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Extinction and Radiation of Life

After a mass extinction evolution works rapidly to create new species that can fill the vacant habitats.

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Extinction and Radiation of Life

Should this pterodactyl be concerned? Should you?

When the dinosaurs were wiped out by an asteroid impact, the mammals were waiting to take over their niches. Could this happen again? Are there other ways species could go extinct and leave open niches for new organisms to fill?


Most of the species that have lived have also gone extinct. There are two ways to go extinct: besides the obvious way of dying out completely, a species goes extinct if it evolves into a different species. Extinction is a normal part of Earth's history.

But sometimes large numbers of species go extinct in a short amount of time. This is a mass extinction. The causes of different mass extinctions are different: collisions with comets or asteroids, massive volcanic eruptions, or rapidly changing climate are all possible causes of some of these disasters (Figure below).

Fossil of a Tyrannosaurus Rex

An extinct Tyrannosaurus rex. This fossil resembles a living organism.

Adaptive Radiation

After a mass extinction, many habitats are no longer inhabited by organisms because they have gone extinct. With new habitats available, some species will adapt to the new environments. Evolutionary processes act rapidly during these times and many new species evolve to fill those available habitats. The process in which many new species evolve in a short period of time to fill available niches is called adaptive radiation. At the end of this period of rapid evolution the life forms do not look much like the ones that were around before the mass extinction. For example, after the extinction of the dinosaurs, mammals underwent adaptive radiation and became the dominant life form.


  • Species go extinct when all of the individuals die out or evolve into a different species.
  • Many species go extinct at roughly the same time during a mass extinction.
  • New habitats become available and species evolve to fill them so that biodiversity increases during adaptive radiation.


  1. Why is extinction considered a normal part of Earth's history?
  2. What are some of the possible causes of mass extinctions?
  3. Why do many new species evolve after a mass extinction?

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Use this resource to answer the questions that follow.

  1. What does a branch on a phylogenetic tree indicate?
  2. What two things could have happened when a species disappears?
  3. What is necessary regarding ability to mate for one species to become two different species?
  4. What two things might a changing environment force to happen? What do each of these to things do to biodiversity?
  5. How did natural selection take place in three-spined stickelbacks at Loberg Lake in Alaska? How is this an example of speciation?
  6. Under what conditions does adaptive radiation take place? How are honeycreepers in Hawaii an example?
  7. How many species that ever existed are now extinct?
  8. How many mass extinctions have been identified according to the teacher? What is a mass extinction?
  9. What is the K-T (Cretaceous-Tertiary) boundary?
  10. What is found at the K-T boundary? What could that mean?

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adaptive radiation An explosion in the diversity of species as vacant niches are filled. This often occurs after a mass extinction.

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