Evolution Notes

Charles Darwin

• Darwin's voyage and excerpts set the stage for understanding evolution.

The Big Picture

• The central question is: How do living things change over time?

Darwin’s Observations and Natural Selection

• Darwin proposed natural selection as the mechanism behind evolution based on key observations:
  • Observation 1: Members of a population often vary in their inherited traits.
  • Observation 2: All species are capable of producing more offspring than the environment can support.
• From these, Darwin inferred:
  • Inference 1: Individuals with inherited traits that give them a higher probability of surviving and reproducing in a given environment tend to leave more offspring than other individuals.
  • Inference 2: This unequal production of offspring will lead to the accumulation of favorable traits in a population over generations.

Evidence of Evolution

• Fossils provide crucial evidence for understanding evolution.

Fossils

• Hard parts of organisms are often preserved:
  • Bones
  • Teeth
  • Shells
• Fossils can be trapped in ice, amber, or tar.
• Soft tissues may be replaced by minerals through petrification.
• Ötzi the Iceman: Oldest known human mummy, lived between 3350-3105 BC.
• Fossils can form imprints on soft mud or sand.
  • Deep imprints create molds.
  • Molds may fill with minerals, forming casts.
• The position of an organism in strata of rock indicates relative age.
• Transitional fossils show change over time.
  • Example: Early whales in Egypt and Pakistan had small hind legs.
• Radiometric Dating: Isotopes decay at a known rate (half-life).
  • Carbon-14 ($C14$): Half-life of 5700 years (accurate for dating materials younger than 50,000 years).
  • Potassium-40 ($K40$): Half-life of 1.3 billion years.
  • Uranium-238 ($U238$): Half-life of 4.5 billion years.
• Error rate for radiometric dating is approximately 10%.
• Relative position and isometric dating provide insight into the age and natural history of an organism.
• 99% of all plant and animal species that ever lived are extinct.

Biogeography

• Biogeography studies the geographic distribution of species.
• Species in similar environments are not necessarily the same.
• Species develop local adaptations.
• Geographic isolation leads to the rise of unique species.

Comparative Anatomy

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

Comparative Embryology

• Embryos of different species develop almost identically.
  • Gill pouches, tails, paws.
• Follow same developmental pathways.
• “Evo-devo”: Evolution/developmental biology.
  • Ontogeny Recapitulates Phylogeny

Molecular Biology

• Infer relatedness based on similarities of DNA sequences.
  • Families are more similar than non-families.
  • Humans are more similar to humans than to chimps.
  • Humans are 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.

Evidence of Evolution

• Evidence of evolution clearly stated and supported by fossil records, comparative anatomy/embryology, and molecular biology.

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; specific mating would be equal to small population).
  • Equal reproductive success.
• This equilibrium is met only if evolution is NOT taking place.

Hardy-Weinberg Equilibrium Calculations

• $p$ = frequency of dominant allele
• $q$ = frequency of recessive allele
• $p + q = 1$
• $p^2 + 2pq + q^2 = 1$
• Example:
  • Tall is dominant to short.
  • 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 individual ($2pq$) = 2 * 0.4 * 0.6 = 0.48
  • Frequency of homozygous dominant individuals ($p^2$) = $(0.4)^2$ = 0.16