1. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Chapter 23 The Evolution of Populations

2. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Overview: Populations evolve, not individuals Natural selection acts on individuals

3. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Genetic variations in populations – Contribute to evolution Figure 23.1

4. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Concept 23.1: Population genetics provides a foundation for studying evolution Microevolution – Is change in the genetic makeup of a population from generation to generation Figure 23.2

5. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Modern Synthesis Population genetics – Is the study of how populations change genetically over time

6. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The modern synthesis – Integrates Mendelian genetics with the Darwinian theory of evolution by natural selection – Focuses on populations as units of evolution

7. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Gene Pools and Allele Frequencies A population – Is a localized group of individuals that are capable of interbreeding and producing fertile offspring MAP AREA ALASKA CANADA Beaufort Sea Porcupine herd range Fairbanks Whitehorse Fortymile herd range NORTHWEST TERRITORIES ALASKA YUKON Figure 23.3

8. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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 population

9. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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 work

10. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Mendelian inheritance – Preserves genetic variation in a population Figure 23.4 Generation 1 C R genotype C W genotype Plants mate All C R C W (all pink flowers) 50% C R gametes 50% C W gametes Come together at random Generation 2 Generation 3 Generation 4 25% C R C R 50% C R C W 25% C W C W 50% C R gametes 50% C W gametes 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

11. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Hardy-Weinberg Equilibrium Hardy-Weinberg equilibrium – Describes a population in which random mating occurs – Describes a population where allele frequencies do not change

12. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings A population in Hardy-Weinberg equilibrium Figure 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%) p2p2 64% C R 16% C R C W 16% C R C W 4% C W qp C R (80%) Eggs C W (20%) pq If the gametes come together at random, the genotype frequencies of this generation are in Hardy-Weinberg equilibrium: q2q2 64% C R C R, 32% C R C W, and 4% C W C W Gametes of the next generation: 64% C R from C R C R homozygotes 16% C R from C R C W homozygotes += 80% C R = 0.8 = p 16% C W from C R C W heterozygotes += 20% C W = 0.2 = q With random mating, these gametes will result in the same mix of plants in the next generation: 64% C R C R, 32% C R C W and 4% C W C W plants p2p2 4% C W from C W C W homozygotes

13. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings If p and q represent the relative frequencies of the only two possible alleles in a population at a particular locus, then – p 2 + 2pq + q 2 = 1 – And p 2 and q 2 represent the frequencies of the homozygous genotypes and 2pq represents the frequency of the heterozygous genotype

14. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Conditions for Hardy-Weinberg Equilibrium The Hardy-Weinberg theorem – Describes a hypothetical population In real populations – Allele and genotype frequencies do change over time

15. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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 selection

16. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Concept 23.2: Two processes, mutation and sexual recombination – Produce the variation in gene pools that contributes to differences among individuals and may lead to evolution.

17. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Mutation Mutations – Are changes in the nucleotide sequence of DNA – Cause new genes and alleles to arise Figure 23.6

18. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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 impact

19. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Mutations That Alter Gene Number or Sequence Chromosomal mutations that affect many loci – Are almost certain to be harmful – May be neutral and even beneficial Gene duplication or deletion – Duplicates or deletes chromosome segments

20. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Mutation Rates 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 microorganisms

21. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Sexual Recombination In sexually reproducing populations, sexual recombination – Is far more important than mutation in producing the genetic differences that make adaptation possible

22. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Concept 23.3: Three major factors alter allele frequencies and bring about most evolutionary change – Natural selection – Genetic drift – Gene flow

23. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Natural Selection Differential success in reproduction – Results in certain alleles being passed to the next generation in greater proportions

24. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Genetic drift – Describes how allele frequencies can fluctuate unpredictably from one generation to the next – Tends to reduce genetic variation – May result from very small population size Figure 23.7 CRCRCRCR CRCWCRCW CRCRCRCR CWCWCWCW CRCRCRCR CRCWCRCW CRCWCRCW CRCWCRCW CRCRCRCR CRCRCRCR Only 5 of 10 plants leave offspring CWCWCWCW CRCRCRCR CRCWCRCW CRCRCRCR CWCWCWCW CRCWCRCW CWCWCWCW CRCRCRCR CRCWCRCW CRCWCRCW 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

25. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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 Original population Bottlenecking event Surviving population Figure 23.8 A (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 population and gold marbles are absent.

26. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Understanding the bottleneck effect – Can increase understanding of how human activity affects other species Figure 23.8 B (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.

27. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Founder Effect The founder effect – Occurs when a few individuals become isolated from a larger population – Can affect allele frequencies in a population

28. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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 time

29. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Concept 23.4: Natural selection is the primary mechanism of adaptive evolution Natural selection – Accumulates and maintains favorable genotypes in a population

30. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Variation Within a Population Both discrete and quantitative characters contribute to variation within a population – Discrete characters can be classified on an either- or basis – Quantitative characters vary along a continuum within a population

31. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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 XX19 13.1710.169.12 8.11 12.193.84.165.146.7 XX 15.18 13.17 11.12 9.10 Figure 23.10

32. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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

33. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Balancing selection – Occurs when natural selection maintains stable frequencies of two or more phenotypic forms in a population – Leads to a state called balanced polymorphism

34. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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 locus

35. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The sickle-cell allele – Causes mutations in hemoglobin but also confers malaria resistance – Exemplifies the heterozygote advantage Figure 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)

36. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Sexual Selection Sexual selection – Is natural selection for mating success – Can result in sexual dimorphism, marked differences between the sexes in secondary sexual characteristics

37. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Evolutionary Enigma of Sexual Reproduction Sexual reproduction – Produces fewer reproductive offspring than asexual reproduction, a so-called reproductive handicap Figure 23.16 Asexual reproduction Female Sexual reproduction Female Male Generation 1 Generation 2 Generation 3 Generation 4

38. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings If sexual reproduction is a handicap, why has it persisted? – It produces genetic variation that may aid in disease resistance and promoting other advantageous adaptations.