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Phenotypic Variation: Definition & Explanation

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  • 0:00 What is Phenotypic Variation?
  • 1:00 Types of Phenotypic Variation
  • 2:10 Causes of Phenotypic Variation
  • 3:40 Natural Selection
  • 5:20 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.

Phenotypic variation in a population of organisms can contribute to natural selection and evolution. Find out what phenotypic variation is, what can cause it, and why it only sometimes contributes to evolution.

What is Phenotypic Variation?

Phenotypes are traits or characteristics of an organism that we can observe, such as size, color, shape, capabilities, behaviors, etc. Not all phenotypes can actually be seen. For example, blood types are phenotypes that we can only observe using laboratory techniques. Phenotypes can be caused by genes, environmental factors, or a combination of both.

Phenotypic variation, then, is the variability in phenotypes that exists in a population. For example, people come in all shapes and sizes: height, weight, and body shape are phenotypes that vary. Hair, eye color, and the ability to roll your tongue are variable phenotypes, too. What about other organisms? All organisms can have phenotypic variation. In plants, flower color and leaf shape are examples of variable phenotypes. In bacteria, resistance to antibiotics is a variable phenotype: some bacteria are resistant and survive antibiotic treatment, while others are susceptible and die when antibiotics are given.

Types of Phenotypic Variation

When a characteristic or phenotype normally exists in a range or gradient, it varies continuously, like shades of gray as opposed to black and white. It's easy to think of examples of phenotypes that vary continuously, such as height and skin color. In between the shortest person in the world and the tallest person in the world, any height is possible, not just four feet, five feet, or six feet. And of course, skin comes in all kinds of shades, not just two or three.

If you would make a frequency graph of the range of heights or skin colors in a group of people, it would look like a bell curve, with intermediate phenotypes being the most common. This is a good way to recognize continuous variation.

In contrast, some phenotypes vary discontinuously. These phenotypes exist only at discrete intervals, like 'black and white' differences. For example, you can have blood type A, B, AB, or O, but there aren't any intermediate blood types in between. Another example is the ability to roll your tongue. Either you can or you can't, so this phenotype varies discontinuously.

What Causes Phenotypic Variation?

As mentioned above, phenotypes can be caused by genes, environmental factors, or both. When we say environmental factors, we aren't necessarily talking about the trees and the climate: environmental factors are things in an organism's surroundings or lifestyle that can influence it in various ways. For example, body weight in humans may be influenced by genes but is also influenced by diet. In this case, diet is an example of an environmental factor. The effects that environmental factors have on phenotypes are hard to pin down, since there are so many possible factors to take into account.

A lot more is known about the relationship between genes and phenotypes. Let's take the example of hair color. Perhaps there is a gene in rabbits that codes for an enzyme that, in turn, makes a brown-colored pigment in hair follicles. Some rabbits may have genetic differences that cause them to have more or less of this enzyme, or an enzyme that works more or less efficiently to produce the pigment. We would expect these rabbits to have different phenotypes, e.g., lighter or darker brown hair, depending on these genetic differences.

Let's take one more example, this time in bacteria. Some bacteria may have a gene that codes for an enzyme that breaks down an antibiotic into a substance that isn't harmful anymore. If you treat these bacteria with the antibiotic, they'll survive: this phenotype is called antibiotic resistance. In contrast, bacteria without that particular gene will be susceptible to the antibiotic.

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