AP Biology 5.5 - Environmental Effects on Phenotype

This section of the AP Biology curriculum looks at the several different ways the environment can change the phenotype that an organism expresses. First, we’ll take a look at what environmental variation is, and how it can take place in an organism. Then, we’ll see how some environmental variations serve a specific purpose, while other variations are simply coincidental interactions between the environment and different aspects of an organism that cause a measurable phenotypic change. After we look at an example of how soil pH can change the color of Hydrangea flowers, we’ll see a trait with phenotypic plasticity that has evolved specifically to increase the fitness of an organism: the seasonal change of coat color in snowshoe hares. These changes are epigenetic, in that they result from a differential expression of various genes based on environmental stimuli. Finally, we’ll take a look at several traits with environmental variability including melanin production in response to UV light and temperature-dependent sex determination in reptiles!

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Naturally occurring diversity among and between components within biological systems affects interactions with the environment.

Explain how the same genotype can result in multiple phenotypes under different environmental conditions.

Environmental factors influence gene expression and can lead to phenotypic plasticity. Phenotypic plasticity occurs when individuals with the same genotype exhibit different phenotypes in different environments.

5.5 Environmental Effects on Phenotype Overview

Pea plants, and many other types of plants, have genes that contribute to how high the plant will grow. But, plants need water to grow. In plants that get water, you may be able to see the difference between a plant with a tall allele and a plant with a short allele. Plants that do not get enough water will undoubtedly be stunted and grow much less than they could if they had water. This is just one simple example of how the environment can change the phenotype of an organism.

In fact, environmental effects on variation account for a large portion of the variation seen in most traits. Some environmental variables simply change the phenotype without altering the genotype in any way, while other environmental variables actually alter the way the DNA is expressed. These concepts will definitely be on the AP Test. So, follow along as we break down the complex effects that the environment has on various phenotypes.

Let’s start by seeing exactly how the environment can affect an organism’s phenotype. In many cases, environmental effects on phenotype are common sense. For example, we know that people come in many different shapes and sizes. To start with a simple example, let’s think about how an environmental variable like food can affect a person’s height and weight. We know that someone who has plenty of food and overeats will become fat. By contrast, someone with not enough food will likely become emaciated. Thus, it is clear that food has an effect on the body type phenotype.

But, did you also know that high-quality nutrients can affect a person’s height as well? Much like plants need water in order to grow to their full potential, animals (including humans) need high-quality food in order to grow to their full potential. In fact, scientists credit food production and availability as one of the reasons that the average height of people has generally increased over the last 100 years. However, we also know that different populations of humans increase and decrease in height at different rates, at different times.

Human height is a complex phenotypic trait determined by both environmental variability and genetic variability.
Human height is a complex phenotypic trait determined by both environmental variability and genetic variability.

So, how can scientists possibly know how much of a trait is determined by the environment and how much is determined by genetics? Scientists start with the following equation:

Total Variation = Genetic Variation + Environmental Variation

V = VE + VG

Knowing that the total variation is made of these two components, we also know that the total variation minus the environmental variation is equal to the genetic variation:

V – VE = VG

This equation allows us to determine how much variation is due to the environment and how much variation is due to genetics. First, we need to experiment to find the environmental variation. Consider an experiment to determine how water can affect plant height in a group of cloned plants. We give one group of clones as much water as they can handle. We give the other group of clones barely enough water to survive. At the end of the experiment, we can measure the variation in height between these two groups. Since these plants are clones, they have the exact same genes. This means that this amount of variation is due entirely to environmental variation, and not at all to genetic variation.

Then, we can go out and measure the variation in a wild group of plants across areas with all different levels of water. If we know the total variation and the environmental variation, then we can calculate the genetic variation and the contribution of both environmental and genetic variability to overall variability.

The answer to the question, “why does the environment affect phenotype?” is not an easy one to answer. Consider the following. The Hydrangea plant can produce flowers in many different color varieties ranging from pink to dark blue. However, only one of these varieties – the white hydrangea – results from a mutated gene. White hydrangeas produce no pigment molecules at all, whereas the other three varieties produce the exact same pigment molecule.

Hydrangea blooms vary in color from red to blue, depending on the soil pH.
Hydrangea blooms vary in color from red to blue, depending on the soil pH.

