23 the evolution of populations


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  • Figure 23.6 One species, two populations.
  • Figure 13.3c The Gal á pagos Islands
  • Figure 23.2 Evidence of selection by food source.
  • Figure 13.11a Gal á pagos finches with beaks adapted for specific diets: large ground finch
  • Figure 23.9 Genetic drift.
  • Figure 23.10 The bottleneck effect.
  • Figure 23.13 Modes of selection.
  • Figure 23.15 Sexual dimorphism and sexual selection.
  • Figure 23.17 Mapping malaria and the sickle-cell allele.
  • 23 the evolution of populations

    1. 1. LECTURE PRESENTATIONSFor CAMPBELL BIOLOGY, NINTH EDITIONJane B. Reece, Lisa A. Urry, Michael L. Cain, Steven A. Wasserman, Peter V. Minorsky, Robert B. Jackson© 2011 Pearson Education, Inc.Lectures byErin BarleyKathleen FitzpatrickThe Evolution of PopulationsChapter 23
    2. 2. Overview: The Smallest Unit of Evolution• One misconception is that organisms evolveduring their lifetimes• Natural selection acts on individuals, but onlypopulations evolve© 2011 Pearson Education, Inc.
    3. 3. • Variation in heritable traits is a prerequisite forevolution• Mendel’s work on pea plants provided evidenceof discrete heritable units (genes)• Genetic variation among individuals is causedby differences in genes or other DNA segments• Phenotype is the product of inherited genotypeand environmental influences• Natural selection can only act on variation witha genetic componentGenetic variation makes evolution possible© 2011 Pearson Education, Inc.
    4. 4. Mutation and sexual recombination produce thevariation that makes evolution possible• Two processes, mutation and sexualrecombination, produce the variation in genepools that contributes to differences amongindividuals
    5. 5. Mutation• Mutations are changes in the nucleotidesequence of DNA• Mutations cause new genes and alleles to arise
    6. 6. Sexual Recombination• Sexual recombination is far more importantthan mutation in producing the geneticdifferences that make adaptation possible• This is particularly true with plants and animals,but clearly isn’t as important in microbes, whodon’t have sex and can obtain genes throughlateral gene transfer (transformation)• In addition, bacteria have the highest mutationrates, so it contributes more to geneticdifferences in them
    7. 7. • Microevolution is a change in allelefrequencies in a population over generations• Three mechanisms cause allele frequencychange:– Natural selection– Genetic drift– Gene flow• Only natural selection causes adaptiveevolution© 2011 Pearson Education, Inc.
    8. 8. Gene Pools and Allele Frequencies• A population is a localized group of individualscapable of interbreeding and producing fertileoffspring• A gene pool consists of all the alleles for all lociin a population• A locus is fixed if all individuals in a populationare homozygous for the same allele© 2011 Pearson Education, Inc.
    9. 9. Figure 23.6Porcupine herdBeaufort SeaPorcupineherd rangeFortymileherd rangeFortymile herdNORTHWESTTERRITORIESALASKACANADAMAPAREAALASKAYUKON
    10. 10. • The frequency of an allele in a population canbe calculated– For diploid organisms, the total number ofalleles at a locus is the total number ofindividuals times 2– The total number of dominant alleles at a locusis 2 alleles for each homozygous dominantindividual plus 1 allele for each heterozygousindividual; the same logic applies for recessivealleles© 2011 Pearson Education, Inc.
    11. 11. • By convention, if there are 2 alleles at a locus,p and q are used to represent theirfrequencies• The frequency of all alleles in a population willadd up to 1– For example, p + q = 1© 2011 Pearson Education, Inc.
    12. 12. • For example, consider a population ofwildflowers that is incompletely dominant forcolor:– 320 red flowers (CRCR)– 160 pink flowers (CRCW)– 20 white flowers (CWCW)• Calculate the number of copies of each allele:– CR= (320 × 2) + 160 = 800– CW= (20 × 2) + 160 = 200© 2011 Pearson Education, Inc.
    13. 13. • To calculate the frequency of each allele:– p = freq CR= 800 / (800 + 200) = 0.8– q = freq CW= 200 / (800 + 200) = 0.2• The sum of alleles is always 1– 0.8 + 0.2 = 1© 2011 Pearson Education, Inc.
    14. 14. The Hardy-Weinberg Principle• The Hardy-Weinberg principle describes apopulation that is not evolving• If a population does not meet the criteria ofthe Hardy-Weinberg principle, it can beconcluded that the population is evolving© 2011 Pearson Education, Inc.
    15. 15. Hardy-Weinberg Equilibrium• The Hardy-Weinberg principle states thatfrequencies of alleles and genotypes in apopulation remain constant from generation togeneration if only Mendelian segregation andrecombination of alleles are at work• In a given population where gametes contributeto the next generation randomly, allelefrequencies will not change• Mendelian inheritance preserves geneticvariation in a population© 2011 Pearson Education, Inc.
