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Stabilizing Selection: Examples, Definition & Graph

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  • 0:00 Evolution by Natural Selection
  • 1:36 What Is Stabilizing Selection?
  • 2:28 Stabilizing Selection Examples
  • 5:35 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.

Evolution, or the gradual change in a population's genetic makeup, is caused by natural selection. One important type of natural selection is stabilizing selection. In this lesson, discover what stabilizing selection is and how it can happen.

Evolution by Natural Selection

Over many generations, the genetic makeup of organisms gradually changes due to evolution by natural selection. In a population of organisms, there is natural variation in genes and the phenotypes that they cause. For example, if we would look at the entire population of pigeons in a city, we'd see that there is a range of sizes, colors and willingness to be near humans. Many of these phenotypes are caused by genetic variations within the population.

Natural selection acts on phenotypic variation. Certain phenotypes can increase or decrease an organism's fitness, or ability to survive and reproduce. In our pigeon example, there may be reasons that a larger size or a browner color would improve pigeons' fitness. These reasons are called selective pressures, and what they are depends very much on the environment an organism finds itself in as well as the specific things it needs to do or avoid doing in order to survive until reproduction.

Individuals that are more fit are more likely to reproduce and pass their genes on to the next generation. Thus, genes that cause favorable phenotypes are selected for during natural selection, and genes that cause unfavorable phenotypes are selected against.

There are several major types of natural selection, such as directional selection, stabilizing selection, disruptive selection and sexual selection. The different types of selection lead to different overall changes in the population in the next generation. Here we'll focus on stabilizing selection.

What Is Stabilizing Selection?

Stabilizing selection happens when extreme phenotypes on both ends of the spectrum are unfavorable. It's easier to understand stabilizing selection by looking at a visual representation. Here's a graph that describes this type of selection:

A graph of phenotypic variation in a population before and after stabilizing selection occurs
Stabilizing selection graph.

The top graph shows the natural variation in a given phenotype in the population before selection occurs. It's more or less a bell curve, with mostly intermediate phenotypes but also some extreme phenotypes at both ends of the spectrum. When stabilizing selection occurs, phenotypes at both extremes are selected against, and intermediate phenotypes are selected for. The bottom graph shows that the curve has become narrower and taller. It is narrower because there is less of a range of phenotypes, and it is taller because there are more organisms with intermediate phenotypes.

Examples of Stabilizing Selection

Stabilizing selection is likely the most common type of natural selection, but it's not always so easy to recognize it, because it doesn't lead to drastic changes in the way that directional selection or disruptive selection does. Let's look at some examples.

Number of Offspring

Let's take a concrete example like birds. The phenotype we are looking at is the number of eggs a mother bird lays. Let's say it ranges from a single egg to twenty eggs on the x-axis. Due to selective pressures in the birds' environment, like predators and scarcity of worms, both extremes of the spectrum get selected against. That means that, as evolution proceeds, there is a narrower range of phenotypes, and most pigeons lay somewhere between five and fifteen eggs, instead of between one and twenty.

Why? If a bird lays too few eggs, there's too great of a chance that predators will eat all of the eggs and her genes won't be passed on to the next generation. On the other hand, if she lays too many eggs, there's a high chance that she won't be able to find enough food to feed them all, and they could all starve, which again wouldn't allow her genes to be passed on.

The same thing happens with other animals that must care for their offspring, such as mammals. Too few offspring means that none may make it to reproduction, but on the other hand, the mother can't effectively care for too many offspring.

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