![Diagram_comparing_bilateral,_radial,_and_spherical_symmetry](https://upload.wikimedia.org/wikipedia/commons/0/0a/Diagram_comparing_bilateral%2C_radial%2C_and_spherical_symmetry.jpg)
Symmetry in biology refers to the symmetry observed in
organism
In biology, an organism () is any living system that functions as an individual entity. All organisms are composed of cells (cell theory). Organisms are classified by taxonomy into groups such as multicellular animals, plants, and ...
s, including plants, animals,
fungi
A fungus ( : fungi or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as the more familiar mushrooms. These organisms are classified as a kingdom, separately from ...
, and
bacteria
Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria were among ...
. External symmetry can be easily seen by just looking at an organism. For example, take the face of a human being which has a plane of symmetry down its centre, or a pine cone with a clear symmetrical spiral pattern. Internal features can also show symmetry, for example the tubes in the human body (responsible for transporting
gas
Gas is one of the four fundamental states of matter (the others being solid, liquid, and plasma).
A pure gas may be made up of individual atoms (e.g. a noble gas like neon), elemental molecules made from one type of atom (e.g. oxygen), or ...
es,
nutrients
A nutrient is a substance used by an organism to survive, grow, and reproduce. The requirement for dietary nutrient intake applies to animals, plants, fungi, and protists. Nutrients can be incorporated into cells for metabolic purposes or excret ...
, and waste products) which are
cylindrical
A cylinder (from ) has traditionally been a three-dimensional solid, one of the most basic of curvilinear geometric shapes. In elementary geometry, it is considered a prism with a circle as its base.
A cylinder may also be defined as an infini ...
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
patterns in nature
Patterns in nature are visible regularities of form found in the natural world. These patterns recur in different contexts and can sometimes be modelled mathematically. Natural patterns include symmetries, trees, spirals, meanders, waves, foa ...
whereby there is near-repetition of the pattern element, either by
reflection Reflection or reflexion may refer to:
Science and technology
* Reflection (physics), a common wave phenomenon
** Specular reflection, reflection from a smooth surface
*** Mirror image, a reflection in a mirror or in water
** Signal reflection, in ...
or
rotation
Rotation, or spin, is the circular movement of an object around a '' central axis''. A two-dimensional rotating object has only one possible central axis and can rotate in either a clockwise or counterclockwise direction. A three-dimensional ...
.
While
sponges
Sponges, the members of the phylum Porifera (; meaning 'pore bearer'), are a basal animal clade as a sister of the diploblasts. They are multicellular organisms that have bodies full of pores and channels allowing water to circulate through ...
and
placozoans
The Placozoa are a Basal (phylogenetics), basal form of marine free-living (non-parasitic) multicellular organism. They are the simplest in structure of all animals. Three genera have been found: the classical ''Trichoplax adhaerens'', ''Hoilung ...
represent two groups of animals which do not show any symmetry (i.e. are asymmetrical), the
body plan
A body plan, ( ), or ground plan is a set of morphological features common to many members of a phylum of animals. The vertebrates share one body plan, while invertebrates have many.
This term, usually applied to animals, envisages a "blueprin ...
s of most
multicellular organism
A multicellular organism is an organism that consists of more than one cell, in contrast to unicellular organism.
All species of animals, land plants and most fungi are multicellular, as are many algae, whereas a few organisms are partially uni- ...
s 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
radial
Radial is a geometric term of location which may refer to:
Mathematics and Direction
* Vector (geometric)
In mathematics, physics, and engineering, a Euclidean vector or simply a vector (sometimes called a geometric vector or spatial vector) ...
(cylindrical),
bilateral
Bilateral may refer to any concept including two sides, in particular:
* Bilateria, bilateral animals
*Bilateralism, the political and cultural relations between two states
*Bilateral, occurring on both sides of an organism ( Anatomical terms of ...
, biradial and
spherical
A sphere () is a geometrical object that is a three-dimensional analogue to a two-dimensional circle. A sphere is the set of points that are all at the same distance from a given point in three-dimensional space.. That given point is the ce ...
symmetry.
While the classification of
viruses
A virus is a submicroscopic infectious agent that replicates only inside the living cells of an organism. Viruses infect all life forms, from animals and plants to microorganisms, including bacteria and archaea.
Since Dmitri Ivanovsky's 1 ...
as an "organism" remains controversial, viruses also contain
icosahedral symmetry
In mathematics, and especially in geometry, an object has icosahedral symmetry if it has the same symmetries as a regular icosahedron. Examples of other polyhedra with icosahedral symmetry include the regular dodecahedron (the dual of the ...
