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Chapter 23

Chapter 23. The Evolution of Populations. Population Genetics. Darwin and Mendel population genetics Gene pool and Allele frequency fixed Heterozygous Mind your p’s and q’s!. Generation 1. X. C R C R. C W C W. genotype. genotype. Calculate allele frequencies: 500 total flowers

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Chapter 23

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  1. Chapter 23 The Evolution of Populations

  2. Population Genetics • Darwin and Mendel • population genetics • Gene pool and Allele frequency • fixed • Heterozygous • Mind your p’s and q’s!

  3. Generation 1 X CRCR CWCW genotype genotype Calculate allele frequencies: 500 total flowers 320 red 160 pink 20 white Freq CR: FreqCW: Plants mate LE 23-4 Generation 2 All CRCW (all pink flowers) 50% CW 50% CR gametes gametes come together at random Generation 3 25% CRCR 50% CRCW 25% CWCW 50% CR 50% CW gametes gametes come together at random Generation 4 25% CWCW 25% CRCR 50% CRCW Alleles segregate, and subsequent generations also have three types of flowers in the same proportions

  4. The Hardy-Weinberg Theorem • Allele frequencies in a population remain unchanged • Basis for understanding long-term evolutionary changes • H-W equilibrium – p2 + 2pq + q2 = 1 • p + q = 1

  5. Gametes for each generation are drawn at random from the gene pool of the previous generation: LE 23-5 80% CR (p = 0.8) 20% CW (q = 0.2) Sperm CR (80%) CW (20%) p2 pq CR (80%) Eggs 64% CRCR 16% CRCW 4% CWCW 16% CRCW CW (20%) qp q2

  6. Conditions for Hardy-Weinberg • Large population size • No gene flow • No mutations • Random mating • No natural selection

  7. Sources of Variation • Mutations • Point mutation • Alter gene number or sequence • duplication • Mutation rates • Sexual recombination

  8. Altering Populations Gene Pool: The Big Players • Natural selection • Genetic Drift • Bottleneck effect • Founder effect • Gene flow

  9. LE 23-7 CWCW CRCR CRCR CRCR CRCR CRCW CRCW CRCR CRCR Only 5 of 10 plants leave offspring Only 2 of 10 plants leave offspring CRCR CRCR CRCR CWCW CWCW CRCR CRCW CRCW CRCR CRCR CRCR CWCW CRCR CRCW CRCR CRCR CRCR CRCW CRCW CRCR CRCW Generation 1 p (frequency of CR) = 0.7 q (frequency of CW) = 0.3 Generation 3 p = 1.0 q = 0.0 Generation 2 p = 0.5 q = 0.5

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

  11. Natural Selection and Adaptive Evolution • Genetic variation • Polymorphism • Geographic variation • Closer look at Natural Selection • Evolutionary fitness • Types of selection • Preservation of genetic variation • Sexual Selection

  12. LE 23-10 1 2.4 3.14 5.18 6 7.15 8.11 9.12 10.16 13.17 19 XX 1 2.19 3.8 4.16 5.14 6.7 9.10 11.12 13.17 15.18 XX

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

  14. LE 23-12a Original population Frequency of individuals Phenotypes (fur color)

  15. LE 23-12b Original population Evolved population Directional selection Disruptive selection Stabilizing selection

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

  17. 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). LE 23-14 Parental population sample 0.6 Experimental group sample 0.5 Phenotypic variation 0.4 Frequency- independent control 0.3 0.2 0 20 40 60 80 100 Generation number Plain background Patterned background

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