Phenotypic plasticity

Phenotypic plasticity is the ability of a living organism to change its appearance, behavior, or physical structure in response to a change in its environment. The effect is most often observed in plants, but it can also occur in more complex animals, including humans. Examples include the changing height of plants in response to access to sunlight or the increased muscle mass of a person who lifts weights. Phenotypic plasticity is not a change in the genetic makeup of an organism, but it is a possible adaptation already encoded into its genetic blueprint. Scientists believe phenotypic plasticity is both a byproduct of evolution and a possible trigger for future evolutionary change.

Background

Plasticity describes the ability of something to be shaped or molded. The term phenotypiccomes from the word phenotype, which is the set of observable physical characteristics of an organism. These can include features such as eye color, height, leaf thickness, or the size of an animal’s tail. A genotype refers to the complete set of genes carried by an organism. A gene is a sequence of chemical molecules that are responsible for producing many of the physical features of an organism. For example, a certain gene carries the genetic instructions that cause blue eyes; another gene may cause an animal to have thicker fur or a plant to have more leaves.

The sequence of molecules that make up a gene is called deoxyribonucleic acid, better known as DNA. Over time, changes, or mutations, can occur in DNA. Most often, these changes have no effect; however, some changes in DNA can alter an organism’s biology in a positive or negative manner. Because genes are passed down through generations from parents to their offspring, the genetic changes in DNA are inherited by the next generation of organisms. If the changes have a negative effect, that organism has a lower chance of surviving to pass the mutation to its offspring. As a result, that specific mutation usually dies out.

If the genetic changes have a beneficial effect, the organism is often better adapted to survive in its environment. These organisms are then more likely to pass down the positive change to their offspring, who pass it to theirs, and so on. In this way, positive mutations survive to change the genetic makeup of a species or create a new species altogether. This process is the driving force behind the theory of evolution.

Overview

Lasting evolutionary changes to an organism’s genotype can take thousands or millions of years. The type of changes that occur in phenotypic plasticity can be seen in the span of an organism’s lifetime or within a generation or two. These changes are brought about by the factors in the environment, such as temperature, diet, moisture, and the presence of predators. Phenotypic plasticity does not cause a genetic mutation in an organism. Two organisms with the exact same genetic makeup can have different phenotypes if they are exposed to different environments.

All organisms on Earth can experience some form of phenotypic plasticity; however, the process occurs most often in plants. Plants are stationary organisms and must adapt to a constant environment to survive. Animals, on the other hand, can often move away from negative environmental conditions. If a plant grows in a relatively open environment, it will reach a certain height range predetermined by its genetic code. If other plants are added to that environment, that plant may grow taller, adapting to the presence of the other plants so that it receives more sunlight.

Some plants that grow in wetland areas can change their leaf structure depending on how dry or wet their surrounding environment is. During times of heavy rain and flooding, the plant’s leaves may become thinner and have no stomata. Stomata are small openings in the leaves through which the plant “breathes” by taking in carbon dioxide from the air and releasing oxygen. If the plant experiences drought conditions, it may develop thicker leaves with more stomata. Plants may also develop more or less stomata if they are moved from one environment to another. Plants in an environment with more carbon dioxide may have fewer stomata, while plants in a low carbon dioxide environment may need more.

Phenotypic plasticity in non-plants can take on many forms. For example, a species of butterfly called the junonia octavia is born with different colored wings depending on the time of year. Butterflies born during the spring have bright orange wings, while those born in the summer have browner wings. This change matches the seasonal environment. Orange wings provide camouflage against predators during the time when more flowers are in bloom. In the drier climate of summer, brown helps the butterfly blend into its environment better. The pink color associated with flamingoes is not a naturally occurring trait but is influenced by chemicals found in the shrimp that are a large part of their diet.

Other changes can be even more dramatic. A species of freshwater snail can grow spines on its shell if it is born and develops in an environment where predators are present. A species of aphid can change its reproductive method from sexual to asexual in the presence of too many females in the environment. While aphids do not normally have wings, they may grow them if the plant they are using as a food source dies or becomes too populated. In this way, they can seek out a new food source. Some fish can even change their sex while they are still in egg form, based on the temperature of the surrounding water. This helps the fish maintain a balance between males and females in the population.

In humans, phenotypic plasticity has produced both long-term and short-term effects on physiological characteristics. The average human in 2024 is taller than the average person was in 1924, most likely because diet and modern medicine are better than a century ago. More immediate effects can be seen in a person who changes his or her body through exercise and physical training. For example, athletes often undergo extensive weight training that can increase muscle mass and efficiency. This not only enhances strength, but can also boost endurance and physical performance. The structure of the brain can also be altered through phenotypic plasticity to improve cognitive ability. By stimulating the brain with mental exercises, humans can create new neural connections in the brain and increase alertness, attention, and overall performance.

While phenotypic plasticity is a scientific fact, researchers are still unsure about the exact mechanism that causes the process. Some believe that an organism’s specific genotype has evolved to include several possible phenotypes that can react to environmental changes. This concept is known as the norm of reaction. In this scenario, the genotype of each organism is encoded with a predisposed range of reactions to specific environments. If that organism is exposed to a higher temperature, for example, it may adopt one set of phenotypes. If the diet of an organism changes, it may move on to another phenotype. Evidence for this theory can be seen in the predetermined orange or brown wings of the junonia octavia butterfly.

Another theory holds that the genetic ability of phenotypic plasticity developed through the process of evolution in direct response to an organism’s environment. Organisms that displayed the ability were better able to adapt to their changing environments and survived longer to pass the trait down to their offspring. In this case, the phenotypic plasticity does not fall in a predetermined range of possibilities but was specifically “chosen” for a particular organism by natural selection. For instance, some tropical plants that have evolved in a year-round warm climate show less phenotypic plasticity than plants that have evolved in changing environments. If a sudden climate change were to occur—as has happened several times in Earth’s history—the plant with the ability to alter its phenotype would have a better chance to survive. Over the course of thousands or millions of years, that plant would pass that ability down through generations of offspring, giving it an evolutionary advantage over other plants.

Many modern researchers believe both theories hold some degree of truth. Evolution most assuredly had a part in developing phenotypic plasticity in living organisms. At the same time, some organisms seem to show an adaptive range that follows a predetermined pattern. Scientists also theorize that phenotypic plasticity in some plants may result in future evolutionary changes brought about by modern climate change. A plant that can adjust the number of its stomata will produce offspring with fewer stomata in an atmosphere with higher levels of carbon dioxide. Over time, the number of plants with more stomata will decease as those with fewer stomata survive to pass their genes to the next generation.

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