.
The importance of symmetry is illustrated by the fact that groups of animals have traditionally been defined by this feature in
taxonomic
Taxonomy is the practice and science of categorization or classification.
A taxonomy (or taxonomical classification) is a scheme of classification, especially a hierarchical classification, in which things are organized into groups or types. ...
groupings. The
Radiata
Radiata or Radiates is a historical taxonomic rank that was used to classify animals with radially symmetric body plans. The term Radiata is no longer accepted, as it united several different groupings of animals that do not form a monophyletic ...
, animals with radial symmetry, formed one of the four branches of
Georges Cuvier
Jean Léopold Nicolas Frédéric, Baron Cuvier (; 23 August 1769 – 13 May 1832), known as Georges Cuvier, was a French natural history, naturalist and zoology, zoologist, sometimes referred to as the "founding father of paleontology". Cuvier ...
's classification of the
animal
Animals are multicellular, eukaryotic organisms in the Kingdom (biology), biological kingdom Animalia. With few exceptions, animals Heterotroph, consume organic material, Cellular respiration#Aerobic respiration, breathe oxygen, are Motilit ...
kingdom
Kingdom commonly refers to:
* A monarchy ruled by a king or queen
* Kingdom (biology), a category in biological taxonomy
Kingdom may also refer to:
Arts and media Television
* ''Kingdom'' (British TV series), a 2007 British television drama s ...
.
Meanwhile,
Bilateria
The Bilateria or bilaterians are animals with bilateral symmetry as an embryo, i.e. having a left and a right side that are mirror images of each other. This also means they have a head and a tail (anterior-posterior axis) as well as a belly and ...
is a taxonomic grouping still used today to represent organisms with
embryo
An embryo is an initial stage of development of a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male spe ...
nic bilateral symmetry.
Radial symmetry
Organisms with radial symmetry show a repeating pattern around a central axis such that they can be separated into several identical pieces when cut through the central point, much like pieces of a pie. Typically, this involves repeating a body part 4, 5, 6 or 8 times around the axis – referred to as tetramerism, pentamerism, hexamerism and octamerism, respectively. Such organisms exhibit no left or right sides but do have a top and a bottom surface, or a front and a back.
George Cuvier classified animals with radial symmetry in the taxon Radiata (''Zoophytes''),
which is now generally accepted to be an assemblage of different animal phyla that do not share a single common ancestor (a
polyphyletic
A polyphyletic group is an assemblage of organisms or other evolving elements that is of mixed evolutionary origin. The term is often applied to groups that share similar features known as homoplasies, which are explained as a result of converg ...
group). Most radially symmetric animals are symmetrical about an axis extending from the center of the oral surface, which contains the mouth, to the center of the opposite (aboral) end. Animals in the phyla
Cnidaria
Cnidaria () is a phylum under kingdom Animalia containing over 11,000 species of aquatic animals found both in freshwater and marine environments, predominantly the latter.
Their distinguishing feature is cnidocytes, specialized cells that th ...
and
Echinoderm
An echinoderm () is any member of the phylum Echinodermata (). The adults are recognisable by their (usually five-point) radial symmetry, and include starfish, brittle stars, sea urchins, sand dollars, and sea cucumbers, as well as the sea ...
ata generally show radial symmetry,
[ although many ]sea anemone
Sea anemones are a group of predation, predatory marine invertebrates of the order (biology), order Actiniaria. Because of their colourful appearance, they are named after the ''Anemone'', a terrestrial flowering plant. Sea anemones are classifi ...
s and some coral
Corals are marine invertebrates within the class Anthozoa of the phylum Cnidaria. They typically form compact colonies of many identical individual polyps. Coral species include the important reef builders that inhabit tropical oceans and sec ...
s within the Cnidaria have bilateral symmetry defined by a single structure, the siphonoglyph The siphonoglyph is a ciliated groove at one or both ends of the mouth of sea anemones and some corals. The siphonoglyph extends into a pharynx and is used to create currents of water into the pharynx. These water currents are important for respirat ...
. Radial symmetry is especially suitable for sessile
Sessility, or sessile, may refer to:
* Sessility (motility), organisms which are not able to move about
* Sessility (botany), flowers or leaves that grow directly from the stem or peduncle of a plant
* Sessility (medicine), tumors and polyps that ...
animals such as the sea anemone, floating animals such as jellyfish
Jellyfish and sea jellies are the informal common names given to the medusa-phase of certain gelatinous members of the subphylum Medusozoa, a major part of the phylum Cnidaria. Jellyfish are mainly free-swimming marine animals with umbrella- ...
