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Invertebrate Characteristics

Eukaryotic, heterotrophic animals that lack a vertebral column and bony skeleton.

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I Thought This Happened Gradually?
Teacher Contributed

I Thought This Happened Gradually?


Tempo of Evolution

Student Exploration

You Want To See Fast?

Many people still feel evolution is a gradual steady process which was the view put forth by Charles Darwin. However, Darwin didn't have the whole story figured out. Go here see what scientists are finding out about the tempo of evolution: http://www.scientificamerican.com/article.cfm?id=see-change-rapid-emergence

In 1972, Niles Eldridge and Stephen Jay Gould put forth a theory that species could go through long periods of relative stasis and then periods of rapid change in response to environmental changes. They called this theory "Punctuated Equilibrium."

You can find a review of gradualism and punctuated equilibrium here

Here is another view point on the importance of this work: http://www.darwinthenandnow.com/2012/07/sea-star-species-defy-darwin/#more-4348

Extension Investigation

If you need help scratching a mental itch, use the resources below:

  1. How does reproduction differ in the two sea stars (Cryptasterina pentagona and C. hystera) mentioned in the above article?
  2. What are two factors that affect the rate of speciaiton? Explain why these factors matter.
  3. What is adaptive radiation?
  4. Can you think of a theory other than the one used by the researchers to explain how this speciation happened so fast?
  5. How does this rapid speciation help us understand biodiversity?
  6. What is "disruptive selection"? How does it differ from "natural selection"?
  7. Take note of the range of dates for the actual split of these two sea stars. What does this tell you about the accuracy of determining dates by molecular clocks? Explain your answer and thinking fully.

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