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Evolutionary Change: Definition and Forms

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  • 0:09 Review of Evolution
  • 0:40 Catastophism vs. Gradualism
  • 2:41 Adaptations vs. Status
  • 4:40 Lesson Summary
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Lesson Transcript
Instructor: Danielle Weber

Danielle teaches high school science and has an master's degree in science education.

You've probably heard of Charles Darwin and evolution. We know that evolution is change over time, but here we will look at how these changes may have occurred during Earth's history.

Review of Evolution

Evolution is a key component of much of our understanding of biology and of life. You may remember that evolution is basically change over time. You previously may have learned about people who studied evolution, such as Charles Darwin and Alfred Wallace, or you may have learned about terms related to evolution, such as 'natural selection' and 'survival of the fittest.' While these people and concepts are essential to our understanding of evolution, here we will focus more on broad patterns of change over time as well as what may have triggered evolution.

Catastrophism vs. Gradualism

All evolution involves change over time, but there are several ways or patterns in which these changes can occur. These theories of how evolution has occurred are largely based on our understanding of Earth's history. Many of the people who proposed these patterns of evolution were geologists, not biologists. This is because they had a solid understanding of the fossil record found within the layers of rocks.

Let's first look at catastrophism. As the name implies, this pattern of evolution is based on the assumption that catastrophic events separate distinctive boundaries in the fossil record. These catastrophes could have been events such as floods, droughts, or even volcanic eruptions. The man who proposed this theory, George Cuvier, thought that these events were local, meaning they would happen in one specific area and only affect the organisms living there. These catastrophes would result in evolutionary change because certain organisms would die while others may survive, resulting in possible changes and adaptations. Sometimes, catastrophic events may have caused mass extinctions, such as the extinction of the dinosaurs.

Rather than sudden events such as those seen in catastrophism, the idea of gradualism proposed that changes take place continually over long periods of time. This means that change did not have to only occur following a huge event, but continually and gradually occurred all the time. The geologist James Hutton used the fossil record to support his theory, which is widely accepted now. Gradualism is when an organism changes just slightly from generation to generation.

For example, with giraffes, the development of a long neck could have happened gradually. That is, in one generation, a giraffe with a slightly longer neck was more successful at gathering food than his friends and was able to reproduce and pass on his long neck. The advantage of having a slightly longer neck continued until all giraffes had long necks. However, it may have taken many generations for this change to take place.

Adaptation vs. Status

Now that we have reviewed both gradualism and catastrophism, let's take a look at how or why things may adapt as well as when they may stay the same. Status, as in 'status quo,' means that things stay as they are. Life forms may not change if they are well-suited for the environment in which they live and if the environment is stable. Between catastrophic events, for example, organisms may have not drastically changed.

Adaptation is when organisms do change. This can be due to countless reasons but, relating it back to our two ideas of how life on Earth has changed over time, the idea of punctuated equilibrium - that there were periods without any major changes and then short, sudden changes that resulted in evolution and even the emergence of new species - fits with catastrophism. One example of a period of intense evolution is seen in the geological record with the Cambrian explosion. This period in Earth's history involved a great diversification of life both in water and on land. Evolution was rapidly occurring.

Punctuated equilibrium isn't the only way that things could have changed over time. Both the idea of adaptive radiation and descent with modification indicate that evolution occurred over time as environments and locations changed. Both adaptive radiation and descent with modification imply that similar organisms shared a common ancestor and then, as organisms moved - or radiated - away from one location, evolution occurred so that they were well-adapted for a new location. These forms of evolutionary change fit more with gradualism than catastrophism.

Slight differences among finch species on the Galapagos Islands is an example of adaptive radiation.
Adaptive radation example

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