1. Chapter 23 The Evolution of Populations

2. Overview: The Smallest Unit of Evolution One misconception is that organisms evolve, in the Darwinian sense, during their lifetimes Natural selection acts on individuals, but only populations evolve Genetic variations in populations contribute to evolution

4. Population genetics provides a foundation for studying evolution Microevolution - change in the genetic makeup of a populations from generation to generation

6. The Modern Synthesis Population genetics - study of how populations change genetically over time Population genetics integrates Mendelian genetics with the Darwinian theory of evolution by natural selection This modern synthesis focuses on populations as units of evolution

7. Gene Pools and Allele Frequencies Population – localized group of individuals capable of interbreeding and producing fertile offspring Gene pool - total of all alleles/genes in a pop at any 1 time gene pool has all gene loci in all individuals of the population

8. LE 23-3 MAP AREA CANADA ALASKA Beaufort Sea Porcupine herd range NORTHWEST TERRITORIES Fairbanks Fortymile herd range Whitehorse ALASKA YUKON

9. The Hardy-Weinberg Theorem The Hardy-Weinberg theorem describes a population that is not evolving Frequencies of alleles & genotypes in a populations gene pool remain constant from generation to generation, provided that only Mendelian segregation & recombination of alleles are at work Mendelian inheritance preserves genetic variation in a population

10. LE 23-4 Generation 3 25% C R C R Generation 4 50% C R C W 25% C W C W 50% C W gametes 50% C R come together at random 25% C R C R 50% C R C W 25% C W C W Alleles segregate, and subsequent generations also have three types of flowers in the same proportions gametes Generation 2 Generation 1 CRCRCRCR CWCWCWCW genotype Plants mate All C R C W (all pink flowers) 50% C R 50% C W gametes come together at random X

11. Hardy-Weinberg Equilibrium HW equilibrium describes a population in which random mating occurs It describes a pop where allele frequencies do not change

12. If p & q represent the relative frequencies of the only 2 possible alleles in a pop at a particular locus, then p2 + 2pq + q2 = 1 p2 & q2 represent the frequencies of the homozygous genotypes & 2pq represents the frequency of the heterozygous genotype

13. LE 23-5 Gametes for each generation are drawn at random from the gene pool of the previous generation: 80% C R (p = 0.8) 20% C W (q = 0.2) Sperm C R (80%) C W (20%) pqp2p2 16% C R C W 64% C R Eggs C W (20%) C R (80%) 16% C R C W qp 4% C W q2q2

14. Conditions for Hardy-Weinberg Equilibrium HW theorem describes a hypothetical population In real populations, allele & genotype frequencies change over time 5 conditions for non-evolving populations; rarely met in nature: Extremely large pop size No gene flow No mutations Random mating No natural selection

15. Population Genetics and Human Health We can use the HW equation to estimate the % of the human pop carrying the allele for an inherited disease If 1 out of every 10,000 babies born is affected by a recessive disorder, what is the frequency of each allele? What is the frequency of heterozygotes?

16. Variation Variation in populations is produced by: mutation sexual recombination

17. Mutation Mutations are changes in the nucleotide sequences of DNA Mutations cause new genes & alleles to arise The original source of genetic diversity

19. Point Mutations A point mutation is a change in 1 base in a gene It is usually harmless but may have significant impact on phenotype

20. Mutations That Alter Gene Number or Sequence Chromosomal mutations that delete, disrupt, or rearrange many loci are typically harmful Gene duplication is nearly always harmful

21. Mutation Rates Mutation rates are low in animals & plants average is ~ 1 mutation in every 100,000 genes per generation Mutations are more rapid in microorganisms

22. Sexual Recombination Sexual recombination is far more important than mutation in producing the genetic differences that make adaptation possible

23. Natural selection, genetic drift, and gene flow can alter a population’s genetic composition 3 factors alter allele frequencies & bring about most evolutionary change: Natural selection Genetic drift Gene flow

24. Natural Selection Differential success in reproduction results in certain alleles being passed to the next generation in greater proportions

25. Genetic Drift The smaller a sample, the > the chance of deviation from a predicted result Genetic drift – describes how allele frequencies fluctuate unpredictably from 1 generation to the next Genetic drift can decrease genetic variation by loss of alleles

26. LE 23-7 CRCRCRCR CRCRCRCR CWCWCWCW CRCRCRCR CRCWCRCW CRCRCRCR CRCWCRCW CWCWCWCW CWCWCWCW CRCWCRCW CRCWCRCW CRCRCRCR CRCWCRCW CRCWCRCW CRCRCRCR CRCRCRCR CRCWCRCW CWCWCWCW CRCWCRCW CRCRCRCR Only 5 of 10 plants leave offspring Only 2 of 10 plants leave offspring CRCRCRCR CRCRCRCR CRCRCRCR CRCRCRCR CRCRCRCR CRCRCRCR CRCRCRCR CRCRCRCR CRCRCRCR CRCRCRCR Generation 2 p = 0.5 q = 0.5 Generation 3 p = 1.0 q = 0.0 Generation 1 p (frequency of C R ) = 0.7 q (frequency of C W ) = 0.3

27. The Bottleneck Effect bottleneck effect - sudden change in environment that may drastically decrease size of a population resulting gene pool may no longer be reflective of original population’s gene pool

