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Understanding the Hardy-Weinberg Theorem in Population Genetics

Explore the Hardy-Weinberg theorem, allele frequencies, gene pool dynamics, and evolutionary changes in populations through genetic drift, natural selection, and more. Learn the conditions for genetic equilibrium and the impact of variations like mutations and sexual recombination.

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Understanding the Hardy-Weinberg Theorem in Population Genetics

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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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