Chapter 18 Notes: Evolution, Natural Selection, and Key Evidence

Darwin and Natural Selection

  • Darwin independently proposed natural selection; Alfred Russel Wallace co-discovered the idea.
  • Origin of Species: outlines evidence for evolution and mechanism (descent modification and natural selection).
  • Natural selection = differential survival and reproduction; environment is the selector, not humans.
  • Fitness = ability to survive and reproduce; not simply physical strength.
  • Lamarck vs Darwin: acquired characteristics are not how evolution works; Darwin refined the mechanism.

Three Conditions for Natural Selection

  • Variation exists within a population.
  • Variation has a genetic basis (heritable).
  • Competition for limited resources leads to differential survival and reproduction.
  • Zusammenfassend:
    • Variation
    • Heritability
    • Competition
    • Natural selection acts via these conditions to produce adaptation. ext{variation}, ext{heredity}, ext{competition}

Sources of Variation

  • Mutations introduce genetic variation; mutations are changes in DNA.
  • In Darwin’s time, genetics was not understood; DNA, chromosomes, and mechanisms of inheritance were later discoveries.

Artificial Selection and Natural Selection

  • Artificial selection: humans select traits (e.g., pigeon breeds, dog breeds).
  • If humans can produce diverse forms by selection, nature can do so as well through natural selection.

Natural Selection in Action and Definition

  • Darwin’s process leads to adaptation over many generations.
  • Simple definition: differential survival and reproduction.
  • “Fitness” is about reproductive success, not physical fitness.
  • Natural selection can lead to new species over time; speciation will be discussed in depth later.

Evidence for Evolution (overview)

  • Multiple, independent lines of evidence should support evolution:
    • Fossil record (transitions between groups)
    • Anatomy (homologous vs analogous structures)
    • Embryology (developmental similarities)
    • Biogeography (distribution of life across geography)
    • Genetics/molecular biology (genetic similarities; universal code)
    • Ecology and natural selection (evidence of selection in action)
    • Cellular evidence (endosymbiosis and other cellular similarities)
  • Tiktaalik as a key fossil illustrating a fish-to-tetrapod transition.

Tiktaalik: Transitional fossil

  • Discovered in Arctic Canada; dated to ~3\times 10^8 years ago (around 300 million years).
  • Found in 2004.
  • Features mix of fish and tetrapod characteristics (gills and fish-like head plus limb-like structures); demonstrates transition between aquatic and terrestrial life.

Homologous vs Analogous vs Vestigial Structures

  • Homologous structures: same bones/origin from a common ancestor; share developmental origin (e.g., forelimbs of humans, bats, whales, cats).
  • Analogous structures: similar function but different evolutionary origin (convergent evolution); e.g., wings of birds, bats, and insects.
  • Vestigial structures: reduced or lost function remnants of past evolution (e.g., appendix; ostrich wings).

Wallace's Line and Biogeography

  • Wallace's Line: a biogeographic boundary proposed by Alfred Russel Wallace separating distinct fauna between Asia and Australia.
  • Explains why nearby islands have very different species due to historical separation and differing evolutionary histories.
  • Plate tectonics later clarified the geographic basis of these patterns (Australia plate vs Asian plates).

Speciation and Reproductive Isolation

  • Speciation arises when populations become geographically or ecologically separated, leading to divergent evolution.
  • Over time, reproductive isolation reduces or eliminates gene flow, producing distinct species.

Quick Classroom Activity (evidence in six areas)

  • Six areas of evidence: fossil record, anatomy, embryology, biogeography, genetics/molecular biology, ecology/natural selection, and cell biology (endosymbiosis).
  • Plan: groups investigate one question per area and report findings; results will be compiled into a PowerPoint slide deck.

Quiz and Homework Prep (from transcript)

  • Quiz on Thursday; five questions; five minutes; questions drawn from Day 1 through today.
  • Content emphasizes vocabulary and notes; review last homework and the instructor’s feedback on open-ended items.
  • Homework tip: apply natural selection steps (variation, heritability, competition) to the organism in question.

Darwin’s Influence and Key Concepts

  • Darwin popularized the idea of evolution and the tree-like pattern of descent with modification.
  • He argued against the ladder (scale of nature) and for a common ancestor and branching diversity.
  • He acknowledged limitations in knowledge of genetics, chromosomes, and DNA; genetics emerged later (late 19th–20th century).

Enduring Questions and Takeaways

  • Evolution is supported by multiple, converging lines of evidence across disciplines.
  • The mechanism is natural selection acting on heritable variation, with mutation providing the raw material.
  • The fossil record, homologous structures, biogeography, and genetics collectively support common descent and diversification.
  • Tiktaalik and Wallace’s Line exemplify key concepts of transitional forms and geographic patterns in evolution.