Charles Darwin and Evidence of Evolution Notes

Charles Darwin

• Darwin's Voyage Map and Excerpts.

The Big Picture

• How do living things change over time?

Darwin’s Observations & Natural Selection

• Darwin proposed Natural Selection as the mechanism behind evolution.
• Observation 1: Members of a population often vary in their inherited traits.
• Observation 2: All species can produce more offspring than the environment can support.
• Inference 1: Individuals with advantageous inherited traits are more likely to survive and reproduce.
• Inference 2: Unequal offspring production leads to the accumulation of favorable traits over generations.

Evidence of Evolution

Fossils

• Hard parts are preserved:
  • Bones
  • Teeth
  • Shells
• Fossils can be trapped in:
  • Ice
  • Amber
  • Tar
• Soft tissues can be replaced by minerals, a process called petrification.
• Examples:
  • Ötzi the Iceman: Lived between 3350-3105 BC.
• Fossils form imprints on soft mud or sand.
  • Deep imprint forms a mold.
  • Molds may fill with minerals, forming a cast.
• Position of organism in strata of rock tells relative age.
• Transitional fossils show change; for example, early whales in Egypt and Pakistan had small hind legs.
• Radiometric Dating: Isotopes decay at a known rate (half-life).
  • C14: Half-life of 5700 years (accurate <50,000 years).
  • K40: Half-life of 1.3 billion years.
  • U238: Half-life of 4.5 billion years.
  • Error rate is approximately 10%.
• Relative position and isometric dating give an idea of age and natural history.
• 99% of all plant and animal species that ever lived are extinct.

Biogeography

• Geographic distribution of species.
• Species in similar environments are not the same; they make local adaptations.
• Geographic isolation gives rise to unique species.

Comparative Anatomy

• Homologous Structures: Same structure modified for a different purpose.
  • Points to common ancestry.
  • “Descent with modification” (Darwin).
  • Consistent with divergent evolution.
• Analogous Structures: Different structure modified for a similar purpose.
  • Happens to unrelated species in similar habitats.
  • Consistent with convergent evolution.
• Imperfection of Adaptation: Structures are not specifically designed for function.
  • Example: Lower backs and knees were designed for 4-legged walking, causing stress in upright walking.
• Vestigial Structures: No longer needed and present in diminished form.
  • Examples:
    • Human tail
    • Appendix
    • Ear muscles
    • Snake pelvic bones and limbs
    • Whale/dolphin hind limbs

Comparative Embryology

• Embryos develop almost identically.
  • Gill pouches, tails, paws.
• Same developmental pathways.
• Science of “evo-devo” – Evolution/developmental biology.
• Ontogeny Recapitulates Phylogeny.

Molecular Biology

• Infer relatedness based on similarities of DNA sequences.
  • Families more similar than non-families.
  • Humans more similar to humans than chimps.
  • Humans more similar to chimps than to mice.
• Sequences of DNA are highly conserved.
• Relatedness can be calculated based on the number of differences in conserved sequences.

Hardy-Weinberg Equilibrium

• Equation: $p^2 + 2pq + q^2 = 1$
• Assumptions:
  • Population is large.
  • Population is isolated (no immigration/emigration).
  • No mutations.
  • Random mating (everyone breeds).
  • Equal reproductive success.
• Equilibrium is only met if evolution is NOT taking place.
• p = frequency of dominant allele
• q = frequency of recessive allele
• $p + q = 1$
• $p^2 + 2pq + q^2 = 1$
• Example Problem:
  • Tall is dominant to short. In a population, 36% of individuals are short.
  • Frequency of homozygous recessive individuals ($q^2$) = 0.36
  • Frequency of recessive allele (q) = $\sqrt{0.36} = 0.6$
  • Frequency of dominant allele (p) = $1 - q = 1 - 0.6 = 0.4$
  • Frequency of heterozygous individuals (2pq) = $2 "." p "." q = 2 "." 0.4 "." 0.6 = 0.48$
  • Frequency of homozygous dominant individuals ($p^2$) = $0.4^2 = 0.16$