28. LE 23-8 Original population Bottlenecking event Surviving population

29. The Founder Effect founder effect – occurs when a few individuals become isolated from a larger pop I.e. part of a population migrates to a different area, becoming geographically isolated from the original population. The individuals of the migrating group are the “founders” of the new population E.g. Polydactyly in Amish population

30. Gene Flow Gene flow – consists of genetic additions or subtractions from a population, resulting from movement of fertile indivividuals or gametes Gene flow causes a population to gain or lose alleles It tends to reduce differences between populations over time

31. Natural selection is the primary mechanism of adaptive evolution Natural selection accumulates & maintains favorable genotypes in a population

32. Genetic Variation Genetic variation occurs in individuals in populations of all species It is not always heritable Like the butterflies on the following page have the same genotype

33. LE 23-9 Map butterflies that emerge in spring: orange and brown Map butterflies that emerge in late summer: black and white

34. Variation Within a Population Both discrete & quantitative characters contribute to variation within a population Discrete characters can be classified on an either-or basis Flowers are either red, pink, or white Quantitative characters vary along a continuum within a population Height Skin tone

35. Polymorphism Phenotypic polymorphism - population in which 2 or more distinct morphs for a character are represented in high enough frequencies to be readily noticeable Genetic polymorphisms - heritable components of characters that occur along a continuum in a population (height)

36. Measuring Genetic Variation Population geneticists measure polymorphisms in a population by determining the amount of heterozygosity at the geneetic & molecular levels Average heterozygosity measures the average % of loci that are heterozygous in a population For ex: Drosophila is heterzygous at ~14% of its loci (so the avg hetero is 14%) So ~1900 of its genes out of 13,700 are hetero

37. Variation Between Populations Most species exhibit geographic variation differences between gene pools of separate populations or population subgroups LE 23-10

38. Some examples of geographic variation occur as a cline Cline - a graded change in a trait along a geographic axis

39. LE 23-11 Heights of yarrow plants grown in common garden Sierra Nevada Range Great Basin Plateau Seed collection sites 100 50 0 3,000 2,000 1,000 0 Mean height (cm) Altitude (m)

40. A Closer Look at Natural Selection From the range of variations available in a population, natural selection increases frequencies of certain genotypes, fitting organisms to their environment over generations

41. Evolutionary Fitness The phrases “struggle for existence” & “survival of the fittest” are commonly used to describe natural selection but can be misleading Reproductive success is generally more subtle & depends on many factors

42. Fitness - contribution an individual makes to the gene pool of the next generation, relative to the contributions of other individuals Relative fitness - contribution of a genotype to the next generation, compared w/ contributions of alternative genotypes for the same locus More successful genotype is set at a value of 1; other genotypes are compared to the more successful genotype

43. Directional, Disruptive, and Stabilizing Selection Selection favors certain genotypes by acting on the phenotypes of certain organisms 3 modes of selection: Directional Disruptive Stabilizing

44. Directional selection favors individuals at 1 end of the phenotypic range Disruptive selection favors individuals at both extremes of the phenotypic range Stabilizing selection favors intermediate variants & acts against extreme phenotypes

45. LE 23-12 Original population Evolved population Phenotypes (fur color) Original population Directional selectionDisruptive selectionStabilizing selection Frequency of individuals

46. 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

47. Balancing selection - natural selection maintains stable freq’s of 2 or more phenotypic forms in a pop; leads to a state called balanced polymorphism 2 kinds: Heterozygote advantage Frequency dependent selection

48. Heterozygote Advantage Heterozygote Advantage – individuals who are heterozygous at a locus have greater fitness than homozygotes Natural selection will tend to maintain 2 or more alleles at that locus E.g. Sickle cell allele

49. LE 23-13 Frequencies of the sickle-cell allele 0–2.5% 2.5–5.0% 5.0–7.5% 7.5–10.0% 10.0–12.5% >12.5% Distribution of malaria caused by Plasmodium falciparum (a protozoan)

50. Frequency-Dependent Selection frequency-dependent selection – fitness of any morph declines if it becomes too common in the population

51. LE 23-14 Parental population sample Experimental group sample On pecking a moth image the blue jay receives a food reward. If the bird does not detect a moth on either screen, it pecks the green circle to continue a new set of images (a new feeding opportunity). Plain backgroundPatterned background 0.6 0.5 0.4 0.3 0.2 020406080100 Generation number Frequency- independent control Phenotypic variation

52. Neutral Variation Neutral variation – genetic variation that appears to confer no selective advantage Such as mutations in introns etc….

53. Sexual Selection Sexual selection – natural selection for mating success Can result in sexual dimorphism - marked differences between the sexes in secondary sexual characteristics Intrasexual selection - competition among individuals of one sex for mates of the opposite sex Intersexual selection - individuals of one sex (usually females) are choosy in selecting their mates from individuals of the other sex Why do you think the female is usually the choosier sex?

55. The Evolutionary Enigma of Sexual Reproduction Sexual reproduction produces fewer reproductive offspring than asexual reproduction, a so-called “reproductive handicap”

56. LE 23-16 Asexual reproduction Female Generation 1 Generation 2 Generation 3 Generation 4 Sexual reproduction Female Male

57. Sexual reproduction produces genetic variation that may aid in disease resistance

58. Why Natural Selection Cannot Fashion Perfect Organisms Why Natural Selection Can’t make Perfect Organisms: Evolution is limited by historical constraints Adaptations are often compromises Chance & natural selection interact Selection can only edit existing variations