Chapter 21 The Origin of Species


Evolutionary theory must explain:

1.      How adaptations evolve in populations

2.      The origin of new species which result in biological diversity


Anagenesis – phyletic evolution; change of an unbranched lineage of organisms

Cladogenesis – branching evolution; budding of one or more new species from a parent species



Species exist in nature as descrete units, usually distinguishable from other species


Linnaeus described species in terms of their physical form – most commonly used today.

Morphospecies – species defined by anatomical features (difficult to apply at times; does not account for discontinuity between species)

Biological species – groups of interbreeding natural populations that are reproductively isolated from other such groups.

ü      Reproductive barriers preventing gene mixing between species

ü      Largest unit of population in which gene flow is possible

ü      Reproductive isolation from other species


ü      Organisms that are completely sexual

o       Asexual produces clones – represent a single organism

ü      Extinct organisms represented only by fossils

ü      Populations which are geographically separated

   - anything that impedes two species from reproducing


Prezygotic Barriers – Pre = before zygote = fertilization

  1. Habitat Isolation – 2 species living in different habitats within the same area (same area but water vs land?)
  2. Temporal Isolation – 2 species that breed at different times of the day, seasons or years (brown & rainbow trout live in same area but breed in different seasons)
  3. Behavioral Isolation – species-specific signals and elaborate behavior to attract males.
    1.  (Male fireflys blink lights in characteristic patterns)
    2. (phermones – distinct chemical signals
    3. (mating songs)
  4. Mechanical Isolation – anatomical incompatibility prevents sperm transfer (dragonflies clasp females during copulation – won’t fit all body forms)
  5. Gametic Isolation – gametes of different species that meet rarely fuse togther. (sperm of a certain organism can not survive in the female) (pairing of chromomsomes)


Postzygotic barriers – post = after zygotic = fertilization

  1. Reduced Hybrid Viability – genetic incompatibility between 2 species
  2. Reduced Hybrid Fertility – if 2 species mate and produce hybrid offspring that are viable but can’t reproduce – genes can’t flow from one species gene pool to the next (different chromosomes number for meiosis) (donkey x horse à mule)
  3. Hybrid Breakdown – sometimes cross-mating produces offspring who are viable and fertile, but the next generation is not.


Introgression – transplantation of alleles between species

Occurs when alleles seep through reproductive barriers – pass between gene pools of closely related species when fertile hybrids mate successfully with one of the parent species (when trying to produce wild-type organisms)




When two or more populations of a species are geographically isolated so they can’t interbreed

            Might occur because of a great distance or physical barrier like a desert or

            Body of water





Chapter 21: The Evolution of Populations

Population Genetics

  1. Explain why it is incorrect to say that individual organisms evolve.
  2. Explain what is meant by "the modern synthesis."
  3. Define a population; define a species.
  4. Explain how microevolutionary change can affect a gene pool.
  5. State the Hardy-Weinberg theorem.
  6. Write the general Hardy-Weinberg equation and use it to calculate allele and genotype frequencies.
  7. Explain why the Hardy-Weinberg theorem is important conceptually and historically.
  8. List the conditions a population must meet to maintain Hardy-Weinberg equilibrium.

Causes of Microevolution

  9. Define microevolution.
  10. Define evolution at the population level.
  11. Explain how genetic drift, gene flow, mutation, nonrandom mating, and natural selection can cause microevolution.
  12. Explain the role of population size in genetic drift.
  13. Distinguish between the bottleneck effect and the founder effect.
  14. Explain why mutation has little quantitative effect on a large population.

Genetic Variation, the Substrate for Natural Selection

  15. Explain how quantitative and discrete characters contribute to variation within a population.
  16. Define polymorphism and morphs. Describe an example of polymorphism within the human population.
  17. Distinguish between gene diversity and nucleotide diversity. Describe examples of each in humans.
  18. List some factors that can produce geographic variation among closely related populations. Define a cline.
  19. Explain why even though mutation can be a source of genetic variability, it contributes a negligible amount to genetic variation in a population.
  20. Describe the cause of nearly all genetic variation in a population.
  21. Explain how genetic variation may be preserved in a natural population.
  22. Briefly describe the neutral theory of molecular evolution and explain how changes in gene frequency may be nonadaptive.

