Symmetry and asymmetry are foundational concepts that describe the universe. An object or system is symmetrical if it remains unchanged after a transformation, such as reflection or rotation, while asymmetry is the absence of this balance. These principles of order and the deviation from it are visible everywhere, from the petals on a flower to the vast structures of galaxies, shaping the form and function of the world.
Patterns of Symmetry in Nature
One of the most common forms of symmetry in the animal kingdom is bilateral symmetry. This pattern is defined by a single plane that divides an organism into two mirror-image halves. This body plan is associated with active, directional movement and the development of a distinct head, a phenomenon known as cephalization. This concentrates sensory organs at the front end, allowing organisms like butterflies, fish, and humans to move forward efficiently.
Another prevalent pattern is radial symmetry, where body parts are arranged around a central axis. This structure is advantageous for animals that are sessile (fixed in one place) or slow-moving, such as sea anemones and starfish. Since threats and food can come from any direction, having sensory and feeding structures distributed around their bodies is a functional adaptation.
A less common pattern is spherical symmetry, where an organism can be divided into identical halves by any plane that cuts through its center. This type of symmetry is mostly found in microscopic organisms like the alga Volvox, which floats or rolls through aquatic environments. This body plan is well-suited for a lifestyle where orientation is not fixed and interaction with the environment is uniform from all directions.
The Purpose of Asymmetry in Living Organisms
While many animals exhibit external bilateral symmetry, their internal organ arrangement is often asymmetrical for functional reasons. Most vertebrates, including humans, have a distinct internal layout. For example, the heart is tilted to the left, while the liver is on the right side of the abdominal cavity. This internal asymmetry allows for the efficient packing of complex organs within the body, preventing them from interfering with one another.
The brain also exhibits functional asymmetry, known as lateralization, where the two cerebral hemispheres specialize in different tasks. In most right-handed individuals, language processing is predominantly localized in the left hemisphere. This division of labor is believed to enhance cognitive capacity by dedicating specific regions to particular functions, allowing the brain to process information more efficiently.
Directional asymmetry is also observed in the external features of some animals for a specific behavioral purpose. The male fiddler crab possesses one dramatically oversized claw used in territorial disputes and to attract females. Another example is the consistent coiling direction of snail shells, a genetically determined asymmetry that affects mating, as snails can only mate with others whose shells coil in the same direction.
Symmetry as a Signal of Health and Development
Small, random deviations from perfect bilateral symmetry, known as fluctuating asymmetry (FA), can provide information about an organism’s life history. These minor differences between the left and right sides of a trait, such as one wing being slightly longer than the other, are considered a measure of “developmental noise.” This noise represents the cumulative impact of stressors encountered during an organism’s growth.
An organism’s ability to maintain symmetrical development reflects its developmental stability, which is the capacity to produce a consistent phenotype despite environmental challenges. Several factors can disrupt development and increase fluctuating asymmetry:
- Parasites
- Toxins
- Nutritional deficiencies
- Genetic stress
A low level of FA indicates that an organism has successfully buffered its development against these influences, signaling its robustness and genetic quality.
This connection between symmetry and robustness plays a role in mate selection in many species. Studies on animals like barn swallows and swordtail fish have shown that individuals with more symmetrical features are often preferred during courtship. For example, female barn swallows tend to favor males with highly symmetrical tail feathers, as this signals their fitness and ability to survive.
Fundamental Principles in Physics and Chemistry
The concepts of symmetry and its absence extend far beyond biology. A deep connection exists between symmetries and conservation laws in physics, a relationship formalized by Noether’s theorem. For example, the fact that the laws of physics are the same everywhere in space (translational symmetry) is directly linked to the law of conservation of momentum. The laws being consistent over time (time-translation symmetry) corresponds to the conservation of energy.
The early universe is thought to have existed in a state of high symmetry, with fundamental forces unified. A process known as symmetry breaking occurred as the universe cooled, causing these forces to separate into the distinct forces we observe today. This series of events was a turning point in cosmic history, allowing for the formation of matter and the complex structures that make up the universe.
In chemistry, asymmetry is manifested in a property called chirality, or “handedness.” Chiral molecules exist as two mirror-image forms that cannot be perfectly superimposed, much like a person’s left and right hands. This is important in biological systems, as organisms are often built from molecules of a specific handedness. The drug thalidomide provides a stark example of this, as one chiral form was an effective treatment for morning sickness while its mirror image caused severe birth defects.