    16. 16. LE 14-10RedCRCRGametesP GenerationCRCWWhiteCWCWPinkCRCWCRGametes CWF1 GenerationF2 Generation EggsCRCWCRCRCRCRCWCRCWCWCWCWSperm121212121212
    17. 17. LE 23-4Generation3 25% CRCRGeneration450% CRCW 25% CWCW50% CWgametes50% CRcome together at random25% CRCR50% CRCW 25% CWCWAlleles segregate, and subsequentgenerations also have three typesof flowers in the same proportionsgametesGeneration2Generation1CRCRCWCWgenotypegenotypePlants mateAll CRCW(all pink flowers)50% CR 50% CWgametes gametescome together at randomX
    18. 18. • If p and q represent the relative frequencies ofthe only two possible alleles in a population ata particular locus, then– p2+ 2pq + q2= 1– where p2and q2represent the frequencies ofthe homozygous genotypes and 2pqrepresents the frequency of the heterozygousgenotype© 2011 Pearson Education, Inc.
    19. 19. Conditions for Hardy-Weinberg Equilibrium• The Hardy-Weinberg theorem describes a hypotheticalpopulation that is not evolving• In real populations, allele and genotype frequencies dochange over time• There are 5 conditions that must be met in order to have anonevolving population:– 1. No natural selection– 2. Extremely large population size– 3. No gene flow– 4. No mutations– 5. Random mating• If any of these processes occur, microevolution will occur© 2011 Pearson Education, Inc.
    20. 20. • Three major factors alter allele frequencies andbring about most evolutionary change:– Natural selection– Genetic drift– Gene flowNatural selection, genetic drift, and geneflow can alter allele frequencies in apopulation© 2011 Pearson Education, Inc.
    21. 21. Natural Selection and Microevolution:Darwin’s Finches• Peter and Rosemary Grant have beendoing research on finches in theGalapagos Islands since 1973• They captured and tagged all of the birdson Daphne Major, an island whereconditions swing between wet and dryyears• They measured the length, width, anddepth of the birds’ beaks
    22. 22. GalápagosIslandsPACIFICOCEANPinta40 miles40 km0 Florenza0FernandinaMarchenaGenovesaEquatorSantiagoDaphne IslandsPinzónEspañolaIsabela SantaCruzSantaFe SanCristobalFigure 13.3c
    23. 23. Darwin’s Finches• When it’s wet, grasses grow and produce a lot ofsmall seeds• When it’s dry, the grasses don’t grow and theavailable seeds from other plants are larger• Smaller beaked birds are unable to crack largerseeds and perish• The Grants found that average beak sizeincreased during dry seasons and decreasedduring wet seasons• Thus, microevolution occurs as the environmentselects for beak size
    24. 24. Figure 23.21976(similar to theprior 3 years)1978(afterdrought)Averagebeakdepth(mm)10980
    25. 25. (a) The large ground finchFigure 13.11a
    26. 26. Genetic Drift• The smaller a sample, the greater the chance ofdeviation from a predicted result• Genetic drift describes how allele frequenciesfluctuate unpredictably from one generation to thenext• Genetic drift tends to reduce genetic variationthrough losses of alleles© 2011 Pearson Education, Inc.
    27. 27. Figure 23.9-35plantsleaveoff-springGeneration 1p (frequency of CR) = 0.7q (frequency of CW) = 0.3CRCR CRCRCRCWCWCW CRCRCRCWCRCRCRCWCRCRCRCWCRCRCWCWCRCWCRCR CWCWCRCWCWCWCRCRCRCWCRCWGeneration 2p = 0.5q = 0.52plantsleaveoff-springCRCRCRCRCRCRCRCRCRCRCRCRCRCRCRCRCRCRCRCRGeneration 3p = 1.0q = 0.0
    28. 28. The Founder Effect• The founder effect occurs when a fewindividuals become isolated from a largerpopulation• Allele frequencies in the small founderpopulation can be different from those in thelarger parent population© 2011 Pearson Education, Inc.
    29. 29. The Bottleneck Effect• The bottleneck effect is a sudden reduction inpopulation size due to a change in theenvironment• The resulting gene pool may no longer bereflective of the original population’s gene pool• If the population remains small, it may be furtheraffected by genetic drift© 2011 Pearson Education, Inc.
    30. 30. Figure 23.10-3OriginalpopulationBottleneckingeventSurvivingpopulation
    31. 31. Effects of Genetic Drift: A Summary1. Genetic drift is significant in small populations2. Genetic drift causes allele frequencies tochange at random3. Genetic drift can lead to a loss of geneticvariation within populations4. Genetic drift can cause harmful alleles tobecome fixed© 2011 Pearson Education, Inc.
    32. 32. Gene Flow• Gene flow consists of the movement of allelesamong populations• Alleles can be transferred through the movementof fertile individuals or gametes (for example,pollen)• Gene flow tends to reduce variation amongpopulations over time© 2011 Pearson Education, Inc.
