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CHAPTER 23

CHAPTER 23. HOW POPULATIONS EVOLVE. OVERVIEW. Natural selection acts on individuals , but only populations evolve . Genetic variations in populations contribute to evolution.

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CHAPTER 23

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  1. CHAPTER 23 HOW POPULATIONS EVOLVE

  2. OVERVIEW • Natural selection acts on individuals, but only populationsevolve. • Genetic variations in populations contribute to evolution. • Microevolution is defined as a change in allele frequencies in a population over time and represents evolutionary change on its smallest scale.

  3. I. Concept 23.1: Genetic Variation Makes Evolution Possible A. Two processes produce the genetic differences that are the basis of evolution: 1. mutation 2. sexual reproduction B. Genetic Variation 1. Variation in individual genotype leads to variation in individual phenotype 2. Natural selection can only act on variation with a genetic component.

  4. NONHERITABLE VARIATIONDifference in caterpillars of same species is due to diet Caterpillars raised on oak flowers resemble the flowers. Their siblings raised on oak leaves resembled oak twigs.

  5. C. Variation Within a Population 1. Both discrete and quantitative characters contribute to variation within a population 2. Discrete characters can be classified on an either-or basis (Ex: flower color) 3. Quantitative characters vary along a continuum within a population (Ex: polygenic traits like skin color) 4. Population geneticists measure genetic variation in a population by determining the amount of heterozygosity at the genelevel and the molecular level of DNA (nucleotide variability)

  6. 5. Average heterozygosity measures the average percent of loci that are heterozygous in a population (gene variability) 6. Nucleotide variability is measured by comparing the DNA sequences of base-pairs of individuals in a population 7. Average heterozygosity tends to be greater than nucleotide variability

  7. D. Variation Between Populations 1. Most species exhibit geographic variation which results from differences in phenotypes or genotypes between populations or between subgroups of a single population that inhabit different areas 2. Some examples of geographic variation occur as a cline, which is a graded change in a trait along a geographic axis based on some environmental variable like temperature

  8. CLINE

  9. E. Mutation 1. Defined as a change in the nucleotide sequence of DNA 2. Mutations cause new genes and alleles to arise 3. Only mutations in cells that produce gametes can be passed to offspring (germ mutation) 4. A point mutation is a change in one base in a gene 5. The effects of point mutations can vary: • Mutations in noncoding regions of DNA are often harmless • Mutations in a gene might not affect protein production because of redundancy in the genetic code

  10. Mutations that result in a change in protein production are often harmful • Mutations that result in a change in protein production can sometimes increase the fit between organism and environment F. Mutations That Alter Gene Number or Sequence 1. Chromosomal mutations that delete, disrupt, or rearrange many gene loci are typically harmful 2. Duplication of large chromosome segments is usually harmful 3. Gene duplication can be an important source of new genetic variation.

  11. G. Mutation Rates 1. Mutation rates are low in animals and plants 2. The average is about one mutation in every 100,000 genes per generation 3. Mutations rates are often lower in prokaryotes and higher in viruses H. Sexual Reproduction 1. Sexual reproduction can shuffle existing alleles into new combinations 2. In organisms that reproduce sexually, recombination of alleles is more important than mutation in producing the genetic differences that make adaptation and evolution possible

  12. 3. Three mechanisms that contribute to the unique combinations of alleles are: a. Crossing over b. Independent assortment of chromosomes c. Fertilization

  13. II. Concept 23.2: Hardy-Weinberg Equation A. Gene Pools 1. A population is a group of individuals that belong to the same species, live in the same area, and interbreed to produce fertile offspring • Individuals near the population’s center are, on average, more closely related to one another than to those individuals on the periphery 2. A gene pool consists of all the alleles for all loci in a population (allele frequencies) 3. A locus is fixed if all individuals in a population are homozygous for the same allele 4. Population Genetics—study of how populations change genetically over time

  14. One Species (caribou); Two Populations

  15. B. Allele Frequencies 1. If there are 2 alleles at a locus, pandqare used to represent their frequencies 2. The frequency of all alleles in a population will add up to 1 • p + q = 1 3. Example: (diploid population) • Imagine a population of 500 wildflower plants with two alleles (CR and CW) at a locus that codes for flower pigment (incomplete dominance) • 20 plants are CWCW—white • 320 plants are CRCR –red • 160 plants are CR CW—pink • What are the allele frequencies for CW and CR?

