Genetic Variation

Genetic Variation Definition

Genetic variation can be described as the differences between organisms caused by alternate forms of DNA. Genetic variation in combination with environmental variation causes the total phenotypic variation seen in a population. The phenotypic variation is what is seen by the observer; the height of a plant for instance. The environmental variation is the difference in what each individual experiences. In our scenario, some plants may get more water and nutrients than others. If you remove the environmental differences from the overall variation, what you have left is the genetic variation. This variation represents the actual differences in DNA sequences between organisms.

Examples of Genetic Variation

Genetic Variation between Individuals

Look at the image of mussel shells below. All of these muscles belong to the same species, meaning they can all interbreed with each other. The differences in their patterns represents the total phenotypic variation in the population. Some of the variation comes from genetics, while some comes from the environment. To sort out what is genetic and what is environmental, scientist would have to conduct a series of experiments.

Two experiments would be required to find the overall genetic variation in the population. In the first, a single mussel would be cloned many times and placed in variable environments. The specimens would be allowed to grow and they would be observed in adulthood. Because their genetics are identical, the variation seen can be attributed solely to environmental variation. In the second experiment, the total variation in a population of wild mussels in the same environment must be observed. At the end of these two experiments, the scientist would have two numbers: one describing the environmental variance and one describing the phenotypic variance.

To get the genetic variation found in this population of mussels, the scientist would simply need to subtract the environmental variance observed with the clone from the total variance observed in the wild population. Another way of calculating the genetic variation is to sample the DNA of the population and measure the differences in the DNA directly. Since genetic variation is produced by differences in the DNA, these differences can be used in reverse to calculate the environmental variation in a population.

Genetic Variation between Species

While the above example discusses genetic variation between members of a population, the concept of genetic variation can be applied on a much grander scale. Consider for instance the Homeobox gene family. This family, known as the “Hox genes” for short, direct and coordinate the positions of body parts during development. These genes, or a variation of them is found among all bilaterally symmetrical animals. This includes everything from insects to fish and mammals. Scientist theorize that an early ancestor developed the Hox genes, which were quickly adapted to many forms of organism. The genetic variation represented in these genes is huge. They produce the different body types of most of the organism on Earth. However, they are still all related and the variance between them can be measured.

Sources of Genetic Variation

With all of the natural variety in the world, it is weird to think that all of the genetic variation comes from only a few simple sources. The simplest source is mutation. As DNA is exposed to the various chemicals and electromagnetic energies of the world, it can mutate. DNA is made of a specific sequence of nucleotides, which produce proteins. Mutations change these proteins by changing the sequence of nucleotides. While mutations are often thought of in a negative context, they drive evolution by putting forth new variants to be tested by the environment.

In fact, genetic variation is so important for species that many species reproduce sexually to aid the process of producing new varieties. Sexually reproducing organisms carry two copies of the genome, allowing mutations to lie dormant or express themselves more subtly. During sexual reproduction, genes are recombined in new ways. This process, known as recombination, shuffles the alleles present and allows different combinations to be expressed. This adds to the total genetic variation. When observing an isolated population, immigration can also be a source of genetic variation. Organism may bring new alleles that have established elsewhere and introduce them to the population.

Quiz

1. What is the formula for quantifying genetic variation?
A. Genetic Variation = Expected variation + Phenotypic variation
B. Genetic Variation = Phenotypic variation – Environmental variation
C. Genetic Variation = Environmental variation + Phenotypic variation

Answer to Question #1

B is correct. To get to the genetic variation, the variation caused by differences in the environment on each individual must be removed from the total variation, a.k.a. phenotypic variation. The only things that cause differences in organisms are their genetics and the environment. Eliminate one and you have the other.

2. Two identical twins go through life separately. At the age of 55, one is healthy and fit while the other is overweight and in danger of having a heart attack. What is responsible for the difference?
A. Genetic Variation
B. Environmental Variation
C. Phenotypic Variation

Answer to Question #2

B is correct. The twins are identical, meaning they have exactly the same DNA. The phenotypic variation is what we observe; one twin is fat, the other is skinny. If we remove the genetic variation from this, we get the environmental variation. The genetic variation is 0, because they are identical. Therefore, the phenotypic variation seen is due entirely to differences in the environment, such as diet and exercise.

3. Polar bears and grizzly bears have been found to interbreed, creating a hybrid species. How is this related to genetic variation?
A. It is not related, because the bears are different species
B. Although there is a lot of genetic variation between the bears, they are still close enough to produce viable offspring
C. There is no genetic variation between the bears, allowing them to interbreed

Answer to Question #3

B is correct. The bears contain a huge amount of genetic variation. So much so that they no longer look alike or act as one species. However, a changing climate is driving polar bears south, where they interact more with the grizzly population. Because the genetic variation is not significant enough to stop the formation and development of offspring, the two species can interbreed. This would not be possible with two species that are very different, such as a giraffe and a duck, for many reasons including the genetic variation.

References

• Hartwell, L. H., Hood, L., Goldberg, M. L., Reynolds, A. E., & Silver, L. M. (2011). Genetics: From Genes to Genomes. Boston: McGraw Hill.


• Lodish, H., Berk, A., Kaiser, C. A., Krieger, M., Scott, M. P., Bretscher, A., . . . Matsudaira, P. (2008). Molecular Cell Biology 6th. ed. New York: W.H. Freeman and Company.

Genetic Variation