Chapter 23~24: Species and Populations
AP Biology
Stoneleigh-Burnham School
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Judith S. de Nuño
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Chapter Objectives

    1. Explain what is meant by the modern synthesis
    2. Explain how microevolutionary change can affect a gene pool
    3. state the Hardy-Weinberg theorem
    4. Write the general Hardy-Weinberg equation and use it to calculate allele and genotype frequencies
    5. Explain the consequences of Hardy-Weinberg equilibrium
    6. Demonstrate that a disequilibrium population requires only one generation of random mating to establish Hardy-Weinberg equilibrium
    7. Describe the usefulness of the Hardy-Weinberg equilibrium model to population genetics
    8. List the conditions a population must meet in order to maintain Hardy-Weinberg equilibrium
    9. Explain how genetic drift, gene flow, mutation, nonrandom mating, and natural selection can cause microevolution
    10. Explain the role of population size in genetic drift
    11. Distinguish between the bottleneck effect and the founder effect
    12. Explain why mutation has little quantitative effect on a large population
    13. Describe how inbreeding and assortive mating affect a population's allele frequencies and genotype frequencies
    14. List factors that produce geographic variation among closely related populations
    15. Explain why even though mutation can be a source of genetic variability, it contributes a negligible amount to genetic variation in a population
    16. Explain how genetic variation may be preserved in a natural population
    17. Describe the neutral theory of molecular evolution and explain how changes in gene frequency may be nonadaptive
    18. Explain what is meant by selfish DNA
    19. Explain the concept of relative fitness and its role in adaptive evolution
    20. Explain why the rate of decline for a deleterious allele depends upon whether the allele is dominant or recessive to the more successful allele
    21. Describe what selection acts on and what factors contribute to the overall fitness of a genotype
    22. Give examples of how an organism's phenotype may be influenced by the environment
    23. Distinguish among stabilizing selection, directional selection, and diversifying selection
    24. Define sexual dimorphism and explain how it can influence evolutionary change
    25. Give at leas 4 reasons why natural selection cannot breed perfect organisms



  1. Distinguish between anagenesis and cladogenesis
  2. Define morphospecies and explain how this concept can be useful to biologists
  3. Define biological species (E. Mayr)
  4. Describe some limitations of the biological species concept
  5. Explain how gene flow between closely related species can be prevented
  6. Distinguish between prezygotic and postzygotic isolating mechanisms
  7. Describe 5 prezygotic isolating mechanisms and give an example of each
  8. Explain why many hybrids are sterile
  9. Explain how hybrid breakdown maintains separate species even if gene flow occurs
  10. Distinguish between allopatric and sympatric speciation
  11. Explain the allopatric speciation model and describe the role of intraspecific variation and geographical isolation
  12. Explain why peripheral isolates are susceptible if geographic barriers arise
  13. Describe the adaptive radiation model and use it to describe how it might be possible to have many sympatric closely related species even if geographic isolation is necessary for them to evolve
  14. Define sympatric speciation and explain how polyploidy can cause reproductive isolation
  15. Distinguish between autopolyploidy and allopolyploidy
  16. List some points of agreement and disagreement between the 2 schools of thought about the tempo of speciation (gradualism vs. punctuated equilibrium)
  17. Describes the origins of evolutionary novelty

Chapter Terms:

Chapter 23 Terms

population genetics

modern synthesis



gene pool

genetic structure

Hardy-Weinberg theorem

Hardy-Weinberg equilibrium

Hardy-Weinberg equation


bottleneck effect


founder effect

gene flow



assortative mating

natural selection


geographical variation


balanced polymorphism

heterozygote advantage


hybrid vigor

frequency-dependent selection

neutral variation

Darwinian fitness

relative fitness

stabilizing selection

directional selection

diversifying selection

sexual dimorphism

sexual selection

Chapter 24 Terms




phyletic evolution


branching evolution


prezygotic barriers

postzygotic barriers

morphological species concept

recognition species concept

cohesion species concept

ecological species concept

evolutionary species concept

allopatric speciation

sympatric speciation

adaptive radiation




hybrid zone

punctuated equilibrium



allometric growth




Chapter Outline Framework

    1. Population Genetics
      1. The modern evolutionary synthesis integrated Darwinian selection and Mendelian inheritance
      2. The genetic structure of a population is defined by its allele and genotype frequencies
      3. The Hardy-Weinberg theorem describes a nonevolving population
    2. Causes of Microevolution
      1. Microevolution is a generation-to-generation change in a population's allele or genotype frequencies
      2. The 5 causes of microevolution are genetic drift, gene flow, mutation, nonrandom mating, and natural selection
    3. Genetic Variation, the Substrate for Natural Selection
      1. Genetic variation occurs within and between populations
      2. Mutation and sexual recombination generate genetic variation
      3. Diploidy and balanced polymorphism preserve variation
    4. Natural selection as the Mechanism of Adaptive Evolution
      1. Evolutionary fitness is the relative contribution an individual makes to the gene pool of the next generation
      2. The effect of selection on a varying characteristic can be stabilizing, directional, or diversifying
      3. Sexual selection may lead to pronounced 2y differences between the sexes
      4. Natural selection cannot fashion perfect organisms
    5. What is a Species?
      1. The biological species concept emphasizes reproductive isolation
      2. Prezygotic and postzygotic barriers isolate the gene pools of biological species
      3. The biological species concept does not work in all situations
    6. Modes of Speciation
      1. Geographical isolation can lead to the origin of species: allopatric speciation
      2. A new species can originate in the geographical midst of the parent species: sympatric speciation
      3. Genetic change in populations can account for speciation
      4. The punctuated equilibrium model has stimulated research on the tempo of speciation
    7. The Origin of Evolutionary Novelty
      1. Most evolutionary novelties are modified versions of older structures
      2. Genes that control development play a major role in evolutionary novelty
      3. An evolutionary trend does not mean that evolution is goal-oriented

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