Extinction

Chapter Summary

1. A species is said to be extinct when all members of that species have died out and left no living representatives. If all species in a genus are extinct, then that genus is extinct. Most species that have ever lived have gone extinct.
2. When estimating extinction dates from the fossil record, evolutionary biologists must be aware of both backward smearing and forward smearing effects.
3. Rates of extinction vary over time. Extinction rates sometimes spike far above normal, or above what is sometimes called background levels. These spikes in extinction rate are called mass extinctions.
4. Many causes for background extinction have been studied, among them, predation, competition, and disease. Both direct and indirect effects of predation, competition, and disease may lead to background extinction.
5. Mass extinctions affect many species over a broad geographic range. At least five (and perhaps as many as eight) such mass extinctions have occurred over 600 million years—at the end of the Ordovician, in the Late Devonian, at the end of the Permian, at the end of the Triassic, and at the Cretaceous–Tertiary (K–T) boundary.
6. Mass extinction not only leads to fewer species and genera, but it also decreases diversity with respect to morphology, behavior, the number of different types of niches inhabited by organisms, and developmental patterns.
7. The best studied and most famous of the mass extinctions is the K–T mass extinction. A large asteroid that collided with Earth approximately 65 million years ago initiated this mass extinction.
8. The Permian extinction occurred approximately 250 million years ago, and 80 to 96% of all marine species went extinct. This mass extinction may have been triggered by a series of huge volcanic eruptions in Siberia that occurred about 251 million years ago in an area known as the Siberian Traps.
9. Two different models—phyletic gradualism and punctuated equilibrium—have been proposed to explain rates of evolutionary change.
10. Across macroevolutionary timescales, evolutionary biologists sometimes find trends—patterns of directional change over time. Some of these trends are passive and some are active. Active trends may arise when the distribution of trait values in a clade shifts because the trait values within each subclade shift in parallel. Alternatively, the average trait value may increase because of species selection.