In biology, symmetry is typically the rule rather than the exception. Our bodies have left and right halves, starfish radiate from a central point and even trees, though not largely symmetrical, still produce symmetrical flowers. In fact, asymmetry in biology seems quite rare by comparison.
Does this mean that evolution has a preference for symmetry? In a new study, an international group of researchers, led by Iain Johnston, a professor in the Department of Mathematics at the University of Bergen in Norway, says it does.
Although symmetrical structures represent only a small fraction of possible forms — in geometry, at least — symmetry pops up everywhere in living organisms. It's not just a body-plan phenomenon, either. Proteins, the molecular machinery within a body, are largely symmetrical as well, often being composed of a series of repeating, modular parts. Repeating structures are often seen in animals, too; think of centipedes, with their repeating body segments. The reason for this apparent "preference" is not driven by aesthetics. Instead, according to the researchers, it comes down to simplicity.
"It can be tempting to assume that symmetry and modularity arise from natural selection," Johnston and his co-authors wrote in the new study. Natural selection can cause beneficial traits to become more common because those traits help survival. However, natural selection can only make a beneficial trait more common or do away with a harmful one; it can't force brand-new ones to appear.
Instead, it can only reinforce the effects of mutations that occur randomly. For example, moths with dark-colored wings might be harder for birds to see than moths with light-colored wings. Predators might therefore be more likely to overlook dark-winged moths, enabling more of those insects to survive, reproduce, and pass that trait along to their offspring. But this doesn’t force black wings into existence; a gene has to mutate in order for that to happen. And if a mutation provides an advantage, it’s more likely to be perpetuated among a population for generations, until it becomes a common trait for the species.
In the same way, natural selection might only seem to favor symmetry because it is mostly given symmetrical forms to work with. The most likely explanation for why proteins and bodies are symmetrical is not because symmetry gives a survival advantage, but because more symmetrical, repeating forms appear in the first place.
So what makes that happen? Symmetrical forms have likely evolved more frequently and then persisted over evolutionary time because they often require less information to produce than asymmetrical forms do.
"Imagine having to tell a friend how to tile a floor using as few words as possible," Johnston said in a statement. "You wouldn't say, 'Put diamonds here, long rectangles here, wide rectangles here.' You'd say something like, 'Put square tiles everywhere.' And that simple, easy recipe gives a highly symmetric outcome."
Johnston and his colleagues tested this simplicity hypothesis by using computational modeling. By running a simulation of protein evolution, the researchers found that random mutations are much more likely to produce simple genetic sequences than complex ones. If those simple structures are good enough to do their jobs, natural selection can then take over and make use of those structures. In the researchers' simulations, as well as in life, high-symmetry structures with low complexity far outnumbered complex structures with low symmetry.
The study puts a new spin on the so-called infinite monkey theorem, an old thought experiment in the field of evolutionary biology. If, as the theorem predicts, a monkey types randomly for an infinite amount of time, it will eventually produce the complete works of Shakespeare (or perhaps the script for "Die Hard"). Essentially, random mutations in DNA are like typing monkeys. Given enough time (and enough monkeys), it is a certainty that some pretty ingenious mutations will appear.
But by the time a hypothetical monkey produces Shakespeare’s entire catalog of work, the industrious creature will have likely already typed a large number of short poems. Similarly, if biology is entirely reliant on genetic instructions generated at random (much like the work of a randomly typing monkey), it is going to generate a very large number of simple instructions, because those will appear much more frequently than complex directions do. As far as natural selection is concerned, complexity is unnecessary when a simple solution is available, study authors concluded.
So, the next time you stop to admire a flower’s radial symmetry, you can also admire the efficiency of the shorter, simpler gene sequences that encoded for that trait.
This study was published March 11 in the journal Proceedings of the National Academy of Sciences (opens in new tab).
Originally published on Live Science.