Bio1151 Chapter 23 The Evolution of Populations
  1. A             is a localized group of individuals of a species.

    One species, two populations.

    These 2 caribou herds in the Yukon are not totally isolated - their ranges overlap.

    Still, a caribou is more likely to breed with members of its own population than with members of the other population.

    Thus the gene pool (total aggregate of alleles) of each population is isolated, with little gene flow between them.

  2. Natural selection acts on individuals, but              evolve, based on variations that are inherited over generations.

    An individual such as this Medium Ground Finch does not evolve, but populations evolve over time by natural selection.

    The basis of evolution, however, is the relative fitness of individuals, determined by their genotypes.

    Non-heritable variation.

    Not all phenotypic variation is heritable.

    Caterpillars of the moth Nemoria arizonaria exhibit different phenotypes based on their diet.

    Caterpillars that feed on oak flowers resemble flowers.

    Those that fed on oak leaves resemble twigs.

  3. As a population evolves, its heritable variation is reflected in change of         and           frequencies.

    Heritable Genetic Variation.

    The Mummichog fish has 2 alleles for the Ldh-B gene (codes for the lactate dehydrogenase B enzyme).

    The enzyme coded by the B^b allele is better adapted to cold temperatures.

    Populations of the fish found in colder waters show a higher frequency of this allele.

  4. A non-evolving population reaches the                 equilibrium due to Mendelian              of alleles.

    Hardy-Weinberg conditions.

    Mendelian inheritance preserves allele frequencies from one generation to the next. This incomplete dominance of flower color gene preserves the ratio of C^R and C^W alleles between generations. Over time, the allele frequencies reach Hardy-Weinberg equilibrium, assuming five idealized conditions:

  5. Extremely large population size.
  6. No gene flow due to migration.
  7. No mutations.
  8. Random mating.
  9. No natural selection.

  10. At Hardy-Weinberg equilibrium, the distribution of 2          p and q in a population can be modeled by two equations.

    Hardy-Weinberg equilibrium.

    Under Hardy-Weinberg equilibrium, allele and genotype frequencies remain constant over generations.

  11. The allele frequencies are described by
  12. p + q = 1, where p is the dominant allele frequency and q is the recessive allele frequency.
  13. The genotype frequencies are described by
  14. p^2 + 2pq + q^2 = 1, where p^2 and q^2 are frequencies of the homozygous genotype and 2pq is the frequency of the heterozygous genotype.
  15. Natural populations may         if conditions deviate from the Hardy-Weinberg model.
    • Genetic        in small populations tends to         genetic variation. In very small populations the              effect can contribute to severe loss of genetic diversity.

      Genetic drift. This small wildflower population has a stable size of 10 plants. Only 5 plants (those in white boxes) of generation 1 produce fertile offspring.

      By chance, only 2 plants of generation 2 manage to leave fertile offspring. The C^W allele frequency (q) increases in generation 2, then falls to zero in generation 3. Genetic variation in this population has been reduced.

      Bottleneck effect. Shaking a few marbles through the narrow neck of a bottle is analogous to a drastic reduction in the size of a population. By chance, blue alleles are over-represented in the new population and gold alleles are lost. Such bottlenecks have reduced genetic diversity in some species.

        Bottleneck effect on greater prairie chicken.

        Loss of prairie habitat caused a severe reduction in the population of greater prairie chickens in Illinois.

        The bottleneck led to reduced genetic variation, as measured by number of alleles per locus .

        The reduced genetic variation is also reflected in lowered egg hatching rates.

    • Gene       from the movement of individuals or gametes between populations can affect allele frequencies and tends to         genetic differences between populations.

      Gene flow due to immigration of people with diverse genetic backgrounds into the U.S. has reduced the genetic differences between the U.S. population and other populations.

      The computer-generated image on this magazine cover illustrates the associated reduction in phenotypic variation.

    •           is the basis for natural selection, and results in unequal reproduction of alleles.

      Mutations can include chromosomal changes as well as point mutations.

      These 2 isolated populations of mice on the island of Madeira exhibit different patterns of fused chromosomes.

      Accumulation of random DNA changes can lead to phenotypic variation and natural selection.

    • Sexually reproductive organisms often exhibit            mating behaviors such as              selection and              selection.

      Intersexual selection.

      Natural selection for mating success may result in sexual dimorphism, marked differences between the sexes in secondary sexual characteristics.

      Many species exhibit intersexual selection (mate choice), when individuals of one sex (usually females) are choosy in selecting their mates.

      Experiments show that peahens prefer to mate with peacocks with greater numbers of eyespots.

      Intrasexual selection.

      Intersexual selection often leads to intrasexual selection, when individuals of one sex (usually males) compete for mates of the opposite sex.

      Intrasexual selection may be observed in physical combat (agonistic behavior) between males.

      Agonistic wolves:

    • Natural selection is the major mechanism that drives adaptive            .

      Modes of natural selection.

      Natural selection favors certain genotypes by acting on the phenotypes of individuals.

      The selection can take place in 3 possible modes.

    • Directional selection favors individuals at one end of the phenotypic range.
    • Disruptive selection favors individuals at both extremes of the phenotypic range.
    • Stabilizing selection favors intermediate phenotypes.

        Directional selection shifts the overall makeup of the population by favoring variants at one extreme of the distribution.

        In this case, darker mice are favored because they live among dark rocks and a darker fur color conceals them from predators.

        Disruptive selection favors variants at both ends of the distribution.

        These mice have colonized a patchy habitat made up of light and dark rocks, with the result that mice of an intermediate color are at a disadvantage.

        Stabilizing selection removes extreme variants from the population and preserves intermediate types.

        If the environment consists of rocks of an intermediate color, both light and dark mice will be selected against.

      Causes of Evolutionary Change: