Animal symmetry, a balanced distribution of body parts, is a widespread feature across the animal kingdom. This arrangement shapes the fundamental body plans of most multicellular organisms. While a few animal groups exhibit irregular forms, the prevalence of symmetry prompts a significant inquiry: why has this characteristic been such a persistent and successful evolutionary trait for animals?
Types of Animal Symmetry
Animals display several distinct types of body symmetry, each suited to different lifestyles. Bilateral symmetry divides an organism into two roughly mirror-image left and right halves along a single plane. This design is common in most animals, including humans, insects, and fish.
In contrast, radial symmetry involves body parts arranged around a central axis, allowing the organism to be divided into identical pieces through multiple planes. Examples include jellyfish and starfish, which can interact with their environment from all directions. Some animals, such as most sponges, exhibit asymmetry, lacking any defined planes of symmetry.
Evolutionary Advantages of Symmetry
The various forms of symmetry provide benefits for an animal’s survival and function. Bilateral symmetry offers advantages for active movement, allowing for a distinct front (anterior) and back (posterior) end, facilitating directional locomotion. This body plan enables efficient navigation, with sensory organs often concentrated at the leading end, enhancing the ability to locate food or detect threats. The paired sensory organs, such as eyes and ears, on either side of the body, also contribute to effective spatial awareness and coordinated responses.
Radial symmetry is advantageous for sessile or slow-moving organisms. This body arrangement allows animals to sense and respond to stimuli equally from all directions, which is beneficial for organisms that are anchored in place or drift in water. For instance, a sea anemone can capture food or defend itself from any approach, and starfish can move across surfaces without a specific forward direction.
Symmetry and Mate Choice
Beyond functional benefits, symmetry plays a role in mate selection, serving as a signal of an individual’s quality. Symmetry, particularly bilateral symmetry, is attractive to potential mates across many species. This preference stems from symmetry indicating developmental stability, meaning the organism successfully navigated environmental stressors and genetic challenges during its growth.
Individuals with more symmetrical features possess good genes and robust health, suggesting a greater capacity to produce viable offspring. For example, studies have shown that more symmetrical individuals in various species often secure more matings. This provides an evolutionary advantage, as organisms seek mates who can contribute to the survival and reproductive success of their progeny.
The Genetic and Developmental Basis of Symmetry
The formation of an animal’s symmetrical body plan is orchestrated by genetic instructions and developmental pathways during embryonic growth. Genes play a role in laying out the body axes and ensuring the balanced distribution of structures. Early cellular divisions and the action of signaling molecules guide the formation of an organized body, establishing the blueprint for symmetry.
For example, specific gene families, such as Hox genes, define the anterior-posterior (head-to-tail) axis and organize repeating body segments in bilaterally symmetrical animals. These genes control the development of structures along the body, contributing to the symmetrical form. The coordinated expression of these genes and subsequent cellular processes allow a symmetrical organism to develop from a single fertilized egg.
Asymmetry in Nature
While symmetry is a dominant theme in the animal kingdom, instances of natural asymmetry exist and are themselves adaptive. Sponges, for example, are asymmetrical, and their irregular shapes are well-suited for their filter-feeding lifestyle. This lack of fixed symmetry allows them to adapt to varied attachment surfaces in their aquatic environments.
Flatfish, such as flounder or sole, undergo a transformation during development. Initially, these fish are bilaterally symmetrical, but as they mature, one eye migrates to the other side of the head, and they adopt a flattened, asymmetrical body form. This adaptation allows them to lie on the seafloor, with both eyes pointing upwards, providing camouflage and an effective ambush predation strategy. Such cases demonstrate that asymmetry can be a specialized and beneficial evolutionary outcome.