment is stable for long periods • Example birth weight of babies.

  6. Stabilizing Selection

  7. 3 Types of Natural Selection: Disruptive Selection • Selects against the average and selects both extremes. • Example: African Seed Crackers

  8. African Seed Crackers Birds feed on two types of seeds one large, one small. Birds with average size bills can’t eat either type efficiently And so aren’t common in the population

  9. Genetic Drift: Another Force for Change • Genetic Drift = Changes in a population’s gene pool that happen by chance - its effect is most profound in small populations - it reduces genetic variation in these populations - this could reduce a population’s ability to survive environmental change

  10. Genetic Drift Continued • An organism can be well suited to its environment can be removed by chance • The result: Organisms that aren’t as well adapted can survive to reproduce by chance - this shifts the gene pool of the population

  11. Genetic Drift • Once an allele is removed from a population, it is unlikely to return

  12. Genetic Drift – The Bottle-neck effect • Something happens to catastrophically reduce a population’s size for at least one generation • Dramatically changes allele frequencies in a small population

  13. Bottle necked species • Elephant seals hunted to near extinction – 20 individuals - which 20 survived? The best adapted or the least desireable? Was their survival random luck? -now number 30,000 but all individuals are descended from the 20 survivors -little genetic diversity this population has a reduced ability to survive an environmental challenge

  14. Founder Effect • Occurs when a small population migrates to a new area or becomes isolated • Limited gene pool – over time some generally rare traits become more common - Huntington’s disease in South Africa - Amish communities • FE most commonly recognized in genetic disorders – occurs in other traits as well

  15. Genetic Drift as a force for change • In small isolated populations - genetic drift could allow otherwise rare alleles to become common - this could cause the isolated population to diverge from the main population and over time become a new species

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