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23 evolutionofpopulations text

  1. 1. The Evolution of Populations • One common misconception about evolution is that individual organisms evolve, in the Darwinian sense, during their lifetimes • Natural selection acts on individuals, but populations evolveCopyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
  2. 2. • Genetic variations in populations – Contribute to evolution Figure 23.1Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
  3. 3. The Modern Synthesis • Microevolution – Is change in the genetic makeup of a population from generation to generation • Population genetics – Is the study of how populations change genetically over time – Darwin’s and Mendel’s ideasCopyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
  4. 4. • The modern synthesis – Integrates Mendelian genetics with the Darwinian theory of evolution by natural selection – Focuses on populations as units of evolutionCopyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
  5. 5. Gene Pools and Allele Frequencies • A population – Is a localized group of individuals that are capable of interbreeding and producing fertile offspring CANADA ALASKA MAP AREA Beaufort Sea Porcupine herd range N TE OR RR TH IT W E O S RI T ES • Fairbanks Fortymile ALASKA • YUKON herd range Whitehorse Figure 23.3Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
  6. 6. • The gene pool – Is the total aggregate of genes in a population at any one time – Consists of all gene loci in all individuals of the populationCopyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
  7. 7. The Hardy-Weinberg Theorem • The Hardy-Weinberg theorem – Describes a population that is not evolving – States that the frequencies of alleles and genotypes in a population’s gene pool remain constant from generation to generation provided that only Mendelian segregation and recombination of alleles are at workCopyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
  8. 8. • Mendelian inheritance – Preserves genetic variation in a population Generation 1 CRCR CWCW genotype genotype Plants mate Generation 2 All CRCW (all pink flowers) 50% CR 50% CW gametes gametes Come together at random Generation 3 25% CRCR 50% CRCW 25% CWCW 50% CR 50% CW gametes gametes Come together at random Generation 4 25% CRCR 50% CRCW 25% CWCW Alleles segregate, and subsequent generations also have three types Figure 23.4 of flowers in the same proportionsCopyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
  9. 9. Preservation of Allele Frequencies • In a given population where gametes contribute to the next generation randomly, allele frequencies will not changeCopyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
  10. 10. Hardy-Weinberg Equilibrium • Hardy-Weinberg equilibrium – Describes a population in which random mating occurs – Describes a population where allele frequencies do not changeCopyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
  11. 11. • A population in Hardy-Weinberg equilibrium Gametes for each generation are drawn at random from the gene pool of the previous generation: 80% CR (p = 0.8) 20% CW (q = 0.2) Sperm CR CW (80%) (20%) p2 pq (80%) CR Eggs p2 64% 16% CRCR CRCW (20%) qp 16% 4% CW CRCW CWCW q2 If the gametes come together at random, the genotype frequencies of this generation are in Hardy-Weinberg equilibrium: 64% CRCR, 32% CRCW, and 4% CWCW Gametes of the next generation: 64% CR from 16% CR from 80% CR = 0.8 = p + CRCW homozygotes = CRCR homozygotes 4% CW from 16% CW from 20% CW = 0.2 = q + CRCW heterozygotes = CWCW homozygotes With random mating, these gametes will result in the same mix of plants in the next generation: Figure 23.5 64% CRCR, 32% CRCW and 4% CWCW plantsCopyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
  12. 12. • 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 – And p2 and q2 represent the frequencies of the homozygous genotypes and 2pq represents the frequency of the heterozygous genotypeCopyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
  13. 13. Conditions for Hardy-Weinberg Equilibrium • The Hardy-Weinberg theorem – Describes a hypothetical population • In real populations – Allele and genotype frequencies do change over timeCopyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
  14. 14. • The five conditions for non-evolving populations are rarely met in nature – Extremely large population size – No gene flow – No mutations – Random mating – No natural selectionCopyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
  15. 15. Population Genetics and Human Health • We can use the Hardy-Weinberg equation – To estimate the percentage of the human population carrying the allele for an inherited diseaseCopyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
  16. 16. • Two processes, mutation and sexual recombination – Produce the variation in gene pools that contributes to differences among individualsCopyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
  17. 17. Mutation • Mutations – Are changes in the nucleotide sequence of DNA – Produces alterations in genes and alleles Figure 23.6Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
  18. 18. Point Mutations • A point mutation – Is a change in one base in a gene – Can have a significant impact on phenotype – Is usually harmless, but may have an adaptive impactCopyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
  19. 19. Mutations That Alter Gene Number or Sequence • Chromosomal mutations that affect many loci – Are almost certain to be harmful – May be neutral and even beneficialCopyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
  20. 20. • Gene duplication – Duplicates chromosome segments • Mutation rates – Tend to be low in animals and plants – Average about one mutation in every 100,000 genes per generation – Are more rapid in microorganismsCopyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
