Gradualism: Definition, Theory & Examples

Instructor: Dominic Corsini
This lesson contains an investigation into evolution via gradualism. Go over examples of the process in action, reasons for its occurrence, geologic references, key terms, and a brief quiz at the end.

Shock Therapy

Back when I was an undergraduate, I had the opportunity to complete an aquatic ecology internship with one of my professors. One of the best parts of the experience involved sampling local fish populations. To do this we used electrofishing equipment! Basically it involved a probe (that looks like a standard metal detector) attached to a backpack generator. A team of people wearing rubber waders would enter a steam and the person wearing the generator would use the probe to send an electric current through the water. The rest of the team would use nets to collect shocked (but still living) fish that floated to the surface.


The purpose of this sampling was to assess physical variation between different fish populations. Each population lived in separate mountain streams. So fish in one stream were isolated from fish in other streams. In essence, we had access to several isolated populations that were evolving independently of one another.

What we found was that each population contained slight variation in their physical appearance. It wasn't enough to call them different species, but it was noticeable. The change was extremely gradual. If the small changes we saw continued for many generations that would be an example of gradualism. Gradualism is a mechanism of evolution that proceeds through the slow accumulation of subtle changes.

Geologic Evidence

Fossils provide an excellent source of evidence for gradualism. Fossils are the preserved remnants of long dead organisms (bone, hair, impressions, etc.). One of the best-preserved fossil records belongs to horses. Take a look at this picture.

Horse Evolution
Horse Evolution

The fossilized remains of horses have been found throughout various layers of rocks. Generally speaking, as you go deeper in rock layers the material you find gets older. Older horse fossils are also smaller. As time goes on, the fossilized remains get larger and larger. Eventually you find fossils that resemble horses today. What this shows is the gradual evolution from small-bodied horses to large-bodied horses. This is an example of gradualism.

Selective Pressures

So what exactly causes organisms to change via gradualism? If we look back at our first example, we could ask why fish exhibited slight variation from one stream to the next.

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