What is Biological Symmetry?
Biological symmetry is the balanced distribution of duplicate body parts or tissue structures arranged around a central axis or across a dividing plane. In both plants and animals, the specific type of symmetry an organism exhibits directly correlates with its ecological niche, motility, and evolutionary history.
Watch our Concept Video: Evolution and Advantages of Body Symmetry in Organisms
|
Symmetry Type |
Definition |
Zoological Examples |
Botanical Examples |
|
Asymmetry |
The complete lack of a central axis
or plane of symmetry; irregular morphological growth. |
Sea sponges (Porifera), Placozoans. |
Many crustose lichens; the foliage
structure of certain creeping aquatic ferns (Salvinia, Azolla) |
|
Radial Symmetry |
Any longitudinal plane passing
through the central vertical axis divides the organism into mirrored halves. |
Cnidarians (jellyfish, anemones),
adult Echinoderms (starfish). |
Actinomorphic flowers (e.g., Hibiscus,
Datura, Solanum). |
|
Bilateral Symmetry |
Only a single sagittal plane
divides the organism into distinct left and right mirrored halves. |
Arthropods (insects, crustaceans),
Vertebrates, Nematodes. |
Zygomorphic flowers (e.g., Eichhornia
crassipes (water hyacinth) , Pisum sativum (Pea), Orchids). |
The Functional Advantage of Asymmetry
- Restricted to morphologically simple organisms, such as sessile filter-feeding sponges.
- Physical form is driven by immediate environmental conditions like water current direction and substrate shape.
- Irregular growth allows these organisms to dynamically adapt their shape to maximize surface area for feeding.
The Functional Advantage of Radial
Symmetry
- Evolves primarily in organisms that are sessile (anchored) or planktonic (drifting).
- Equips organisms to interact with their environment equally from all 360 degrees, compensating for their inability to actively hunt or flee.
- In plants, radial (actinomorphic) flowers offer multiple approach angles, an adaptation designed to attract a wide variety of unspecialized pollinators.
The Functional Advantage of Bilateral
Symmetry
- Serves as the prerequisite for active, directional movement requiring streamlined architecture.
- Drives cephalization—the evolutionary concentration of nervous tissue and sensory organs at the anterior (front) end to process new environments as the organism moves forward.
In plants: Bilateral
(zygomorphic) flowers—like orchids and water hyacinths—co-evolved with specific
insect vectors.
Provides a specialized
physical "landing pad" that forces pollinators to approach from one
precise angle, significantly increasing the efficiency of pollen transfer.