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  1. 1. Lesson Overview 17.2 Evolution as Genetic Change in Populations
  2. 2. Lesson Overview Evolution as Genetic Change in Populations THINK ABOUT IT Insect populations often contain a few individuals that are resistant to a particular pesticide. Those insects pass on their resistance to their offspring and soon the pesticide-resistant offspring dominate the population. The relationship between natural selection and genetics explains how pesticide resistance develops.
  3. 3. Lesson Overview Evolution as Genetic Change in Populations How Natural Selection Works How does natural selection affect single-gene and polygenic traits?
  4. 4. Lesson Overview Evolution as Genetic Change in Populations How Natural Selection Works How does natural selection affect single-gene and polygenic traits? Natural selection on single-gene traits can lead to changes in allele frequencies and, thus, to changes in phenotype frequencies. Natural selection on polygenic traits can affect the distributions of phenotypes in three ways: directional selection, stabilizing selection, or disruptive selection.
  5. 5. Lesson Overview Evolution as Genetic Change in Populations How Natural Selection Works Evolutionary fitness is the success in passing genes to the next generation. Evolutionary adaptation is any genetically controlled trait that increases an individual’s ability to pass along its alleles.
  6. 6. Lesson Overview Evolution as Genetic Change in Populations Natural Selection on Single-Gene Traits Natural selection for a single-gene trait can lead to changes in allele frequencies and then to evolution. For example, a mutation in one gene that determines body color in lizards can affect their lifespan. So if the normal color for lizards is brown, a mutation may produce red and black forms.
  7. 7. Lesson Overview Evolution as Genetic Change in Populations Natural Selection on Single-Gene Traits: The example of Lizard Color
  8. 8. Lesson Overview Evolution as Genetic Change in Populations Natural Selection on Single-Gene Traits If red lizards are more visible to predators, they might be less likely to survive and reproduce. Therefore the allele for red coloring might not become common.
  9. 9. Lesson Overview Evolution as Genetic Change in Populations Natural Selection on Single-Gene Traits Single-Gene Traits: The allele for red coloring might not become common.
  10. 10. Lesson Overview Evolution as Genetic Change in Populations Natural Selection on Single-Gene Traits Black lizards might be able to absorb sunlight. Higher body temperatures may allow the lizards to move faster, escape predators, and reproduce.
  11. 11. Lesson Overview Evolution as Genetic Change in Populations Natural Selection on Single-Gene Traits Single-Gene Traits: The allele for black color might become more common.
  12. 12. Lesson Overview Evolution as Genetic Change in Populations Natural Selection on Polygenic Traits Polygenic traits have a range of phenotypes that often form a bell curve. The fitness of individuals may vary from one end of the curve to the other. Natural selection can affect the range of phenotypes and hence the shape of the bell curve.
  13. 13. Lesson Overview Evolution as Genetic Change in Populations Directional Selection Directional selection occurs when individuals at one end of the curve have higher fitness than individuals in the middle or at the other end. The range of phenotypes shifts because some individuals are more successful at surviving and reproducing than others.
  14. 14. Lesson Overview Evolution as Genetic Change in Populations Directional Selection For example, if only large seeds were available, birds with larger beaks would have an easier time feeding and would be more successful in surviving and passing on genes.
  15. 15. Lesson Overview Evolution as Genetic Change in Populations Stabilizing Selection Stabilizing selection occurs when individuals near the center of the curve have higher fitness than individuals at either end. This situation keeps the center of the curve at its current position, but it narrows the overall graph.
  16. 16. Lesson Overview Evolution as Genetic Change in Populations Stabilizing Selection For example, very small and very large babies are less likely to survive than average-sized individuals. The fitness of these smaller or larger babies is therefore lower than that of more average-sized individuals.
  17. 17. Lesson Overview Evolution as Genetic Change in Populations Disruptive Selection Disruptive selection occurs when individuals at the upper and lower ends of the curve have higher fitness than individuals near the middle. Disruptive selection acts against individuals of an intermediate type and can create two distinct phenotypes.
