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11.5: Theory of Evolution by Natural Selection

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How did dark deer mice become lighter over time?

A species changes color over time if its environment changes. Many other types of changes can take place too. The darker deer mice evolved into the lighter deer mice in some locations. What do you think changed about the environment?

Biological Evolution

Adaptations in a species add up over time. These adaptations will mean that many genes have changed. If the environment is stable, the species won't change. But if the environment is changing, the species will need to adapt. Many adaptations may be necessary. In time, the species may change a lot. The descendants will be very different from their ancestors. They may even become a new species. This process is called biological evolution, but is usually referred to just as evolution .

Organisms alive today evolved from earlier life forms. We can learn about this from fossils. For example, horse fossils from 60 million years ago are very different from modern horses. Ancient horses were much smaller than they are today ( Figure below ). The horses’ teeth and hooves have also changed. The horses evolved because of changes in their environment.

Ancient horses were quite different from present-day horses.

Although basic horse evolution appears to be linear, there were many more branches. Horse evolution is very well understood.

Natural Selection

Evolution happens because of natural selection . Good traits become more common in a population. Bad traits become less common. The deer mouse, species Peromyscus maniculatus gives an example of evolution by natural selection. In Nebraska this mouse is typically brown. But in places where glaciers dropped lighter sand over the darker soil, the mice are light. Why? Because predators could more easily spot the dark mice on light sand. The lighter mice were more likely to survive and have offspring. Natural selection favored the light mice. Over time, the population became light colored. Enough changes may take place over time that the two types of mice become different species.

This story is covered in more detail here: http://news.harvard.edu/gazette/story/2009/08/mice-living-in-sand-hills-quickly-evolved-lighter-coloration/ .


  • evolution : Change through time. The change in the genetic makeup of a population of organisms over time such that a new species is often the result.
  • natural selection : The mechanism for evolution.


  • Changes in the genes of a species may result in a new species. This is biological evolution.
  • Natural selection can change a species or even make a new species. Natural processes favor some traits over others in a population. This causes those traits to be more common in subsequent generations.
  • Evolution just means change over time.


Use these resources to answer the questions that follow.

http://www.hippocampus.org/Biology \rightarrow Non-Majors Biology \rightarrow Search: Natural Selection and Evolution

  1. What is natural selection?
  2. What are genes?
  3. How does natural selection effect the individual?
  4. What does natural selection cause in populations?

Mechanisms of Evolution: How Does Variety Arise? at http://www.youtube.com/watch?v=-mPCqYxB4d4 (2:51)

  1. How does variety occur in an individual?
  2. How are genomes diversified and passed on to offspring?


  1. Why are some deer mice brown and some lighter colored in Nebraska?
  2. How does natural selection work?
  3. How does biological evolution work?

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Jan 04, 2013

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Jun 03, 2014
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