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The Evolution of Populations

This chapter explores the evolution of populations, delving into fundamental concepts such as populations, species, and gene pools. It defines population variations, geographic variation and how new alleles arise. Learn about mutations essential for inherited traits, the Hardy-Weinberg theorem, and the conditions necessary for genetic equilibrium. It discusses genetic drift, its impact on small populations, and the concepts of founder and bottleneck effects. Explore selection types such as directional, disruptive, and stabilizing, illustrating how evolution is influenced by chance and environmental factors.

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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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