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The Evolution of Populations

The Evolution of Populations. Chapter 23 Biology – Campbell • Reece. What is a population? Species Gene pool. Population. Variations within a population AND Geographic variation How does variation occur? What is the ultimate source of new alleles?

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The Evolution of Populations

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  1. The Evolution of Populations Chapter 23 Biology – Campbell • Reece

  2. What is a population? • Species • Gene pool Population

  3. Variations within a population AND • Geographic variation • How does variation occur? • What is the ultimate source of new alleles? • Where must mutations occur in order to be passed to the next generation? Genetic Variation & Evolution

  4. Allele frequency example… • Red flower (R) is dominant over white flower (r) • In a population of 500, 20 have white flowers (rr) • The other 480 have red flowers (RR or Rr) • 320 are RR, 160 are Rr • The dominant allele (R) accounts for 800 or 80% of the total (1000) number of genes • The recessive allele (r) accounts for 200 or 20% Alleles in a Population

  5. Describes a nonevolving population • The frequencies of alleles and genotypes in a population’s gene pool remain constant over generations • Chance of RR – 0.8 x 0.8 = .64 • Chance of Rr – 0.8 x 0.2 = .16 + .16 (for rR) = .32 • Chance of rr – 0.2 x 0.2 = 0.04 • The allele frequency does not change Hardy-Weinberg Theorem

  6. Hardy-Weinberg Theorem

  7. Hardy-Weinberg Theorem

  8. p = one allele, q = other allele • p + q = 1 • Frequency of RR = p2 • Frequency of Rr/rR = 2pq • Frequency of rr = q2 • Hardy-Weinberg Equation: • p2 + 2pq + q2 = 1 Hardy-Weinberg Equilibrium

  9. Very large population size. • No migration. • No net mutations. • Random mating. • No natural selection. • We do not really expect a natural population to be in H-W equilibrium 5 Conditions for H-W Equilibrium

  10. What might cause the allele frequencies to change? Altering Allele Frequencies

  11. What is genetic drift? • What size population is most likely to be affected? • Founder effect • Bottleneck effect Genetic Drift

  12. Genetic Drift

  13. Bottleneck Effect

  14. Bottleneck Effect

  15. 4 key points: Genetic drift… • is significant in small populations • can cause allele frequencies to change at random • can lead to a loss of genetic variation within populations • can cause harmful alleles to become fixed Genetic Drift

  16. What is gene flow? • What results from gene flow? Gene Flow

  17. Directional Selection

  18. Disruptive Selection

  19. Stabilizing Selection

  20. Selection can act only on existing variations • Evolution is limited by historical constraints • Adaptations are often compromises • Chance, natural selection, and the environment interact No ‘Perfect’ Organisms

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