Evolution, Natural Selection, and a Gnarly Skin Infection

What is Evolution?

  • Descent with modification

  • Change in allele frequencies over time

Buffon’s Observations B

  • Buffon’s studies on animals showed anatomical similarity

  • Despite a wide diversity of form and function, the basic architecture was the same

  • Possible result of common ancestry?

Lamarck’s ideas of evolution

  • Lamarck believed that the use or disuse of traits could cause them to be altered over an individual’s lifetime

  • He suggested these differences could be inherited by offspring

Charles Darwin & The Origin

  • evolution as a concept was known in the pre-Darwinian world

  • His book, The Origin provided a suitable mechanism to explain how evolution can occur

Darwin’s Voyage

  • At age 22, Charles Darwin began a five-year, round-the-world voyage aboard the HMS Beagle

  • In his role as ships naturalist, he collected and examined the species that inhabited the regions the ship visited

Darwin’s Observations

  • Many species of finch on the Galapagos on the island were clearly from similar origin

  • Why so different?

Darwin and Natural Selection

  • Darwin’s Theory of Natural Selection

    • Variation

    • Heritability

    • Differential reproductive success

Darwin’s Theory

  • Observation 1: All populations have the
    potential to grow exponentially

  • Observation 2: A population does not grow
    exponentially

  • Inference 1: There is a competition for
    resources

  • Observation 3: Variation exists within a
    population:

  • Observation 4: This variation is heritable (i.e.,
    offspring resemble parents)

  • Inference 2: Some traits help an individual
    better compete for resources. These traits will
    become more abundant in a population.

  • Conclusion: Natural selection favors traits in a
    population that are adaptive. Less
    advantageous traits will decrease in
    frequency. The population changes (evolves)
    as a result of this selection.

Natural Selection Distilled Down

  • Natural selection has three important points

    • A trait must be variable

    • A trait must be heritable

    • This trait must contribute to the owners’ achieving
      greater reproductive success

Individuals do not evolve, only populations

  • Population is the basic unit of evolution

Directional Selection

  • The change in a phenotype or genotype of a population in one direction away from the mean (average) in a particular environment over time.

  • Prior to the Industrial Revolution, trees were covered with light-colored lichens, which camouflaged the light-colored moths. With the advent of industry, pollution caused many trees to become dark, favoring dark-colored moths.

Stabilizing Selection

  • form of natural selection wherein individuals with moderate or average phenotypes are more fit (more likely to survive and reproduce)

  • Selection acts against extreme variants

  • Results in a population with a smaller range of
    variation

Disruptive Selection

  • produces a population that has two extreme versions of a trait as the dominant phenotype.

  • Intermediate values are
    selected against

  • Extreme values are
    favored, resulting in a
    bimodal distribution of
    variants

Sexual Selection

  • Natural selection favors
    those traits that allow
    individuals to spread
    their alleles more

  • This can be
    accomplished even at
    the expense of
    survivorship, in a special
    form of natural
    selection called sexual
    selection

Evolution is a change in allele frequencies

  • All of the alleles of a given population contribute to its
    gene pool

  • Changes in the frequencies of these alleles = evolution

    • Microevolution
      – Small-scale changes in allele frequencies

    • Macroevolution
      – Evolutionary patterns on a long time scale

Gene Flow

  • The movement of individuals or gametes from
    one population to another

  • This movement of genes brings a novel
    distribution of alleles that changes a population’s
    allele frequencies

Genetic Drift

  • The random sampling of
    alleles in a population
    can cause a shift in
    allele frequencies

  • Affects small
    populations more than
    large

Evolution Can Occur in Several Ways

  • Mutation: a change in the sequence of an organisms DNA

  • Gene flow: the movement of genes from one population to another

  • Genetic drift: the evolutionary mechanism whereby random fluctuation in allele frequencies occurs across generations by chance

  • Natural selection: the process where organisms with traits that better suit their environment are more likely to survive and reproduce, causing those beneficial traits to become more common within a population over time

  • Sexual selection (Non-random mating): mechanism of evolution that describes how organisms compete for access to mates and reproduce

Drift: Founders Effect

  • A small, colonizing population cannot
    represent the allelic diversity of a large
    population

Hardy-Weinberg Equilibrium

  • They concluded that the allele
    frequencies of a population could be
    predicted by:
    p + q = 1

  • And the genotype frequencies
    could be predicted by:
    p2 + 2pq + q2 = 1