Overview
This lecture covers the parallel journeys of Charles Darwin and Alfred Russel Wallace as they independently discovered the process of evolution by natural selection, shaping the modern understanding of the origin of species.
The Search for the Origin of Species
- Alfred Wallace's Amazon specimens and notes are destroyed in a ship fire, losing valuable data on species distribution.
- The central scientific question: Where do species come from?
- Charles Darwin and Wallace approach the answer through separate expeditions.
Darwin’s Voyage and Discoveries
- Darwin joins the HMS Beagle as a naturalist, exploring global flora, fauna, and geology.
- He observes similarities between fossils (glyptodon, giant sloths) and modern animals (armadillos, sloths).
- In the Galapagos Islands, differences among island tortoises and mockingbirds suggest species can change.
- Darwin sketches a family tree of life, proposing that new species arise from existing ones.
- His idea that species are not fixed challenges the doctrine of special creation.
Wallace’s Expeditions and Insights
- Wallace survives his ordeal at sea and later explores the Malay Archipelago.
- He notices distinct but related species of butterflies and birds on different islands, mirroring Darwin’s Galapagos observations.
- Wallace develops a "law of nature": new species arise near similar existing species.
- The "Wallace Line" divides Asian-type and Australian-type animals, explained by historical land connections.
Mechanism of Evolution: Natural Selection
- Both Darwin and Wallace realize: Individuals in species vary, and more are born than can survive.
- Variation plus massive death leads the best-adapted to survive and reproduce—a process called natural selection.
- Wallace independently writes to Darwin outlining this theory, prompting the joint presentation of their ideas.
Legacy and Impact
- Darwin publishes On the Origin of Species (1859), launching modern evolutionary biology.
- Wallace and Darwin remain collaborators and friends, recognizing their shared contributions.
Key Terms & Definitions
- Special Creation — The belief that each species was individually created by God and is unchanging.
- Natural Selection — The process where individuals with advantageous traits survive and reproduce more, driving evolutionary change.
- Vestigial Structures — Anatomical features that serve no current function, remnants of ancestral forms.
- Wallace Line — A biogeographical boundary separating Asian and Australian animal species in the Malay Archipelago.
- Tree of Life — Darwin’s metaphor illustrating evolutionary relationships among species.
Action Items / Next Steps
- Review the definitions and key ideas of natural selection and biogeography.
- Read additional chapters from Darwin’s On the Origin of Species as assigned.
- Prepare notes on how evidence from fossils and biogeography supports evolution for next class discussion.