Phenotypic Variations

Definition of phenotypic variation

The phenotype of an organism refers to its observable traits- its physical appearance, behavior, learning ability, mode of reproduction, and so on.

Figure 1 below shows four phenotypes of foxglove, each with petals of different colors. Coloration is an example of a phenotypic trait.

Figure 1. Four phenotypes of foxglove, each with petals of a different color.

Causes of phenotypic variation

The causes of phenotypic variation can be summarized in two main factors: genotypic variation and environmental influence (Fig. 2).

Figure 2. The phenotype of an organism is determined by its genotype and its interaction with the environment.

Genotypic variation

Genotype refers to the genetic information of an organism. Specifically, it is the combination of alleles (gene variants) that an organism has. The genotype contributes to the phenotype of an organism. Organisms can either be homozygous or heterozygous for a specific genotype (Fig. 3):

• Homozygous: where two alleles of a gene are identical

• Heterozygous: where two alleles of a gene are different

Fig. 3. Example of a genotype variation that has phenotypic variation in fly wings.

It is important to distinguish between homozygous and heterozygous genotypes because these affect how phenotypes are passed on from parent to offspring. This will be further discussed in the examples later.

Differences in genotype (or genotypic variations) contribute to phenotypic variation. Genotypic variations are caused by mutations, gene flow, and sexual reproduction:

• Mutation is a change in the sequence of genes in DNA. It is the ultimate source of new alleles.

• Gene flow is the introduction of genes from one population of organisms to another. This can occur when organisms migrate to and reproduce with a different population or when pollen or seeds are dispersed to a population that is geographically separated.

• Sexual reproduction promotes genetic variation by creating new combinations of genes.

Environmental influence

Environmental conditions such as climate, availability of food, and interactions with other organisms can influence the development of inherited traits, thus contributing to phenotypic variation. The ability of a genotype to produce different phenotypes in response to different environmental conditions is called phenotypic plasticity.

For example, differences in the availability of food can result in differences in size and weight among organisms of the same genotype. Similarly, differences in climate (dry season vs. wet season) can result in differences in crop yield. In both cases, stunted growth could result from a lack of nutrition.

It is important to note that phenotypic variations due to environmental influence are not passed on from parent to offspring.

Types of phenotypic variation in a population

Phenotypic variation in a population can be classified as either discrete or continuous. Examples of each type will be discussed in the following section.

Discrete variations

Discrete variations are traits with qualitative differences. These are distinct and separate categories with nothing ‘in between’. Think blood type: you can have only one of four possible types: A, B, AB, or O.

In discrete variations, the combination of alleles at a single gene locus (the position of a gene on a chromosome) have a significant impact on the phenotype. Some examples include the inheritance of sickle cell anemia in humans and the stem color of tomato plants. Let's briefly discuss these examples.

Continuous variations

Continuous variations are traits expressed as quantitative differences with a wide range of values. This is because, for continuous variations, the combination of alleles at a single gene locus has small or additive effects on the phenotype. Examples of continuous variations in humans are height, weight, and skin color.

Examples of phenotypic variations

Now, let's see some examples of phenotypic variations both discrete and continuous.

Figure 4. A diagram showing the inheritance of sickle cell hemoglobin in humans.

Example of discrete phenotypic variation in humans: sickle cell anemia

Suppose a gene determines whether or not a human has sickle cell anemia. The alleles of this gene could either lead to normal or sickle-cell hemoglobin. As mentioned earlier, a genotype could either be homozygous or heterozygous, resulting in the following genotypes (Fig. 4):

• A homozygous genotype will have alleles that are either both normal or sickle-cell.
  • A homozygous genotype with two sickle-cell alleles will have sickle cell anemia. All of their hemoglobin will have difficulty transporting oxygen.
  • A homozygous genotype with two normal alleles will have completely normal hemoglobin.
• A heterozygous genotype will have one normal allele and one sickle-cell allele.
  • They will have a sickle-cell trait but will appear normal.

Other examples of discrete variations in humans are the ability to roll the tongue (whether one can or cannot) and the hand used for writing (left- or right-dominant).

Example of discrete phenotypic variation in plants: color of tomato stem

There are also instances where the phenotype of a heterozygous organism is affected by only one allele.

For example, the stem color of a tomato plant is determined by two alleles: one produces green stems and the other produces purple stems. A tomato plant that has one allele for purple stems and one allele for green stems will have purple stems identical to a tomato plant that has two alleles for purple stems.

In this example, the allele that produces purple stems is dominant, while the allele that produces green stems is recessive.

• Dominant allele: affects the phenotype
• Recessive allele: does not affect the phenotype

Example of continuous phenotypic variation

Let’s say the height of an organism is determined by two genes: Aa and Bb, where the dominant alleles (A and B) add 2x cm each and the recessive alleles add 1x cm each.

This means:

• A homozygote recessive (aabb) is potentially 4x cm tall.
• A homozygote dominant (AABB) is potentially 8x cm tall.
• The other 14 genotypes will fall within this range. For example:
  • A heterozygote with AaBb alleles is potentially 6x cm tall.
  • A heterozygote with AABb alleles is potentially 7x cm tall.

In reality, more genes are involved in determining the height of the organism, each having an additive effect. Traits determined by multiple genes are called polygenic traits.

Why is phenotypic variation necessary for evolution by natural selection?

Phenotypic variation is necessary for evolution because natural selection acts on the phenotype of an organism, and not on its genotype (although they are related). Phenotypic variations lead to different survival and reproduction rates among organisms. Evolution by natural selection--where individuals with traits that are more adapted to the environment have more chances of survival and reproduction--can only take place when these differences are present. Without phenotypic variation, a population cannot evolve.

Some factors can lead to reduced phenotypic variation. An example of this is inbreeding, or when closely-related individuals reproduce. This could take place in nature when a population becomes so small that the individuals capable of reproducing are closely related. This could also take place when humans practice artificial selection, the process of breeding organisms with desirable traits. Inbreeding will cause a reduction in the number of alleles in the gene pool of the population, leading to loss in genetic variation.

Another example is genetic drift. Genetic drift is when chance events cause allele frequencies to change at random. This means that the alleles that are passed onto succeeding generations are not the most well-adapted to the environment, potentially causing individuals with these inherited traits to die out hence reducing genetic variation.

With less genetic variation, it is less likely that there are individuals with the traits needed to survive changes in environmental conditions (for instance, climate change or the emergence of infectious diseases), which can lead to the extinction of the population or even the entire species.