Selective Pressure: Definition, Example & Quiz

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Katy Metzler

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

Because of selective pressures, organisms with certain phenotypes have an advantage when it comes to survival and reproduction. Over time, this leads to evolution. In this lesson, you'll learn about various selective pressures and their consequences.

We also recommend watching Selectively Permeable Membranes: Definition, Examples & Quiz and Cluster Random Samples: Definition, Selection & Examples

What is Selective Pressure?

Organisms have many different phenotypes, or observable characteristics. Your hair color and eye color are phenotypes, for instance. Many phenotypic variations are neutral: they don't give organisms a benefit or disadvantage when it comes to survival and reproduction. People with blue eyes are not any likelier or unlikelier to survive and reproduce than people with other eye colors.

However, some phenotypes are either selected for or against by the conditions in which an organism lives. For example, people that live in places with strong sunlight, like near the equator, are likelier to survive and reproduce if they have dark skin to protect them from UV damage.

A selective pressure is any reason for organisms with certain phenotypes to have either a survival benefit or disadvantage. In the example above, strong sunlight is a selective pressure that favors darker-skinned people: lighter skin would be a disadvantage in these regions.

Selective Pressure Leads to Evolution

Selective pressures drive natural selection: some members of the population will not survive and reproduce, and thus will not pass on their genes into the next generation. Gradually, the population changes, and genes that improve survival and reproduction will become more common while genes that are disadvantageous to survival and reproduction will become more rare. This change in the genetic makeup of a population is called evolution.

Examples of Selective Pressures

Selective pressures can take many forms, including environmental conditions, availability of food and energy sources, predators, diseases, and even direct human influence.

Climate

Let's take climate as an example. In a cold climate, animals need certain characteristics to survive, such as perhaps a warm furry coat, the ability to make burrows to live in and the ability to collect and store food for the winter. The selective pressure of cold weather means that animals that don't have these characteristics are less likely to survive and reproduce. In a hot, dry climate, plants will have an advantage if they have phenotypes such as the ability to store water, large root systems to absorb what little water is in the soil, and perhaps ways to prevent water loss even at high temperatures.

Food and Energy Sources

What about food and energy sources as selective pressures? In a dense rainforest, plants on the forest floor will survive and reproduce better if they are able to gather as much light as possible, perhaps by having very large leaves. Thus, light availability can be a selective pressure for plants. Food acquisition is also a selective pressure: for example, sharp teeth and the ability to hunt prey are advantages for carnivores.

Predation

On the flip side of the coin, those very carnivores are a selective pressure for their prey. Animals that have sharp eyesight, are poisonous to their predators, can run very fast or can camouflage themselves or hide from predators will be likelier to survive and reproduce than animals without these phenotypes.

Diseases

Diseases can also be selective pressures. One well-known example is the sickle cell trait in humans, caused by having one copy of the mutated hemoglobin allele that causes sickle cell anemia. People that have two copies of this mutated allele are likely to die of anemia at a young age, so you'd think that the allele would be strongly selected against, right? However, people with only one copy of the sickle cell allele have a survival advantage in malarial regions because they are more resistant to malaria. Scientists don't yet know exactly why they are more resistant, but in these regions, malaria is a selective pressure that keeps the sickle cell allele circulating in the population.

The Immune System

Now let's put ourselves in the pathogens' shoes: for microbes such as the malaria parasite, as well as many other bacteria, viruses and fungi, animals' immune systems are a major selective pressure. Pathogens that have phenotypes that increase their resistance to immune defenses are likelier to replicate and go on to infect another host. This leads pathogens to evolve very interesting abilities, such as disguising themselves from the immune system by changing their outer coats, or even hijacking our immune cells to make them into comfortable places to live.

Human manipulation

Let's have one last example: what was that about direct human influence being a selective pressure? For hundreds and perhaps thousands of years, people have been domesticating and breeding animals and plants. How do they do this? By choosing organisms with specific phenotypes and allowing them to reproduce and pass on their genetic material. This means that human manipulation is a selective pressure that leads to so-called artificial selection.

Selective Pressure and Reproduction

One very important concept about selective pressure is that it must occur before reproduction in order to have any effect on the population.

Many selective pressures are present throughout an organism's life, such as predators and other threats in the environment, diseases that are common in a population, and things that an organism must be able to do to survive and reproduce.

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