WHAT IS BILATERAL SYMMETRY? (WITH EXAMPLES) - BIOLOGY - 2025

The bilateral symmetry, also called sagittal plane symmetry, is that condition of a structure by which it is divided into two equal halves. They are usually left and right halves and are mirror images of each other (like reflection in a mirror).

In nature, flowers like the orchid and seeds like the pea are examples of bilateral symmetry. This symmetry is better adapted to active organisms, that is, in movement. This condition leads to a greater balance of the bodies and is the most common among animals.

Monarch butterfly, example of bilateral symmetry

This symmetry helps in the formation of the main nerve centers and sensory organs of animals. In addition, it allows cephalization, which is the evolutionary development of the head, as explained below.

When animals move in any direction, they necessarily have a front side or front. This front end is the one that first makes contact with the environment, as the individual moves.

The organs of perception (such as the eyes) are located in the front, and also the mouth, to facilitate the search for food. Therefore, the head with sensory organs in connection with a central nervous system is common in bilateral symmetrical beings, this is called cephalization.

Regarding the external aspect of the organisms, the existing symmetry is a reflection and inside them there might not be symmetry in the organs. However, on each side there is a sensing organ and a group of limbs.

When animals have bilateral symmetry, it occurs in a single plane (sagittal) so the body is divided vertically into two halves: right and left.

Approximately 99% of animals have bilateral symmetry, including humans, in which facial symmetry is directly related to the phenomenon of attraction.

What is bilateral symmetry?

Symmetry is the similarity between the parts of an organism so that when a straight cut is made through a point or along a line, equal halves are formed as reflected in a mirror.

Bilateral symmetry is also known as zygomorphic (from the Greek zigo: yoke), dorsiventral or lateral. It is common in 33% of dicotyledonous plants and in 45% of monocotyledonous plants.

The condition of bilaterality has evolved in the species, appearing and disappearing on many occasions. This uniqueness occurs because the change in symmetry can happen very easily and is related to one or two genes.

When a living being moves, a difference is immediately generated between the front-rear concepts, likewise, by the action of gravity, the difference between dorsal-ventral and right-left is established.

Therefore, all animals that have bilateral symmetry have a ventral region, a dorsal region, a head, and a tail or caudal region. This condition allows a simplification that reduces the resistance to the medium, facilitating movement.

By having symmetry, organisms have an axis in their structure, both bilateral and radial. This line or geometric axis can pass through a cavity, any interior anatomical structure or a central vesicle.

Bilateral symmetry is present in large metazoans (multicellular, heterotrophic, mobile organisms formed by differentiated cells grouped in tissues), which are almost all animals in nature. Only sponges, jellyfish and echinoderms do not have bilateral symmetry.

Examples of bilateral symmetry

In some species of animals, symmetry is linked to sex and biologists assume that it is a type of mark or signal for a certain aptitude.

In the case of a species of swallows, the males have a long tail similar to a serpentine and the females prefer to mate with the males that have more symmetrical tails.

In the phylum Echinodermata (the starfish) and in sea urchins, the larval stage shows bilateral symmetry and the adult forms have fivefold symmetry (pentamerism).

The Mollusca phylum (octopus, squid, mussel and clam) has bilateral symmetry.

The variety of emperor moth Saturnia pavonia has a deimatic pattern (threatening behavior) with bilateral symmetry.

The bee orchid (Ophrys apifera) is bilaterally symmetrical (zygomorphic) and has a lip-shaped petal that resembles the abdomen of the female bee. This characteristic favors pollination when the male tries to mate with her.

In some families of flowering plants such as orchids, peas, and most fig trees there is bilateral symmetry.

origins

The appearance of bilateral symmetry (balance between arms, legs and organs distributed to the right and left) is considered to be a distinctive characteristic of higher animals. It is considered to be one of the most important breakthroughs in the history of life.

In June 2005, a group of paleontologists managed to identify the oldest example of bilateral symmetry, in fossils belonging to a 600 million-year-old quarry in southern China.

Jun Yuan Chen, from the Nanjing Institute of Geology and Paleontology, and his colleagues collected and analyzed samples of Vernanimalcula guizhouena, a microorganism that was likely a seabed inhabitant feeding on bacteria.

The scientists observed signs of a mouth in the anterior region and a group of paired digestive canals on each side of the intestine. This would be an indication that the first animals with symmetry appeared 30 million years earlier than previously believed.

This means that long before the Cambrian explosion, about 540 million years ago, a great diversity of hard-bodied animals appeared, of which there are fossil records.

There are paleontologists who believe that the symmetry found in this species could have originated in a petrification process. David Bottjer from the University of California, who worked with Chen, believes that the fossils of this microorganism were located in an unusual mineral environment that preserved them exceptionally.

The ancient origin of symmetry makes sense, in Bottjer's words, since all animals, except for the most primitive ones, have been bilateral at some stage in their life. This would confirm that symmetry is an early evolutionary innovation.

Differences between bilateral and radial symmetry

In nature there is a great variety of flowers that can be classified into two large groups, according to their symmetry: radial, like the lily, and bilateral, like the orchid.

Studies carried out on floral fossils and botanical genetics show that radial symmetry is an ancestral condition; on the other hand, bilateral symmetry is the result of evolution and has repeatedly varied independently in many families of plants.

By making observations in the evolutionary process of the flower, it is concluded that natural selection favors bilateral symmetry because pollinating insects prefer it.

Study with

To corroborate the previous statement, reference is made to a study carried out at the University of Granada, Spain. José Gómez and his team experimented with the Erysimum mediohispanicum plant, typical of the mountains of southeastern Spain.

This plant produces flowers with both radial and bilateral symmetry, in the same specimen. The observation of the insects that pollinate the flowers showed that the most frequent visitor is a small beetle: Meligethes maurus.

In a count of 2000 visits in which the three-dimensional shape of the flowers was measured, using the geometric morphometry technique, the team found that the most visited flowers were those with bilateral symmetry.

It was also determined that the plants with bilateral symmetry flowers produced more seeds and more daughter plants, during the time that the study was carried out. This means that, for many generations, more flowers of bilateral symmetry would be present than radial.

The resulting question is about the preference of insects for flowers of bilateral symmetry, the answer could be related to the location of the petals, as it provides them with a better landing platform.

References

1. Symmetry, biological, from The Columbia Electronic Encyclopedia (2007).
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3. Balter, M. (2006). Pollinators Power Flower Evolution. Science.
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5. Weinstock, M. (2005). 88: Mirror-Image Animals Found. Discover.
6. Willmer, P. (2011). Pollination and Floral Ecology. New Jersey: Princeton University Press.