The Four Forces of Evolution

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  • 0:02 Forces of Evolution Defined
  • 0:33 Mutation
  • 1:13 Gene Flow
  • 2:03 Genetic Drift
  • 2:49 Natural Selection
  • 3:33 Lesson Summary
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Lesson Transcript
Instructor: Terry Dunn

Terry has a master's degree in environmental communications and has taught in a variety of settings.

As you might have guessed, evolution in the natural world is not a simple proposition. There are four different forces that propel evolution onward. In this lesson, you will learn what those forces are and how they work.

Forces of Evolution Defined

May the force be with you. Or, in this case, may the four forces be with you. It's a life or death, high-stakes game of evolution, and every creature, every plant, is going to need all the help it can get. In an ever-changing world, species are trying to adapt, compete, and sometimes just get lucky, in an effort to continue their existence.

So what are these mysterious four forces of evolution? They include mutation, gene flow, genetic drift, and natural selection.


Mutations happen just by chance. Somewhere along the line, a change in a gene or a chromosome shows up as a physical or a behavioral difference in an individual. Because it affects a gene or chromosome, it can then be passed to the next generation. Maybe you think of a mutation as a creepy-looking new feature, but not always. A cat that has six toes on each foot has a mutation that has been passed on from a cat far in the past. Okay that might be a little creepy, but here's one that's not: It's believed that red hair in people was a mutation. Redheads are only 1 to 2 percent of the population. It may be rare, but not too creepy!

Gene Flow

In gene flow, two populations mix genes. Over time there's less distinction between those populations. So, let's say there are two populations of polar bears that have not met in the recent past. Each population was existing on their own, separate chunks of land for a very long time. One population lived to the south and one population lived in the north. There was no reason, or perhaps no way, to have met before. Then, as the climate started to change and more of the sea ice started to melt, the polar bears in the south were not finding the habitats they needed to survive. They started to shift their population towards the north and met the other population of polar bears. Once they started interbreeding, the genetic distinctions decreased. True story. Scientists are seeing this example of gene flow right now.

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