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16.2 notes

16.2 notes. Essential Question How does evolution occur?. Vocabulary Preview. Immigration Emigration Gene flow Genetic drift Sexual selection Stabilizing selection Disruptive selection Directional selection. Objectives. List five conditions under which evolution may take place .

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16.2 notes

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  1. 16.2 notes Essential Question How does evolution occur?

  2. Vocabulary Preview • Immigration • Emigration • Gene flow • Genetic drift • Sexual selection • Stabilizing selection • Disruptive selection • Directional selection

  3. Objectives • List five conditions under which evolution may take place. • Explain how migration can affect the genetics of populations. • Explain how genetic drift can affect populations of different sizes. • Contrast the effects of stabilizing selection, directional selection, and disruptive selection on populations over time. • Identify examples of nonrandom mating.

  4. Five Conditions that Cause Evolution • Mutation • Gene flow • Genetic drift • Nonrandom mating • Natural selection

  5. Mutations and Evolution • Evolution is the change in a population’s genetic material (allele frequencies) • A mutation is a change in the DNA structure • Mutations introduce new alleles into a gene pool. • If mutation is beneficial, it will be selected for. • Over time, the frequency and variety of alleles in the population will change. • This change is evolution. • Mutations that are beneficial to the survival of a species are a vital part of evolution.

  6. Evolution and Populations • Evolution may take place when populations are subject to genetic mutations, gene flow, genetic drift, nonrandom mating, or natural selection.

  7. Gene Flow and Gene Frequencies • Population numbers can change due to immigration (movement of species into the population) and emigration (movement of species out of the population). • Gene flow is the process of genes moving from one population to another. • Emigration and immigration cause gene flow between populations and can thus affect gene frequencies.

  8. Genetic Drift • Genetic drift is a change in allele frequencies due to random events. • Genetic drift operates most strongly in small and medium-sized populations. • Because the gene pool is smaller in smaller populations, a particular allele may disappear in a short time – over one – two generations • In larger populations, the allele could be present in enough individuals to be maintained in the population.

  9. Nonrandom Mating • Mating is nonrandom whenever individuals may choose partners. • Mate selection is often influenced by geographic proximity which can amplify certain traits and can result in offspring with disorders caused by recessive genes. • Individuals may also select a mate that has traits similar to their own traits. This mate would probably have some similar genes. • Selection of a mate based on similarity is called assortative mating. • Nonrandom mating also affects which alleles will be combined within individuals, but it does not affect the overall allele frequencies within a population.

  10. Sexual Selection • Sexual selection occurs when certain traits increase an individual’s success at mating. • Sexual selection explains the development of traits that improve reproductive success but that may harm the individual.

  11. Natural Selection • Natural selection can influence evolution in one of three general patterns. • Stabilizing selection • Disruptive selection • Directional selection

  12. Stabilizing Selection • Stabilizing selection favors the formation of average traits having the highest fitness. • Most common kind of selection. • Shows a shift toward the center of the bell curve

  13. Disruptive Selection • Disruptive selection favors extreme variations of a trait rather than individuals with the average form of the trait. • It is a shift in both directions away from the center.

  14. Natural Selection, continued • Disruptive Selection • Disruptive selection favors extreme traits rather than average traits.

  15. Directional Selection • Directional selection favors the formation of more-extreme traits. • Individuals that display a more extreme from of a trait have greater fitness than individuals with an average form of the trait. • It is a shift in one direction only

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