Darwin and the Theory of Evolution: Lecture Notes

Darwin and the Theory of Evolution

2.I Setting the Stage

  • Historical Background Before Darwin

    • Early Greek philosophers were among the first to attempt systematic explanations for the diversity of life based on natural processes.
    • Most Greek philosophers perceived the world as eternal and unchanging, which contradicted evolutionary concepts.
    • These ideas persisted until the Middle Ages and Renaissance, when European scholars adapted them to fit a Christian creation narrative based on literal interpretations of the Bible.
    • This included the belief that the Earth was very young, roughly dated to 4000 BCE based on genealogies in the Old Testament.
    • An 'argument from design' was promoted by medieval scholar St. Thomas Aquinas (1224-1274) and later by William Paley (1743-1805) through his "watchmaker analogy."
    • This analogy suggested that the complex structure of living organisms and their remarkable adaptations are evidence of an intelligent designer.

  • Natural Theology and Classification

    • Beginning in the 1600s, natural theologians developed systematic classification systems based on similarities among organisms.
    • Carl Linnaeus (1707-1778) created a taxonomic system arranging organisms in hierarchical groupings based on their similarities, though he had no theoretical basis beyond believing it represented a divine plan.
    • At this time, species were considered fixed and unchanging, reflecting God's wisdom.
    • Earth was thought to be very young.

  • Changing Ideas

    • By the 10th century, geological circles began arguing that the Earth is ancient based on observed rock formations.
    • James Hutton (1795) proposed that geological processes, including erosion and sedimentation, determined Earth's age.
    • Charles Lyell explained existing geological features through processes observed today, promoting uniformitarianism over catastrophism.
    • Fossils played a critical role in establishing the concept of extinction.
    • French naturalist Georges Cuvier established the occurrence of extinction through fossil anatomy comparisons.
    • Notable fossil discoveries were made by British paleontologist Mary Anning (1799-1847), who showed that some extinct species had no living counterparts.
    • The recognition of extinction challenged the idea that Earth's flora and fauna were constant, hinting at change over large time scales.

  • Historical Context of Women in Evolutionary Biology

    • Women faced significant barriers to scholarship in evolutionary biology.
    • Many Anning, from a working-class background, learned paleontology through family engagement in fossil collecting despite lacking formal education.
    • Anning's expertise in fossils and geology often surpassed that of her wealthier colleagues who published her findings without mentioning her contributions.
    • Despite historical sexism, women are increasingly prominent in contemporary evolutionary biology, contributing significantly to the field.

2.2 - Darwin and the Voyage of the Beagle

  • The Beagle Voyage

    • Post-Bachelor's degree, Charles Darwin received a letter from Henslow, proposing him as an unofficial naturalist on the HMS Beagle, captained by Robert FitzRoy.
    • Voyage began in 1831, initially planned for 2 years, lasting 5 years, during which Darwin suffered from acute sea sickness.
    • He visited three continents and various islands, collecting fossils and studying local fauna and flora, particularly in the ecologically rich Brazilian jungles.
    • Geological Observations
    • Started with catastrophist perspective (e.g., Great Flood).
    • Fitzroy provided Darwin with a copy of Charles Lyell's "Principles of Geology" which shifted his focus to geological questions.
    • Observed uplifted sedimentary rock beds in Cape Verde Islands and experienced land elevation from earthquakes off Chile, integrating these observations into geological understanding.
    • Studied coral reefs and the formation of atolls through coral growth on seabeds.
    • By the return in 1836, Darwin fully embraced Lyell's and Hutton's uniformitarianism.

  • Fauna of the Galapagos Islands

    • The Galapagos archipelago, consisting of 21 volcanic islands about 970km off Ecuador, exhibited unique and varied fauna.
    • Observed significant differences among species occupying similar habitats on different islands, indicating independent evolution.
    • Collected various birds he initially thought were diverse types, only to find upon review that they represented distinct finch species related yet unique to the islands.

