Punctuated Equilibrium: Definition, Theory & Examples

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  • 0:04 Punctuated Equilibrium Defined
  • 1:29 How Does It Work
  • 3:19 Is This the Answer?
  • 4:10 Lesson Summary
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Lesson Transcript
Instructor: Chelsea Schuyler
In this lesson, we'll take a look at the theory of punctuated equilibrium developed by the notable paleontologists Niles Eldredge and Stephen Jay Gould. We'll define punctuated equilibrium, explain its basic tenets and see some simple examples.

Punctuated Equilibrium Defined

Scientists attempt to explain the world around us. They do this through a series of proposed explanations for observed phenomena called hypotheses. These hypotheses are then tested over a period of years to determine if they are supported by facts or if they need to be revised.

Big, complex concepts and observations often have many different hypotheses attempting to explain bits and pieces of the concept all at once. Take, for example, evolution. Scientists have long studied how new species come to be in the world and how these new species descend from ancestors. The exact mechanism that drives this speciation still eludes scientists to some extent, so they are constantly attempting to come up with hypotheses to try and explain their observations.

One major observation about the world around us has to do with when new species appear in the fossil record. If you look through the record, you'll notice that there is a long time where there doesn't seem to be any new life forms on Earth, and then suddenly, boom! We get all these new species, all at once. This is punctuated equilibrium. Punctuated equilibrium is a hypothesis devised to explain a pattern of change in the fossil record and how closely related species appear in nature. The hypothesis states that individual species tend to show little or no change over a long period of geological time and then enter a period of rapid change, which gives rise to new species.

How Does It Work?

Look around you. Do you see all those animals, plants, bacteria, fungi, and viruses around your house and yard? Do you notice any one of them changing? Probably not. Most species are in stasis, a state of stability in which all forces are equal and, therefore, cancel each other out. When a species is in stasis, it's not experiencing any evolutionary pressures, those environmental forces that change birth and death rates in living things, causing species to have to evolve to better handle the pressures. An organism in stasis has no need to evolve - everything is just fine and dandy, so why waste the effort?

But what if something changes in your yard? What if the pH changes in the soil, or suddenly all the water evaporates and it never rains again? All those plants, animals, bacteria, and fungi would experience a harsh environmental change. Many would die. Some would mutate and evolve to handle the new pH levels or the new, drier environment. After several thousand generations, those that survived would probably look different than those that came before, and you would see a whole mess of new species in your backyard.

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