Symmetry in biology

Symmetry in biology refers to the symmetry observed in <a href="/facts/Organism/j8JkL09u">organisms</a>, including plants, animals, <a href="/facts/Fungi/BHo5DP59">fungi</a>, and <a href="/facts/Bacteria/wC8xSCsU">bacteria</a>. External symmetry can be easily seen by just looking at an organism. For example, the face of a human being has a plane of symmetry down its centre, or a pine cone displays a clear symmetrical spiral pattern. Internal features can also show symmetry, for example the tubes in the human body (responsible for transporting <a href="/facts/Gas/hVVAoodV">gases</a>, <a href="/facts/Nutrient/34bn4skK">nutrients</a>, and waste products) which are <a href="/facts/Cylindrical/0wQPPr3k">cylindrical</a> and have several planes of symmetry.
Biological symmetry can be thought of as a balanced distribution of duplicate body parts or shapes within the body of an organism. Importantly, unlike in mathematics, symmetry in biology is always approximate. For example, plant leaves – while considered symmetrical – rarely match up exactly when folded in half. Symmetry is one class of <a href="/facts/Patterns_in_nature/handzeJh">patterns in nature</a> whereby there is near-repetition of the pattern element, either by <a href="/facts/Reflection_(geometry)/5JHWnrql">reflection</a> or <a href="/facts/Rotation_(geometry)/6gtkkcy0">rotation</a>.
While <a href="/facts/Sponge/E9T2WFWA">sponges</a> and <a href="/facts/Placozoa/F9NcYdAz">placozoans</a> represent two groups of animals which do not show any symmetry (i.e. are asymmetrical), the <a href="/facts/Body_plan/TykqU5bY">body plans</a> of most <a href="/facts/Multicellular_organism/rk8QBF7o">multicellular organisms</a> exhibit, and are defined by, some form of symmetry. There are only a few types of symmetry which are possible in body plans. These are <a href="/facts/Rotational_symmetry/bpvJqcnS">radial</a> (cylindrical) symmetry, <a href="/facts/Reflection_symmetry/eSeR8nbD">bilateral</a>, biradial and <a href="/facts/Circular_symmetry/gEgSEFxy">spherical</a> symmetry. While the classification of <a href="/facts/Virus/mf88Bcbl">viruses</a> as an "organism" remains controversial, viruses also contain <a href="/facts/Icosahedral_symmetry/pDlwAy23">icosahedral symmetry</a>.
The importance of symmetry is illustrated by the fact that groups of animals have traditionally been defined by this feature in <a href="/facts/Taxonomy_(biology)/Qkg8wuyw">taxonomic</a> groupings. The <a href="/facts/Radiata/89tVkW0k">Radiata</a>, animals with radial symmetry, formed one of the four branches of <a href="/facts/Georges_Cuvier/4mmE7NzN">Georges Cuvier</a>'s classification of the <a href="/facts/Animal/4b8J33nd">animal</a> <a href="/facts/Kingdom_(biology)/QGRKbHos">kingdom</a>. Meanwhile, <a href="/facts/Bilateria/nfUwVYrx">Bilateria</a> is a taxonomic grouping still used today to represent organisms with <a href="/facts/Embryo/9QHnLVLN">embryonic</a> bilateral symmetry.

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Symmetry in biology