Unit 4: Extinction and Origin of Higher Taxa
Table of Contents
1. Extinction: Background vs. Mass Extinction
Extinction is the complete disappearance of a species from Earth. It is a natural part of evolution, but it occurs at varying rates.
Background Extinction
The standard, low-level rate of extinction that occurs continuously throughout geological time. It is usually caused by local environmental changes, competition, or gradual loss of habitat.
Mass Extinction
A widespread and rapid decrease in biodiversity on Earth. It involves the loss of a large percentage of all living species across various groups within a relatively short geological period.
2. Major Mass Extinctions and the K-T Boundary
Earth has witnessed five major mass extinction events (the "Big Five").
| Event | Period | Major Impact |
|---|---|---|
| Permian Extinction | End of Permian | The "Great Dying"; ~96% of marine species lost. |
| K-T Extinction | End of Cretaceous | Extinction of Dinosaurs; allowed for the rise of mammals. |
The K-T Boundary (Cretaceous-Tertiary)
The K-T boundary is a thin layer of sediment found worldwide, marking the end of the Mesozoic Era. It is famous for the Alvarez Hypothesis, which suggests a massive asteroid impact caused the extinction of non-avian dinosaurs.
- Evidence: High concentrations of Iridium (rare on Earth but common in asteroids) in the boundary clay.
- Consequences: Global cooling ("impact winter"), acid rain, and massive wildfires.
3. Origin of Higher Taxa: Simpson's View
George Gaylord Simpson, a key figure in the Modern Synthesis, explained how higher taxonomic groups (Families, Orders, Classes) originate through Quantum Evolution.
- Adaptive Zones: Evolution often involves moving from one "adaptive zone" (a set of ecological niches) to another.
- Quantum Evolution: Rapid evolutionary change that occurs when a population moves into a new adaptive zone where it faces strong new selective pressures. This leads to the origin of higher taxa.
4. Phyletic Gradualism and Punctuated Equilibrium
There are two primary models regarding the tempo of evolution.
Phyletic Gradualism
The traditional Darwinian view that evolution occurs at a constant, slow, and steady rate. Transformation is continuous and incremental.
Punctuated Equilibrium
Proposed by Niles Eldredge and Stephen Jay Gould. It suggests that species remain stable for long periods (stasis), "punctuated" by brief periods of rapid change during speciation.
5. Exam Focus Enhancements
Exam Tips
- Iridium: If a question asks for the primary evidence of an asteroid impact at the K-T boundary, the answer is Iridium spike.
- The "Big Five": While you should know there are five, focus your detailed study on the Permian (largest) and Cretaceous (dinosaurs).
- Punctuated Equilibrium: Remember the term "Stasis"—it describes the long periods where no evolutionary change occurs in this model.
Frequently Asked Questions
- Differentiate between background extinction and mass extinction.
- Explain the significance of the K-T boundary in evolutionary history.
- Discuss Simpson's concept of the origin of higher taxa.
- Compare the models of Phyletic Gradualism and Punctuated Equilibrium.