  16. Total number of alleles—1000 • CR—800 alleles • The frequency of CR allele in the gene pool of this population is 800/1000 = 0.8 or 80%. • Therefore frequency of CW = ? • 0.2 or 20% WHY????? • p = 0.8 • q = 0.2 4. Allele and genotype frequencies can be used to test whether evolution is occurring in a population

  17. C. The Hardy Weinberg Principle 1. Describes the gene pool of a population that is not evolving 2. If a population does not meet the criteria of the Hardy-Weinberg principle, it can be concluded that the population is evolving 3. The Hardy-Weinberg principle states that frequencies of alleles and genotypes in a population remain constant from generation to generation unless acted upon by agents other than Mendelian segregation and recombination of alleles 4. Such a gene pool is said to be in Hardy-Weinberg equilibrium

  18. Selecting alleles at random from a gene pool

  19. 5. If p and q represent the relative frequencies of the only two possible alleles in a population at a particular locus, then p2 + 2pq + q2 = 1 AA Aaaa where p2 and q2 represent the frequencies of the homozygous genotypes and 2pq represents the frequency of the heterozygous genotype 6. This equation can be used to determine the frequencies of the possible genotypes if we know the frequencies of alleles, or we can calculate the frequencies of alleles in a gene pool if we know the frequencies of genotypes.

  20. Hardy-Weinberg Principle Gametes for each generation are drawn at random from the gene pool of the previous generation If the gametes come together at random, the genotype frequencies of this generation are in Hardy-Weinberg equilibrium.

  21. 7. The Hardy-Weinberg theorem describes a hypothetical population 8. In real populations, allele and genotype frequencies do change over time 9. The five conditions for nonevolving populations are rarely met in nature: • No mutations • Random mating • No natural selection • Extremely large population size • No gene flow (migration)

  22. 10. Natural populations can evolve at some loci, while being in Hardy-Weinberg equilibrium at other loci D. Applying the Hardy-Weinberg Principle 1. We can assume the locus that causes phenylketonuria (PKU) is in Hardy-Weinberg equilibrium. (PKU is recessive) 2. The occurrence of PKU is 1 per 10,000 births • q2 = 0.0001 • q = 0.01 3. The frequency of normal allele is • p = 1 – q = 1 – 0.01 = 0.99 4. The frequency of carriers is • 2pq = 2 x 0.99 x 0.01 = 0.0198 • or approximately 2% of the U.S. population

  23. III. Concept 23.3: Three Mechanisms That Directly Alter Allele Frequencies A. Any condition that is a deviation from the 5 criteria for the Hardy-Weinberg Theorem has the potential to cause evolution. B. Three major factors that alter allele frequencies and bring about evolutionary change are: 1. Natural selection (adaptive) 2. Genetic drift (nonadaptive) 3. Gene flow (nonadaptive)

  24. C. Natural Selection • Differential success in reproduction results in certain alleles being passed to the next generation in greater proportions D. Genetic Drift 1. Defined as changes in the gene pool (allele frequencies) of a small population due to chance 2. The smaller a sample, the greater the chance of deviation from a predicted result 3. Genetic drift tends to reduce genetic variation through losses of alleles

  25. GENETIC DRIFT

  26. GENETIC DRIFT

  27. GENETIC DRIFT

  28. 4. May occur in small populations (less than 100) as the result of two situations: • Bottleneck effect • Founder effect 5. Bottleneck effect • Defined as a sudden reduction in population size due to a change in the environment (Ex: disaster) • Alleles may be lost • New gene pool may differ drastically from original

  29. BOTTLENECK EFFECT

  30. Case Study: Impact of Genetic Drift on the Greater Prairie Chicken • Loss of prairie habitat caused a severe reduction in the population of greater prairie chickens in Illinois • The surviving birds had low levels of genetic variation, and only 50% of their eggs hatched • Researchers used DNA from museum specimens to compare genetic variation in the population before and after the bottleneck • The results showed a loss of alleles at several loci • Researchers introduced greater prairie chickens from population in other states and were successful in introducing new alleles and increasing the egg hatch rate to 90%

