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 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

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4. 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

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6. The Modern Synthesis Population genetics is the 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. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Gene Pools and Allele Frequencies A population is a localized group of individuals capable of interbreeding and producing fertile offspring The gene pool is the total aggregate of genes in a population at any one time The gene pool consists of 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. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Hardy-Weinberg Theorem The Hardy-Weinberg theorem describes a population that is not evolving It states that 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 Mendelian inheritance p reserves 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. 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 It describes a population where allele frequencies do not change

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. 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

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 Mutations cause new genes and alleles to arise Animation: Genetic Variation from Sexual Recombination Animation: Genetic Variation from Sexual Recombination

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

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

23. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Mutation Rates Mutation rates are low in animals and plants The average is about one mutation in every 100,000 genes per generation Mutations are more rapid in microorganisms

24. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Sexual Recombination 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 The smaller a sample, the greater the chance of deviation from a predicted result Genetic drift describes how allele frequencies fluctuate unpredictably from one generation to the next Genetic drift tends to reduce genetic variation through losses of alleles Animation: Causes of Evolutionary Change Animation: Causes of Evolutionary Change

28. 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

29. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Bottleneck Effect The bottleneck effect is a sudden change in the environment that may drastically reduce the size of a population The resulting gene pool may no longer be reflective of the original population’s gene pool

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