A Closer Look at Natural Selection as the Mechanism of Adaptive Evolution

  23. Distinguish between Darwinian fitness and relative fitness.
  24. Describe what selection acts on and what factors contribute to the overall fitness of a genotype.
  25. Describe examples of how an organism's phenotype may be influenced by the environment.
  26. Distinguish among stabilizing selection, directional selection, and diversifying selection.
  27. Describe the advantages and disadvantages of sexual reproduction.
  28. Define sexual dimorphism and explain how it can influence evolutionary change.
  29. Distinguish between intrasexual selection and intersexual selection.
  30. Describe at least four reasons why natural selection cannot breed perfect organisms.


·         The modern evolutionary synthesis integrated Darwinian selection and Mendelian inheritance (p. 446) The modern synthesis focuses on populations as units of evolution.

·         A population’s gene pool is defined by its allele frequencies (pp. 446-447) A population, a localized group of organisms belonging to the same species, is united by its gene pool, the aggregate of all alleles in the population.

·         The Hardy-Weinberg theorem describes a nonevolving population (pp. 447-449, FIGURE 23.3) The frequencies of alleles in a population will remain constant if Mendelian segregation is the only process that affects the gene pool. If p and q represent the relative frequencies of the dominant and recessive alleles of a two-allele locus, respectively, then p2 + 2pq + q2 = 1, where p2 and q2 are the frequencies of the homozygous genotypes and 2pq is the frequency of the heterozygous genotype. For Hardy-Weinberg equilibrium to apply, the population must be very large, be totally isolated, have no net mutations, show random mating, and have equal reproductive success for all individuals.


·         Microevolution is a generation-to-generation change in a population’s allele frequencies (p. 450) Microevolution can occur when one or more of the conditions required for Hardy-Weinberg equilibrium are not met.


·         The two main causes of microevolution are genetic drift and natural selection (pp. 450-452, FIGURE 23.4, 23.5) Natural selection and chance effects, called genetic drift, can change allele frequencies. Migration and mutation also influence allele frequencies in a population.


·         Genetic variation occurs within and between populations (pp.453-454, FIGURE 23.7-FIGURE 23.9) Genetic variation includes individual variation in discrete and quantitative characters within a population, as well as geographic variation between populations.


·         Mutation and sexual recombination generate genetic variation (pp.454-456) Most mutations have no effect or are harmful, but some are adaptive. Sexual recombination produces most of the genetic variation that makes adaptation possible in populations of sexually reproducing organisms.


·         Diploidy and balanced polymorphism preserve variation (pp. 456-457, FIGURE 23.10, 23.11) Diploidy maintains a reservoir of latent variation in heterozygotes. Balanced polymorphism may maintain variation at some gene loci as a result of heterozygote advantage or frequency-dependent selection.


·         Evolutionary fitness is the relative contribution an individual makes to the gene pool of the next generation (pp.457 -458) One genotype has a greater relative fitness than another if it leaves more descendants. Selection favors certain genotypes in a population by acting on the phenotype of individual organisms.

·         The effect of selection on a varying characteristic can be directional, diversifying, or stabilizing (pp. 458-459, FIGURE 23.12) Natural selection can favor relatively rare individuals on one end of the phenotypic range (directional selection), can favor individuals at both extremes of the range over intermediate phenotypes (diversifying selection), or can act against extreme phenotypes (stabilizing selection).

·         Natural selection maintains sexual reproduction (pp. 459-460, FIGURE 23.15) Enhanced disease resistance based on genetic variation may help explain how sex can overcome its twofold disadvantage compared to asexual reproduction.

·         Sexual selection may lead to pronounced secondary differences between the sexes (pp. 460-461) Sexual selection leads to the evolution of secondary sex characteristics, which can give individuals an advantage in mating.

·         Natural selection cannot fashion perfect organisms (p. 461) Structures result from modified ancestral anatomy; adaptations are often compromises; the gene pool can be affected by genetic drift; and natural selection can act only on available variation.