What is Directional Selection? - Examples, Definition & Graph

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  • 0:00 Phenotypes and Natural…
  • 1:35 Directional Selection
  • 3:25 Directional Selection Examples
  • 5:45 Artificial Directional…
  • 6:20 Lesson Summary
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Lesson Transcript
Instructor: Katy Metzler

Katy teaches biology at the college level and did her Ph.D. work on infectious diseases and immunology.

Natural selection is the driving force behind evolution - the gradual change in organisms' genetic makeup over time. In this lesson, you'll learn about directional selection, review some examples, and end with a quiz.

Phenotypes and Natural Selection

People come in all shapes, sizes, and colors. What's more, they have different blood types, different metabolisms, and different physical abilities. Of course, this is not only true for humans. For any given phenotype, or observable characteristic, there is natural variation within a population of organisms.

These phenotypic variations are often neutral, but sometimes certain phenotypes provide an advantage for an organism. For example, giraffes that have long necks can reach leaves that are higher up on trees and thus have more food available to them than shorter-necked giraffes. Conversely, some phenotypes can put an organism at a disadvantage. A bright white rabbit might stick out visually in a wooded habitat and thus be an easier target for predators than its brown or gray counterparts.

Fitness is simply defined as an organism's chance of surviving until it reproduces. When these advantages and disadvantages make a difference in an organism's fitness, natural selection occurs. Only the individuals that reproduce will pass their genetic material on to the next generation. So, genes that cause phenotypes that increase fitness will be selected for and will be more common in the population in the next generation. The same occurs the other way around, too: genes that cause disadvantageous phenotypes will be selected against and will be less common in the next generation. Over many, many generations, natural selection drives evolution, or gradual change in organisms' genetic makeup.

Directional Selection

There are several different types of natural selection, like stabilizing selection, directional selection, disruptive selection, and sexual selection. Each type leads to different evolutionary consequences. In other words, different types of selection lead to different changes in the overall genetic makeup of the population in the next generation. In this lesson, we will focus on directional selection. In order to understand directional selection, it helps to look at a visual representation.

Natural selection forms a bell curve
Bell curve of natural variation

The top graph here shows the natural variation in a particular phenotype within a population. It could be the varying lengths of giraffes' necks in a certain region of Africa. As you can see, it looks like a bell curve, with a lot of giraffes with intermediate neck lengths, a few with really long necks, and a few with really short necks.

When directional selection occurs, phenotypes at one end of the spectrum are selected against, and phenotypes at the other end of the spectrum are selected for. The bottom graph shows what happens after directional selection. The curve is skewed to the right, meaning that the necks of the giraffes as a group have gotten longer. There is still variation in the lengths, but overall, the population has evolved to have longer necks.

Directional selection favors one phenotype over another
Graph of directional selection

Directional selection can happen in response to certain selective pressures, which are basically reasons that certain phenotypes lead to a fitness advantage or disadvantage. In our giraffe example, the selective pressure could be decreased food availability because of, say, a disease that killed a lot of smaller bushes and shrubs in the region. As a result, the giraffes needed to eat leaves on taller trees. Of course, giraffes with longer necks would have more food and would be likelier to survive and reproduce, passing on their long neck genes to the next generation.

Examples of Directional Selection

Directional selection is fairly easy to recognize, because it leads to dramatic shifts toward extreme phenotypes over time. There are many examples in nature, and also examples caused by human manipulation.

One famous example of directional selection for color is the evolution of peppered moths in London at the beginning of the Industrial Revolution. Before the Industrial Revolution, most of the moths were white, and they blended in well with the bark of the light-colored trees in the city. This camouflage helped them avoid being eaten by predators. Some moths were darker in color as well, but the dark phenotype was less frequent.

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