Natural Selection & Adaptation Processes

Instructor: Lisa Roundy

Lisa has taught at all levels from kindergarten to college and has a master's degree in human relations.

Through the process of adaptation, natural selection shapes the variety of life that exists around us. This lesson will explore some of the ways this process can take place.

Variation, Adaptation, and Natural Selection

A population of beetles lives in a field with red flowers. The beetles live inside the flowers. Usually, the beetles are also red in color, but occasionally you will find a beetle that is yellow. When a trait is expressed in more than one way, such as the different colors of the beetles, it is called variation. Variation is essential to a species' potential for adaptation, or ability to adjust to change.

In order to understand this, let's look at what might happen if the red flowers in the field were replaced by yellow flowers. When the flowers were red, the red beetles were camouflaged. Because they could blend in with the environment the red beetles were less likely to be eaten by predators and more likely to reproduce successfully. Thus, there were more red beetles.

Now that the flowers are yellow, the red beetles stand out like a sore thumb, and hungry birds are scooping them up for dinner. It is the few yellow beetles that have an advantage. Since the yellow beetles are now more likely to avoid being eaten and reproduce successfully, the main color of the population of beetles slowly shifts from red to yellow.

In our example of the beetles we see natural selection at work. Natural selection is the process in which the organism better suited for its environment will thrive and produce more offspring. This process plays a central role in evolutionary theory. As the demands of the environment act upon specific traits in a population, changes occur to improve the survival of the organism. The demands of the environment either benefit or work against specific traits that already exist within the population.

Variation provides the means for change. The red and yellow beetles in our previous example were just different variations of the same species. Over thousands, or even millions of years, evolutionary changes can have a significant effect. This occurs in different ways. Let's take a look at various types of natural selection.

Directional Selection

Directional selection takes place when traits on one end of a range of variation are favored over the others. Imagine early giraffes without the long necks they have today. Most of the early giraffe necks were average in length, but some necks were shorter and some were longer. This created a range of neck lengths.

Long-necked giraffes were at the far end of this range. The giraffes with the longer necks were able to reach leaves higher in the trees. Because of this, the early giraffes with longer necks were better able to survive when food was scarce. With each generation the average neck length of giraffes became longer. This continued to occur until all giraffes gained the long necks they have today. In other words, natural selection occurred in one direction -- in this case toward longer necks!

Directional selection pushes trait selection in one direction away from the average.
directional selection

Disruptive Selection

Disruptive selection takes place when traits at both ends of a range of variation are favored. Let's use our beetle example again to illustrate this. If our beetles lived without predators, many of the beetles would be orange because the red and yellow beetles would interbreed. In other words we would have a variation in beetle color from red at one end of the range, through all the shades of orange, to yellow at the other end.

Birds that eat the beetles are introduced to fields where our beetles live. Some fields in the area have red flowers and others have yellow flowers. Because they are camouflaged, the red beetles survive well in the fields with red flowers. The same is true of the yellow beetles in the fields with the yellow flowers. However, the beetles that are orange are easy prey for the birds in both fields. Natural selection is occurring toward opposite ends of the range of variation -- in this case toward the red and yellow beetles!

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