    33. 33. • Evolution by natural selection involves bothchange and “sorting”– New genetic variations arise by chance– Beneficial alleles are “sorted” and favored bynatural selection• Only natural selection consistently results inadaptive evolutionNatural selection is the only mechanism thatconsistently causes adaptive evolution© 2011 Pearson Education, Inc.
    34. 34. A Closer Look at Natural Selection• Natural selection brings about adaptiveevolution by acting on an organism’sphenotype• The phrases “struggle for existence” and“survival of the fittest” are misleading as theyimply direct competition among individuals• Reproductive success is generally more subtleand depends on many factors© 2011 Pearson Education, Inc.
    35. 35. • Relative fitness is the contribution anindividual makes to the gene pool of the nextgeneration, relative to the contributions of otherindividuals• Selection favors certain genotypes by acting onthe phenotypes of certain organisms© 2011 Pearson Education, Inc.
    36. 36. Directional, Disruptive, and StabilizingSelection• Three modes of selection:– Directional selection favors individuals at oneend of the phenotypic range– Disruptive selection favors individuals at bothextremes of the phenotypic range– Stabilizing selection favors intermediatevariants and acts against extreme phenotypes© 2011 Pearson Education, Inc.
    37. 37. Figure 23.13Original populationPhenotypes (fur color)FrequencyofindividualsOriginalpopulationEvolvedpopulation(a) Directional selection (b) Disruptive selection (c) Stabilizing selection
    38. 38. • Natural selection increases the frequencies ofalleles that enhance survival and reproduction• Adaptive evolution occurs as the match betweenan organism and its environment increases• Because the environment can change, adaptiveevolution is a continuous process© 2011 Pearson Education, Inc.
    39. 39. • Genetic drift and gene flow do not consistentlylead to adaptive evolution as they can increaseor decrease the match between an organismand its environment© 2011 Pearson Education, Inc.
    40. 40. Sexual Selection• Sexual selection is natural selection for matingsuccess• It can result in sexual dimorphism, markeddifferences between the sexes in secondarysexual characteristics© 2011 Pearson Education, Inc.
    41. 41. Figure 23.15
    42. 42. • Intrasexual selection is competition amongindividuals of one sex (often males) for matesof the opposite sex• Intersexual selection, often called matechoice, occurs when individuals of one sex(usually females) are choosy in selecting theirmates• Male showiness due to mate choice canincrease a male’s chances of attracting afemale, while decreasing his chances ofsurvival© 2011 Pearson Education, Inc.
    43. 43. Diploidy• Diploidy maintains genetic variation in the formof hidden recessive alleles• Heterozygotes can carry recessive alleles thatare hidden from the effects of selection© 2011 Pearson Education, Inc.
    44. 44. • Heterozygote advantage occurs whenheterozygotes have a higher fitness than doboth homozygotes• Natural selection will tend to maintain two ormore alleles at that locus• The sickle-cell allele causes mutations inhemoglobin but also confers malaria resistanceHeterozygote Advantage© 2011 Pearson Education, Inc.
    45. 45. Natural Selection and Microevolution:Sickle-Cell Anemia• A mutation in the gene for hemoglobin resultsin sickle-cell anemia if an individual has 2copies of the gene (one from each parent)• Red blood cells form a sickle shape and areineffective at transporting oxygen• They tend to clump together in joints, causepain, and may break (hence, the anemia)• Death before reaching reproductive age isquite likely
    46. 46. Sickle-Cell Anemia• If sickle-cell disease often causes death, shouldn’t thefrequency of the allele in the human populationdecrease?• It should be selected against, but in areas of the worldwith high malaria rates, the frequency of the alleleincreases• Tony Allison showed that the sickle-cell allele inheterozygotes provides protection against malaria,known as the heterozygote advantage• The parasite that causes malaria invades red bloodcells (RBCs) but doesn’t do well in sickle-shaped cells,and about half of a heterozygote’s RBCs will sickle
    47. 47. Sickle-Cell Anemia• Thus, nature has selected for sickle-cellanemia because it protects people fromthe parasite that causes malaria• A heterozygous baby born in an area witha high incidence of malaria has a 26%better chance of surviving their birth andchildhood than a baby that is homozygousfor normal hemoglobin
    48. 48. Figure 23.17Distribution ofmalaria caused byPlasmodium falciparum(a parasitic unicellular eukaryote)KeyFrequencies of thesickle-cell allele0–2.5%2.5–5.0%5.0–7.5%7.5–10.0%10.0–12.5%>12.5%
    49. 49. Why Natural Selection Cannot FashionPerfect Organisms1. Selection can act only on existing variations2. Evolution is limited by historical constraints3. Adaptations are often compromises4. Chance, natural selection, and theenvironment interact© 2011 Pearson Education, Inc.