1. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings PowerPoint Lectures for Biology, Seventh Edition Neil Campbell and Jane Reece Lectures by Chris Romero Chapter 23 The Evolution of Populations

2. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Overview: The Smallest Unit of Evolution One common misconception about evolution is that individual organisms evolve, in the Darwinian sense, during their lifetimes Natural selection acts on individuals, but populations evolve

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 – Reconciled Darwin’s and Mendel’s ideas

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 Preservation of Allele Frequencies In a given population where gametes contribute to the next generation randomly, allele frequencies will not change

12. 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

13. 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

14. 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

15. 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

16. 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

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

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

19. 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

20. 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

21. 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

22. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Gene duplication – Duplicates chromosome segments

23. 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

24. 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

25. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Concept 23.3: Natural selection, genetic drift, and gene flow can alter a population’s genetic composition Three major factors alter allele frequencies and bring about most evolutionary change – Natural selection – Genetic drift – Gene flow

26. 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

27. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Genetic Drift Statistically, the smaller a sample – The greater the chance of deviation from a predicted result

28. 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 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

29. 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.

30. 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.

31. 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

32. 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