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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.

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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.When the air became polluted because of industry, the bark of the trees became coated with dark soot.

The darker-colored moths had a fitness advantage, because they could camouflage themselves better than the white moths could. Directional selection occurred, and the population shifted towards the dark phenotype.The same thing happens with other animals that can avoid predation by being a certain color. For example, there are cases of non-poisonous butterflies evolving directional color changes that make them look like poisonous butterflies because their predators tend to avoid the color of the poisonous butterflies.Abilities that an organism needs in order to survive and reproduce can also be selected for in a directional way.

Again, we can easily come up with examples that have to do with predation. For example, cheetahs have evolved the ability to run extremely fast in order to effectively hunt their prey. Slow speeds would be selected against in cheetahs because they wouldn’t be able to get enough food to survive. Another example is hawks’ sharp eyesight. Hawks with excellent vision have a fitness advantage because they can find prey from far above as they soar through the sky.There are also examples of directional selection in the microscopic world.

Bacteria and viruses that need to infect animal hosts in order to reproduce encounter many obstacles. For example, in modern times, the use of antibiotics leads to directional selection in bacteria. Bacteria that have drug-resistance genes are far more likely to survive and reproduce than bacteria that are susceptible to antibiotics. This means that scientists continually need to search for new antibiotics that bacteria have not evolved resistance to yet.

Artificial Directional Selection

Directional selection is also commonly used by humans to breed crops and animals with preferred phenotypes. For example, cows can be bred for their milk production, racehorses for their speed, or show dogs for a certain appearance. By choosing which animals to allow to mate and with whom, humans apply selective pressure that selects for phenotypes that are often on an extreme end of the spectrum. In the same way, fruits and vegetables can be bred for certain colors, sizes, or flavors.

Lesson Summary

In this lesson, we have seen that directional selection happens when extreme phenotypes on one end of the spectrum are more favorable than phenotypes at the other extreme. This type of selection results in noticeable directional changes in a population of organisms over the course of evolution.

Learning Outcomes

When you are finished, you should be able to:

  • Recall what a phenotype is
  • Discuss the relationship between fitness, natural selection, and evolution
  • Explain how directional selection works on a population
  • State why humans would use artificial directional selection

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