Punctuated Equilibrium — Definition & Overview

What Is Punctuated Equilibrium?

Punctuated equilibrium is a theory of evolution that tries to model the rate of speciation. The term was coined by biologists Stephen Jay Gould and Niles Eldredge.

Punctuational evolution describes species as being mostly in a stable equilibrium. They change little over time. These long periods of stability are punctuated by short bursts of rapid evolution.

This happens because most species are already well-adapted to their current environment. They only need to gain new adaptations if their environment changes.

This model explains why all organisms on Earth have a ton of genetic material in common. Evolution happens only when it is necessary. It also explains why we don't see small changes in a species within the fossil record.

How Is It Different from Gradualism?

The other major theory describing rate of speciation is gradualism. It says that species keep adding up small variations. As a result, speciation happens gradually.

Models of Speciation

Punctuated equilibrium is a theory of evolution that tries to model the rate of speciation. It was developed through research by scientists Eldredge and Gould. According to this theory, species are mostly in equilibrium. That is, populations stay more or less the same over time. These long periods of stability are punctuated by short periods of rapid evolution.

Why Would Populations Be Stable Most of the Time?

Have you heard the phrase "If it ain't broke, don't fix it"? Life pretty much follows the same rule. Most organisms are pretty well adapted to their environment. Even if an enzyme could be more efficient, the genetic code is resistant to change. That's why there's so much evidence of shared ancestry in distantly related organisms.

Mutations occur at random and most mutations are harmful. Mutations that lower an organism's fitness are selected against. Harmful mutations will be quickly erased from the population if they are dominant. Their frequency will steadily drop over time if they are recessive. Beneficial mutations and mutations that cause huge changes (good or bad) in an organism are rare.

What Causes the Sudden Periods of Evolution?

Suppose a new mutation causes a large selective advantage. Thanks to natural selection, organisms with that mutation will be more likely to survive and produce more offspring. Organisms without this beneficial mutation do worse. They are less likely to survive and/or pass on their genes to offspring. Within a few generations the new, fitter trait(s) become common in the population. If there is strong selective pressure, the mutation may even replace the less fit version of the trait entirely.

Evolutionary processes can also be driven by isolated populations. Sudden, drastic changes in the environment have an effect as well. According to the theory of punctuated equilibrium, these changes cause quick speciation events over short periods of time. Don't be fooled though. When we say short periods of time, we are still talking about millions of years in most cases.

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What's the Alternative Theory?

Gradualism, the other major theory describing rate of speciation, basically says the opposite thing. Gradualism says that species are gradually evolving over time. This model is more similar to what Charles Darwin proposed in his book On the Origin of Species.

(Video) Process of Evolution | Gradualism vs. Punctuated Equilibrium vs. Catastrophism

by 2 Minute Classroom

There are two main theories of evolutionary biology. They are known as gradualism and punctuated equilibrium. These theories model the rate of speciation. Check out this video to learn about how they are different.