  21. 21. Sexual Recombination • In sexually reproducing populations, sexual recombination – Is far more important than mutation in producing the genetic differences that make adaptation possibleCopyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
  22. 22. • Three major factors alter allele frequencies and bring about change – Natural selection – Genetic drift – Gene flowCopyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
  23. 23. Natural Selection • Differential success in reproduction – Results in certain alleles being passed to the next generation in greater proportionsCopyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
  24. 24. Genetic Drift • Statistically, the smaller a sample – The greater the chance of deviation from a predicted resultCopyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
  25. 25. • Genetic drift – Describes how allele frequencies can fluctuate unpredictably from one generation to the next – Tends to reduce genetic variation CRCR CRCR CW CW CRCR CRCR CRCW Only 5 of CRCW Only 2 of CRCR CRCR 10 plants 10 plants leave leave CWCW CRCR offspring CW CW offspring CRCR CRCR CRCR CRCW CRCW CRCR CRCR CRCR CRCW CW CW CRCR CRCR CRCR CRCW CRCW CRCW CRCR CRCR Generation 1 Generation 2 Generation 3 p (frequency of CR) = 0.7 p = 0.5 p = 1.0 q (frequency of CW) = 0.3 q = 0.5 q = 0.0 Figure 23.7Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
  26. 26. The Bottleneck Effect • In the bottleneck effect – A sudden change in the environment may drastically reduce the size of a population – The gene pool may no longer be reflective of the original population’s gene pool (a) Shaking just a few marbles through the narrow neck of a bottle is analogous to a drastic reduction in the size of a population after some environmental disaster. By chance, blue marbles are over-represented in the new Original Bottlenecking Surviving population and gold marbles are absent. population event population Figure 23.8 ACopyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
  27. 27. • Understanding the bottleneck effect – Can increase understanding of how human activity affects other species (b) Similarly, bottlenecking a population of organisms tends to reduce genetic variation, as in these northern elephant seals in California that were once hunted nearly to extinction. Figure 23.8 BCopyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
  28. 28. The Founder Effect • The founder effect – Occurs when a few individuals become isolated from a larger population – Can affect allele frequencies in a populationCopyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
  29. 29. Gene Flow • Gene flow – Causes a population to gain or lose alleles – Results from the movement of fertile individuals or gametes – Tends to reduce differences between populations over timeCopyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
  30. 30. • Natural selection is the primary mechanism of adaptive evolution • Natural selection – Accumulates and maintains (preserves) favorable genotypes in a populationCopyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
  31. 31. Genetic Variation • Genetic variation – Occurs in individuals in populations of all species – Is not always heritable (a) Map butterflies that (b) Map butterflies that emerge in spring: emerge in late summer: orange and brown black and white Figure 23.9 A, BCopyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
  32. 32. • Polymorphism • Phenotypic polymorphism – Describes a population in which two or more distinct morphs for a character are each represented in high enough frequencies to be readily noticeable • Genetic polymorphisms – Are the heritable components of characters that occur along a continuum in a populationCopyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
  33. 33. • Measuring Genetic Variation • Population geneticists – Measure the number of polymorphisms in a population by determining the amount of heterozygosity at the gene level and the molecular level • Average heterozygosity – Measures the average percent of loci that are heterozygous in a populationCopyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
  34. 34. Variation Between Populations • Most species exhibit geographic variation – Differences between gene pools of separate populations or population subgroups 1 2.4 3.14 5.18 6 7.15 8.11 9.12 10.16 13.17 19 XX 1 2.19 3.8 4.16 5.14 6.7 9.10 11.12 13.17 15.18 XX Figure 23.10Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
  35. 35. • Some examples of geographic variation occur as a cline, which is a graded change in a trait along a geographic axis Heights of yarrow plants grown in common gardenEXPERIMENT Researchers observed that the average sizeof yarrow plants (Achillea) growing on the slopes of the SierraNevada mountains gradually decreases with increasing Mean height (cm)elevation. To eliminate the effect of environmental differencesat different elevations, researchers collected seedsfrom various altitudes and planted them in a commongarden. They then measured the heights of theresulting plants. RESULTS The average plant sizes in the common garden were inversely correlated with the altitudes at which the seeds were collected, although the height differences were less than in the plants’ natural environments. Atitude (m) Sierra Nevada Great Basin Range Plateau CONCLUSION The lesser but still measurable clinal variation in yarrow plants grown at a common elevation demonstrates the Seed collection sites role of genetic as well as environmental differences. Figure 23.11Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
  36. 36. A Closer Look at Natural Selection • From the range of variations available in a population – Natural selection increases the frequencies of certain genotypes, fitting organisms to their environment over generations Most Creationists would completely agree with this assessment of natural selection, and would use this phenomenon to explain the appearance of skin tones and other human phenotypes.Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