  18. 18. Lesson Overview Evolution as Genetic Change in Populations Disruptive Selection For example, in an area where medium-sized seeds are less common, birds with unusually small or large beaks would have higher fitness. Therefore, the population might split into two groups—one with smaller beaks and one with larger beaks.
  19. 19. Lesson Overview Evolution as Genetic Change in Populations Genetic Drift What is genetic drift?
  20. 20. Lesson Overview Evolution as Genetic Change in Populations Genetic Drift What is genetic drift? In small populations, individuals that carry a particular allele may leave more descendants than other individuals, just by chance. Over time, a series of chance occurrences can cause an allele to become more or less common in a population.
  21. 21. Lesson Overview Evolution as Genetic Change in Populations Genetic Drift Genetic drift occurs in small populations when an allele becomes more or less common simply by chance. Genetic drift is a random change in allele frequency.
  22. 22. Lesson Overview Evolution as Genetic Change in Populations Genetic Bottlenecks The bottleneck effect is a change in allele frequency following a dramatic reduction in the size of a population. For example, a disaster may kill many individuals in a population, and the surviving population’s gene pool may contain different gene frequencies from the original gene pool.
  23. 23. Lesson Overview Evolution as Genetic Change in Populations The Founder Effect The founder effect occurs when allele frequencies change as a result of the migration of a small subgroup of a population.
  24. 24. Lesson Overview Evolution as Genetic Change in Populations The Founder Effect Two groups from a large, diverse population could produce new populations that differ from the original group.
  25. 25. Lesson Overview Evolution as Genetic Change in Populations Evolution Versus Genetic Equilibrium What conditions are required to maintain genetic equilibrium?
  26. 26. Lesson Overview Evolution as Genetic Change in Populations Evolution Versus Genetic Equilibrium What conditions are required to maintain genetic equilibrium? According to the Hardy-Weinberg principle, five conditions are required to maintain genetic equilibrium: (1) The population must be very large; (2) there can be no mutations; (3) there must be random mating; (4) there can be no movement into or out of the population, and (5) no natural selection.
  27. 27. Lesson Overview Evolution as Genetic Change in Populations Evolution Versus Genetic Equilibrium A population is in genetic equilibrium if allele frequencies in the population remain the same. If allele frequencies don’t change, the population will not evolve.
  28. 28. Lesson Overview Evolution as Genetic Change in Populations The Hardy-Weinberg Principle The Hardy-Weinberg principle describes the conditions under which evolution does not occur. The Hardy-Weinberg principle states that allele frequencies in a population remain constant unless one or more factors cause those frequencies to change.
  29. 29. Lesson Overview Evolution as Genetic Change in Populations Large Population Genetic drift can cause changes in allele frequencies in small populations. Genetic drift has less effect on large populations, such as the seals shown. Large population size helps maintain genetic equilibrium.
  30. 30. Lesson Overview Evolution as Genetic Change in Populations No Mutations If mutations occur, new alleles may be introduced into the gene pool, and allele frequencies will change.
  31. 31. Lesson Overview Evolution as Genetic Change in Populations Random Mating All members of the population must have an equal opportunity to produce offspring. Individuals must mate with other members of the population at random. In natural populations, however, mating is not random. Female peacocks, for example, choose mates on the basis of physical characteristics such as brightly patterned tail feathers. Such nonrandom mating means that alleles for those traits are under selection pressure.
  32. 32. Lesson Overview Evolution as Genetic Change in Populations No Movement Into or Out of the Population Individuals who join a population may introduce new alleles into the gene pool. Individuals who leave may remove alleles from the gene pool. Thus, for no alleles to flow into or out of the gene pool, there must be no movement of individuals into or out of a population.
  33. 33. Lesson Overview Evolution as Genetic Change in Populations No Natural Selection All genotypes in the population must have equal probabilities of surviving and reproducing. No phenotype can have a selective advantage over another.
  34. 34. Lesson Overview Evolution as Genetic Change in Populations Sexual Reproduction and Allele Frequency Meiosis and fertilization do not change the relative frequency of alleles in a population. The shuffling of genes during sexual reproduction produces many different gene combinations but does not alter the relative frequencies of alleles in a population.

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