2.3 - Darwin's Insights: Evolution and Natural Selection

  • Initial Insights

    • Darwin concluded that species on different islands were sometimes distinct yet closely related to nearby species, leading him to question creationist explanations.
    • Realized that species were not fixed and that the Earth had undergone significant changes over extensive time frames.
    • However, he initially lacked a definitive mechanism for explaining this change, which he later developed upon returning to England.

  • Publication and Collaboration

    • Darwin had the foundational pieces of his theory by the late 1830s but did not publish until a joint letter from Alfred Russell Wallace in 1858 prompted action.
    • Wallace proposed similar ideas regarding species change and natural selection, leading to a joint presentation of their findings at the Linnean Society in July 1858.
    • Darwin’s eventual publication, "On the Origin of Species by Means of Natural Selection," came a year later in 1859.

  • The Concept of Evolution

    • In Darwin's view, evolution entails a gradual change in the characteristics (traits/phenotypes) of populations over time.
    • Modern definitions include changes in heritable characteristics or allele frequencies within populations across generations.
    • Key takeaway: populations evolve, individuals do not.
    • All species are interconnected through a lineage resembling a family tree, descended from common ancestors.

  • Evidence of Descent with Modification

    • Homology: Refers to structural similarities between species, indicating shared ancestry despite functional differences.
    • Original understanding tied to shared structural traits inherited from a common ancestor.
    • Transitional Fossils: Fossil records provide evidence of species changing over time, predicted to include transitional forms.
    • Biogeography: Noted distribution patterns where species grouping reflected geographical proximity and resemblance, further supporting descent from common ancestors.

  • Natural Selection

    • Natural selection serves as the primary mechanism for evolution, explaining how traits conducive to survival are favored.
    • Competitive interactions for finite resources among members of a species drive this process, leading to successful traits becoming more common in succeeding generations.

Critical Observations for Natural Selection

  1. Excess Fertility
    • More offspring are produced than the environment can support, creating competition for resources.
  2. Variation Among Individuals
    • No two individuals are identical; some traits provide advantages in survival.
  3. Heritability of Traits
    • Traits that enhance survival and reproduction can be passed down, becoming prevalent over generations.

Mechanisms of Natural Selection

  • Natural selection requires the following conditions:

    1. Variation in traits within the population.
    2. Non-random association between traits and reproductive success.
    3. Heritability of the advantageous traits.

  • If met, outcomes include changes in allele frequencies across generations.

  • Evidence Supporting Natural Selection

    • Consistent observations align with Darwin's arguments, illustrating how traits enhance survival across environments.
    • Artificial selection has long been an observable process, as illustrated with domestic species bred for specific traits (e.g., pigeons).

2.4 - Evidence for and Misconceptions About Natural Selection

  • The Accumulation of Evidence

    • From the 1930s and 40s, substantial evidence emerged supporting natural selection as a contributor to evolution through observational and manipulative studies.

  • Contemporary Examples

    • Millerian mimicry in Heliconius butterflies exemplifies natural selection at work in response to predator behavior.
    • Tests demonstrated that morphs aligning with local co-models had better survival rates.

  • Misconceptions

    • Natural selection is not goal-driven nor linear, and no species is 'higher' or 'lower.'
    • Misapplications of evolutionary theory (e.g., social Darwinism) lack scientific basis and underline the 'naturalistic fallacy.'
    • Many scientists reconcile their beliefs in evolution with religious faith, viewing science and religion as separate realms.

  • Creationism vs. Evolution

    • Creationism suggests life originated from supernatural events and conflicts with scientific explanations of origins, often invoked through arguments of design.
    • Intelligent design lacks falsifiability and cannot compete with established scientific knowledge.

  • Ethical Implications

    • Misuses of evolutionary principles have justified poor policies such as eugenics and racial segregation, emphasizing the need for ethical scrutiny in scientific interpretations.