Surprisingly, this pigment molecule responds to aluminum ions in the soil. The availability of aluminum ions in the leaves is dependent upon soil pH. When there is an acidic soil pH, aluminum ions are easily absorbed into the plant. These aluminum atoms cause a chemical reaction within the pigment molecule that causes each pigment molecule to reflect blue light. If the exact same plant is transferred to a more basic soil, then aluminum binds to hydroxide ions in the soil and cannot be transported into the plant at all. Thus, the pigment molecule is never modified and remains a red color. At a neutral soil pH, there is a small amount of aluminum in the plant and some of the pigment gets modified, leading to the purple coloration.

So, while scientists have answered the proximate question of why the environment changes phenotypes, there is still no answer to the ultimate question of what purpose this change serves in the plant. As of now, there is no known evolutionary advantage to having a flower that serves as a functional pH test.

However, as we will see in a few paragraphs, there are many environmentally-caused phenotypic changes that do have clear purposes and increase an organism’s fitness.

To see a purposeful and driven form of environmental variation, consider the snowshoe hare. Like many animals that live in cold, snowy regions, the snowshoe hare goes through a seasonal molting event. Though the hare is pure white during the winter months, the hair will become brown in the summer. These two different color phenotypes help the hare stay camouflaged in both seasons. This is known as phenotypic plasticity.

A hare during a molting event, halfway between the white winter coat and the brown summer coat.
A hare during a molting event, halfway between the white winter coat and the brown summer coat.

Unlike some of the environmental variations we have seen, these changes are epigenetic. This means that signals in the environment actually change which genes are expressed in different environments. In fact, scientists have identified over 600 genes that change their expression patterns due to signals from the environment. For instance, as springtime starts to replace the winter weather, several environmental cues hit the rabbit. There is more sunlight, more nutritious foods start to sprout, and the temperature starts to rise.

These environmental changes stimulate a number of signal transduction pathways that, in turn, lead to changes in the expression of a large number of genes. For example, genes related to hair growth are turned on, as are genes related to the production of pigment molecules. This stimulates a new coat of hair to begin growing, leading to the molting phase seen between the winter and summer phenotypes. However, it also affects genes related to the metabolism and the release of hormones that trigger the reproductive cycle. Ultimately, all of these changes lead to the phenotype of the hare we see in the summertime.

Unlike the change in flower color we observed previously, these environmentally-driven variations have a clear evolutionary advantage – they not only keep the hare well camouflaged, but they prepare the hare for different seasons and ensure the hare is reproducing at the appropriate time of year.

Let’s go through a few quick examples of other forms of environmental variation. Let’s start by taking a look at the process of skin tanning. Tanning is a form of phenotypic change from lighter skin to darker skin, stimulated by UV rays. Contrary to popular belief, all people undergo the process of tanning, even if they already have naturally dark skin. Dark skin and tan skin are both caused by the pigment molecule melanin, released by melanocytes located several cell layers deep within your skin. Melanin makes its way through the skin cells to the surface of the skin, where it can block harmful UV rays. In fact, the process of melanin production is stimulated by UV rays passing through unprotected skin and creating reactive oxygen species in the cells. These reactive oxygen species damage the DNA and stimulate signal transduction pathways that lead to increased expression of the melanin gene.

The process of tanning is caused by the release of melanin stimulated by UV light damage.
The process of tanning is caused by the release of melanin stimulated by UV light damage.

However, this process takes about 10 days to become fully activated after initial sun exposure. That is why many people get a sunburn on their first exposure to the sun, instead of instantly getting more tan. This redness is extreme damage caused by UV light bombarding your skin cells. Considering that tanning can only happen after UV damage, you should always wear sunscreen and consider a spray-on tan to avoid the possibility of skin cancer.

Let’s consider another interesting type of environmental variation: temperature-dependent sex determination in reptiles! In mammals, sex is determined by X and Y chromosomes. In birds, a similar, but opposite set of chromosomes known as Z and W determine the sex of each individual. However, in reptiles, each individual receives the exact same set of chromosomes. If the egg is relatively cold as it incubates, the chromosome will create male hormones and the offspring will be male. If the egg is relatively warm as it incubates, the chromosomes release female hormones and the offspring will be female.

In a nest of turtle eggs, temperature variations lead to a differential expression of sex hormones, leading to male and female offspring.
In a nest of turtle eggs, temperature variations lead to a differential expression of sex hormones, leading to male and female offspring.

In the nest, this works out well because parts of the nest are relatively warm while other parts are cooler. This creates a mixture of male and female offspring from each nest. While this temperature-dependent form of sex determination has worked out well for reptiles for hundreds of millions of years, global warming may lead to more females and fewer males, possibly endangering the survival of many different kinds of reptiles.