Lecture 3

Introduction: Uniformitarianism

• Definition: The idea that the same natural processes operating today have always functioned in the past.

Charles Darwin: Life, Work, and the Road to Evolution

Early Life and Education

• Born February 12, 1809 (same day as Abraham Lincoln).

• Came from a wealthy English family.

• Initially studied medicine at Edinburgh but switched to theology at Cambridge.

• Became interested in natural history through mentor John Henslow (botanist).

• Influenced by geologist Charles Lyell, who proposed the Earth was much older than previously thought.

Voyage on HMS Beagle (1831–1836)

• Assigned as the ship’s naturalist for a five-year expedition.

• Key stops and observations:

  1. South America – Studied geological formations and collected fossils.

  2. Galápagos Islands – Observed unique species (e.g., finches, giant tortoises).

  3. Australia and other regions – Collected more biological specimens.

• Major realization: Species differed slightly from island to island, suggesting adaptation to local environments.

• Initial thoughts: Did not immediately develop the theory of evolution but noted the patterns of species variation.

Developing the Theory of Natural Selection

Post-Beagle Research (1836–1859)

• Spent 8 years studying barnacles to gain credibility as a scientist.

• Maintained a notebook on species transmutation (how species change over time).

• Read Thomas Malthus’s Essay on Population (1798):

  • Inspired idea of competition for survival.

  • Population growth is exponential, while resources grow linearly → leads to a struggle for existence.

• Worked in secrecy for 20 years, fearing backlash from religious and scientific communities.

Alfred Wallace’s Letter and Joint Publication (1858)

• Alfred Wallace (naturalist in Indonesia) independently developed the same idea of natural selection.

• Sent a letter to Darwin outlining his theory.

• Darwin, fearing he would be “scooped,” consulted friends.

• Solution:

  • Both Darwin and Wallace’s papers were presented at the Royal Society in 1858.

  • Irony: The Royal Society President’s annual summary stated, “Nothing important happened this year.”

• Darwin rushed to publish On the Origin of Species in 1859.

Key Arguments in On the Origin of Species

1. Variation Under Domestication

• Observation: Domesticated plants and animals show variation in traits.

• Examples:

  • Some cows produce more milk than others.

  • Dog breeds differ drastically in size and shape.

• Key Point: Farmers can select desired traits over generations (artificial selection).

• Heritability: Offspring tend to resemble their parents, but at the time, mechanisms of inheritance were unknown.

  • Gregor Mendel (1865) later discovered genetic inheritance, but Darwin was unaware.

• Pigeon Breeding: Darwin studied pigeon clubs in London, where breeders selected for specific traits.

2. Variation in the Wild

• Key Argument: Variation isn’t limited to domesticated species—it also exists in wild species.

• Natural History as a Popular Hobby:

  • Many Victorian-era middle-class families had curio cabinets filled with fossils, feathers, and specimens.

  • This made Darwin’s argument relatable to the public.

• Implication: Just as farmers select traits artificially, nature must also “select” certain traits naturally.

3. The Struggle for Existence

• Influence from Malthus:

  • Organisms produce more offspring than can survive.

  • Resources are limited, leading to competition.

• Common Misconception at the Time:

  • Natural Theology: The belief that nature was harmonious and perfectly designed.

  • Darwin’s Counterargument:

    • Nature is brutal and full of competition (e.g., lions chasing zebras).

    • More organisms are born than can survive, leading to natural selection.

4. Natural Selection: The Core Idea

• Definition: Traits that increase survival and reproduction become more common over generations.

• Key Components:

  1. Variation: Individuals in a population differ in traits.

  2. Heritability: Traits must be passed to offspring.

  3. Differential Survival/Reproduction: Some individuals survive and reproduce more than others.

• Difference from Lamarck:

  • Lamarck suggested individuals evolve (e.g., giraffes stretching necks).

  • Darwin argued species evolve, but individuals do not change their genetic traits.

Addressing Criticisms of Evolution

1. Where Are the Transitional Fossils?

• Darwin’s Prediction: If species evolved from common ancestors, there should be fossils showing gradual changes.

• Challenges:

  • Fossilization is rare—most organisms don’t fossilize.

  • Fossils only form in certain environments (e.g., muddy seabeds preserve fossils better than forests).

  • Soft-bodied organisms rarely fossilize (e.g., worms leave little fossil evidence).

• Example of Transitional Fossil: Archaeopteryx

  • A “missing link” between dinosaurs and birds.

  • Had feathers (like birds) but teeth and a long bony tail (like reptiles).

2. Large Evolutionary Transitions (e.g., Flight, Eyes)

• Criticism: “What good is half an eye or half a wing?”

• Darwin’s Response:

  • Eyes exist in different forms across species:

    1. Light-sensitive patches (flatworms).

    2. Cup-shaped eyes (some mollusks) – provide directional light sensing.

    3. Pinhole-camera eyes (nautilus) – better focus.

    4. Lens-based eyes (vertebrates) – evolved for sharp vision.

  • Wings may have evolved for other purposes first:

    • Feathers evolved in dinosaurs before flight—possibly for insulation or display.

    • Hollow bones in dinosaurs existed before birds evolved flight.

The “Wait, There’s More!” Argument

• Final Selling Point: If natural selection is true, it explains many unrelated biological phenomena.

• Example: Why Study Mice for Human Medicine?

  • Mice and humans share common ancestors, meaning we share many genetic pathways.

  • Evolution explains why medical research on mice is useful for human medicine.

Final Takeaways

• Darwin’s Book Changed Science Forever:

  • Introduced the mechanism of natural selection.

  • Explained adaptation without supernatural intervention.

  • Laid the foundation for modern genetics and evolutionary biology.

• Evolution Continues to Be Studied Today:

  • Fossil discoveries continue to support Darwin’s ideas.

  • Genetics and molecular biology have strengthened evolutionary theory.