Nature’s Affinity for Symmetry

Scanning electron micrograph of various symmetrical pollen grains

Biological Strategy

Common to all organisms

Sam Stier

Image: Dartmouth College Electron Microscope Facility / Wikimedia Commons / Public Domain - No restrictions

Physically Assemble Structure

Living systems use physical materials to create structures to serve as protection, insulation, and other purposes. These structures can be internal (within or attached to the system itself), such as cell membranes, shells, and fur. They can also be external (detached), such as nests, burrows, cocoons, or webs. Because physical materials are limited and the energy required to gather and create new structures is costly, living systems must use both conservatively. Therefore, they optimize the structures’ size, weight, and density. For example, weaver birds use two types of vegetation to create their nests: strong, a few stiff fibers and numerous thin fibers. Combined, they make a strong, yet flexible, nest. An example of an internal structure is a bird’s bone. The bone is comprised of a mineral matrix assembled to create strong cross-supports and a tubular outer surface filled with air to minimize weight.

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Animals

Kingdom Animalia (“having breath or soul”): Mammals, reptiles, sponges, corals, insects, birds

This kingdom captures a staggering variety of life—from the smallest coral polyp to the largest elephant. Although animal body shapes and life histories vary, there are a few defining characteristics. Animals are heterotrophs, meaning they gain energy from eating other organisms, and have developed from single-celled ancestors to highly diversified and coordinated multicellular forms. Unlike plant and fungi cells that have rigid cell walls, animal cells are bound by more flexible combinations of proteins. Most animals have cells organized into tissues and are able to move within their environment.

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Symmetrical body plans are a form of efficiency given the rules of genetic coding.

Introduction

Symmetry is common in nature, and often very useful; Human tool use developed through the coordination of our two mirror-image hands, for example. Symmetry enables things like movement (by alternating appendages), depth perception (by overlapping two eyes’ fields of view), and resiliency (if one hand is injured, we can still use the other). People, pelicans, pineapples––why exactly is symmetry so common in nature? Is it natural selection, which explains biological form so often? When it comes to the wide-ranging phenomenon of symmetry, surprisingly, natural selection is not the whole picture.

The Strategy

To a degree, the prevalence of symmetry in nature begins as an artifact of information dynamics. This is because it takes less genetic information to code for a repeating structure, such as a pair of symmetrical antennae, than to code for a non-repeating (or asymmetrical) structure. Random chance, then, on average, will favor symmetry, at least initially, as fewer mutations are required to produce it. In other words, evolution initially has a greater chance of discovering symmetrical structures than non-symmetrical ones. If such structures then function competitively, these symmetrical forms may become preferentially inherited after processes of natural selection kick in.

Importantly, symmetry doesn’t have to be simply bilateral as in vertebrates, where the left side mirrors the right. It can also be radial as in starfish, where one form is repeated angularly in any number of repetitions before completing a circle.

One way researchers have confirmed this dynamic is using simulations of evolution with sets of shapes, starting with equal numbers of symmetrical and asymmetrical forms. Even when the probability of reproduction or survival (that is, “fitness”) is set to be the same between these two classes of shapes, much higher proportions of symmetrical shapes result from these simulations, just as we see in the natural world.

See Related Strategy

Biological Strategy

Controlled Crystal Symmetry

Calcareous sponge

Crystals in glass sponges exhibit controlled growth due to biologically induced reduction in calcitic crystal symmetry.

photograph of pink starfish underwater suctioned onto glass

Biological Strategy

Skeleton Components Arranged Efficiently

Sea stars

The skeletons of some echinoderms arrange their calcium carbonate plates efficiently using pentaradial symmetry.

The Potential

Nature’s bias towards producing structures exhibiting repetition (such as symmetry) as an artifact of informational efficiency has a number of applications to human design. The design of software using repeating subroutines or loops is already considered an important feature of algorithmic efficiency. Other techniques capitalizing on repetition (for example, fractal compression) are used to reduce the time and energy requirements of transmitting graphic data.

Further emphasis on the use of repetition whenever possible will likely further increase efficiency in informational technologies. But the advantages of symmetry also extend to more three-dimensional technologies, just as it does in nature. For instance, when otherwise equally functional, designing repetitive structures over non-repeating structures may facilitate ease of manufacturing and ease of repair of human-made things. This can be especially important for sustainable manufacturing, in contexts such as rural technology, or in space exploration.

Last Updated November 9, 2023

References

“Symmetric phenotypes typically need less information to encode algorithmically, due to repetition of subunits… Upon random mutations these symmetric phenotypes are much more likely to arise as potential variation, so that a strong bias toward symmetry may emerge even without natural selection for symmetry.” (Johnston et al. 2022: 1)

“The arguments above are general enough to suggest that many biological systems, beyond the examples we provided, may favor simplicity and, where relevant, high symmetry, without requiring selective advantages for these features.” (Johnston et al. 2022: 6)

Journal article

Symmetry and simplicity spontaneously emerge from the algorithmic nature of evolution

Proceedings of the National Academy of Sciences | 2022 | Johnston, I.G., Dingle, K., Greenbury, S.F., Camargo, C.Q., Doye, J.P., Ahnert, S.E. and Louis, A.A.

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