  37. 37. Evolutionary Fitness • The phrases “struggle for existence” and “survival of the fittest” – Are commonly used to describe natural selection – Can be misleading • Reproductive success – Is generally more subtle and depends on many factorsCopyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
  38. 38. • Fitness – Is the contribution an individual makes to the gene pool of the next generation, relative to the contributions of other individuals • Relative fitness – Is the contribution of a genotype to the next generation as compared to the contributions of alternative genotypes for the same locusCopyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
  39. 39. Directional, Disruptive, and Stabilizing Selection • Selection – Favors certain genotypes by acting on the phenotypes of certain organisms • Three modes of selection are – Directional – Disruptive – StabilizingCopyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
  40. 40. • Directional selection – Favors individuals at one end of the phenotypic range • Disruptive selection – Favors individuals at both extremes of the phenotypic range • Stabilizing selection – Favors intermediate variants and acts against extreme phenotypesCopyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
  41. 41. • The three modes of selection Frequency of individuals Original population Original Evolved Phenotypes (fur color) population population (a) Directional selection shifts the overall (b) Disruptive selection favors variants (c) Stabilizing selection removes makeup of the population by favoring at both ends of the distribution. These extreme variants from the population variants at one extreme of the mice have colonized a patchy habitat and preserves intermediate types. If distribution. In this case, darker mice are made up of light and dark rocks, with the the environment consists of rocks of favored because they live among dark result that mice of an intermediate color are an intermediate color, both light and rocks and a darker fur color conceals them at a disadvantage. dark mice will be selected against. from predators. Fig 23.12 A–CCopyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
  42. 42. The Preservation of Genetic Variation • Various mechanisms help to preserve genetic variation in a population • Diploidy – Maintains genetic variation in the form of hidden recessive alleles • Balancing selection – Occurs when natural selection maintains stable frequencies of two or more phenotypic forms in a population – Leads to a state called balanced polymorphismCopyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
  43. 43. Heterozygote Advantage • Some individuals who are heterozygous at a particular locus – Have greater fitness than homozygotes • Natural selection – Will tend to maintain two or more alleles at that locusCopyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
  44. 44. • The sickle-cell allele – Causes mutations in hemoglobin but also confers malaria resistance – Exemplifies the heterozygote advantage Frequencies of the sickle-cell allele 0–2.5% 2.5–5.0% Distribution of 5.0–7.5% malaria caused by 7.5–10.0% Plasmodium falciparum 10.0–12.5% (a protozoan) >12.5% Figure 23.13Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
  45. 45. • Frequency-Dependent Selection • In frequency-dependent selection – The fitness of any morph declines if it becomes too common in the populationCopyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
  46. 46. • An example of frequency-dependent selection On pecking a moth image the blue jay receives a food reward. If the bird Parental population sample does not detect a moth on either screen, it pecks the green circle to continue to a new set of images (a new feeding opportunity). Experimental group sample 0.06 Phenotypic diversity 0.05 0.04 Frequency- 0.03 independent control 0.02 0 20 40 60 80 100 Generation number Plain background Patterned background Figure 23.14Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
  47. 47. Neutral Variation • Neutral variation – Is genetic variation that appears to confer no selective advantage • Sexual selection – Is natural selection for mating success – Can result in sexual dimorphism, marked differences between the sexes in secondary sexual characteristicsCopyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
  48. 48. • Intersexual selection – Occurs when individuals of one sex (usually females) are choosy in selecting their mates from individuals of the other sex – May depend on the showiness of the male’s appearance Figure 23.15Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
  49. 49. The Evolutionary Enigma of Sexual Reproduction • Sexual reproduction – Produces fewer reproductive offspring than asexual reproduction, a so-called reproductive handicap Asexual reproduction Sexual reproduction Female Generation 1 Female Generation 2 Male Generation 3 Generation 4 Figure 23.16Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
  50. 50. • If sexual reproduction is a handicap, why has it persisted? – Evolutionary answer: It produces genetic variation that may aid in disease resistance The appearance of sexual reproduction has baffled evolutionists for many decades and poses a challenge to the theory. Read article!Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
  51. 51. Why Natural Selection Cannot Fashion Perfect Organisms • Evolution is limited by historical constraints • Adaptations are often compromises • Chance and natural selection interact • Selection can only edit existing variations Natural selection explains the ‘survival of the fittest’ but not the ‘arrival of the fittest.’Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

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