  31. 6. Founder effect • Occurs when a few individuals colonize a new habitat or become isolated from a larger population • Allele frequencies in the small founder population can be different from those in the larger parent population

  32. 7. Effects of Genetic Drift Summary a. Genetic drift is significant in small populations b. Genetic drift causes allele frequencies to change at random c. Genetic drift can lead to a loss of genetic variation within populations d. Genetic drift can cause harmful alleles to become fixed

  33. E. Gene Flow • Defined as the transfer of alleles among populations due to the migration of fertile individuals or gametes (pollen) • Tends to reduce differences between populations over time • More likely than mutation to alter allele frequencies directly • Can increase or decrease the fit between between organism and environment • Can introduce new alleles into a population

  34. IV. Concept 23.4: Natural Selection • Only natural selection leads to the adaptation of an organism to its environment (adaptive evolution) • Natural selection brings about adaptive evolution by acting on an organism’s phenotype A. Relative Fitness 1. Defined as the contribution an individual makes to the gene pool of the next generation, relative to the contributions of other individuals 2. Natural selection acts on the genotype indirectly by how the genotype affects the phenotype 3. How an organism benefits from a particular allele depends on the genetic and environmental contexts in which it is expressed

  35. B. Three Modes of Selection: • Depends on which phenotypes in a population are favored. 1. Directional Selection • Favors phenotype of one extreme • Most common when species migrate to new and different habitats or during major environmental change

  36. 2. Disruptive Selection • Favors individuals at both extremes of the phenotypic range

  37. 3. Stabilizing Selection • Favors intermediate variants and acts against extreme phenotypes

  38. THREE MODES OF NATURAL SELECTION

  39. C. Key Role of Natural Selection in Adaptive Evolution 1. Natural selection increases the frequencies of alleles that enhance survival and reproduction 2. Adaptive evolution occurs as the match between an organism and its environment increases 3. Because the environment can change, adaptive evolution is a continuous process 4. Genetic drift and gene flow do not consistently lead to adaptive evolution as they can increase or decrease the match between an organism and its environment

  40. D. Sexual Selection 1. Defined as natural selection for mating success 2. Can result in sexual dimorphism (marked differences between the sexes in secondary sexual characteristics not directly associated with reproduction) 3. Intrasexual selection is competition among individuals of one sex (often males) for mates of the opposite sex 4. Intersexual selection, often called mate choice,occurs when individuals of one sex (usually females) are choosy in selecting their mates

  41. SEXUAL SELECTION

  42. E. Mechanisms for Preserving Genetic Variation in a Population 1. Diploidy • Maintains genetic variation in the form of hidden recessive alleles 2. Balancing selection • Occurs when natural selection maintains stable frequencies of two or more phenotypic forms in a population (polymorphism) • Two mechanisms that help maintain balanced polymorphism a. Heterozygote advantage --Occurs when heterozygotes have a higher fitness than do both homozygotes

  43. --Natural selection will tend to maintain two or more alleles at that locus --The sickle-cell allele causes mutations in hemoglobin but also confers malaria resistance --Defined in terms of the genotype, not the phenotype b. Frequency-dependent selection --the fitness of a phenotype declines if it becomes too common in the population --Selection can favor whichever phenotype is less common in a population

  44. c. Neutral Variation --genetic variation that appears to confer no selective advantage or disadvantage F. Why Natural Selection Cannot Fashion Perfect Organisms 1. Selection can act only on existing variations 2. Evolution is limited by historical constraints 3. Adaptations are often compromises 4. Chance, natural selection, and the environment interact

  45. You should now be able to: • Explain why the majority of point mutations are harmless • Explain how sexual recombination generates genetic variability • Define the terms population, species, gene pool, relative fitness, and neutral variation • List the five conditions of Hardy-Weinberg equilibrium • Apply the Hardy-Weinberg equation to a population genetics problem

  46. 6. Explain why natural selection is the only mechanism that consistently produces adaptive change 7. Explain the role of population size in genetic drift • Distinguish among the following sets of terms: directional, disruptive, and stabilizing selection; intrasexual and intersexual selection • List four reasons why natural selection cannot produce perfect organisms

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