, and slow moving organisms such as starfish
Starfish or sea stars are star-shaped echinoderms belonging to the class Asteroidea (). Common usage frequently finds these names being also applied to ophiuroids, which are correctly referred to as brittle stars or basket stars. Starfish ...
; whereas bilateral symmetry favours locomotion
Locomotion means the act or ability of something to transport or move itself from place to place.
Locomotion may refer to:
Motion
* Motion (physics)
* Robot locomotion, of man-made devices
By environment
* Aquatic locomotion
* Flight
* Locomo ...
by generating a streamlined
Streamlines, streaklines and pathlines are field lines in a fluid flow.
They differ only when the flow changes with time, that is, when the flow is not steady.
Considering a velocity vector field in three-dimensional space in the framework of ...
body.
Many flowers are also radially symmetric, or "actinomorphic
Floral symmetry describes whether, and how, a flower, in particular its perianth, can be divided into two or more identical or mirror-image parts.
Uncommonly, flowers may have no axis of symmetry at all, typically because their parts are spirall ...
". Roughly identical floral structures – petal
Petals are modified Leaf, leaves that surround the reproductive parts of flowers. They are often advertising coloration, brightly colored or unusually shaped to attract pollinators. All of the petals of a flower are collectively known as the ''c ...
s, sepal
A sepal () is a part of the flower of angiosperms (flowering plants). Usually green, sepals typically function as protection for the flower in bud, and often as support for the petals when in bloom., p. 106 The term ''sepalum'' was coined b ...
s, and stamen
The stamen (plural ''stamina'' or ''stamens'') is the pollen-producing reproductive organ of a flower. Collectively the stamens form the androecium., p. 10
Morphology and terminology
A stamen typically consists of a stalk called the filame ...
s – occur at regular intervals around the axis of the flower, which is often the female reproductive organ
A sex organ (or reproductive organ) is any part of an animal or plant that is involved in sexual reproduction. The reproductive organs together constitute the reproductive system. In animals, the testis in the male, and the ovary in the female, a ...
containing the carpel
Gynoecium (; ) is most commonly used as a collective term for the parts of a flower that produce ovules and ultimately develop into the fruit and seeds. The gynoecium is the innermost whorl of a flower; it consists of (one or more) ''pistils'' ...
, style
Style is a manner of doing or presenting things and may refer to:
* Architectural style, the features that make a building or structure historically identifiable
* Design, the process of creating something
* Fashion, a prevailing mode of clothing ...
and stigma.
Subtypes of radial symmetry
Some jellyfish, such as ''Aurelia Aurelia may refer to:
People
* Version of feminine given name Aurélie
* Aurelia (mother of Caesar)
* Aurelia gens, a Roman family
* Aurelia Browder, American civil rights activist
* Astrud Aurelia, American drag queen
Science
* ''Aurelia'' (c ...
marginalis'', show tetramerism with a four-fold radial symmetry. This is immediately obvious when looking at the jellyfish due to the presence of four gonads
A gonad, sex gland, or reproductive gland is a mixed gland that produces the gametes and sex hormones of an organism. Female reproductive cells are egg cells, and male reproductive cells are sperm. The male gonad, the testicle, produces sperm ...
, visible through its translucent
In the field of optics, transparency (also called pellucidity or diaphaneity) is the physical property of allowing light to pass through the material without appreciable scattering of light. On a macroscopic scale (one in which the dimensions a ...
body. This radial symmetry is ecologically
Ecology () is the study of the relationships between living organisms, including humans, and their biophysical environment, physical environment. Ecology considers organisms at the individual, population, community (ecology), community, ecosy ...
important in allowing the jellyfish to detect and respond to stimuli
A stimulus is something that causes a physiological response. It may refer to:
* Stimulation
** Stimulus (physiology), something external that influences an activity
** Stimulus (psychology), a concept in behaviorism and perception
* Stimulus (eco ...
(mainly food and danger) from all directions.
Flowering plant
Flowering plants are plants that bear flowers and fruits, and form the clade Angiospermae (), commonly called angiosperms. The term "angiosperm" is derived from the Greek words ('container, vessel') and ('seed'), and refers to those plants th ...
s show five-fold symmetry, or pentamerism, in many of their flowers and fruits. This is easily seen through the arrangement of five carpel
Gynoecium (; ) is most commonly used as a collective term for the parts of a flower that produce ovules and ultimately develop into the fruit and seeds. The gynoecium is the innermost whorl of a flower; it consists of (one or more) ''pistils'' ...
s (seed pockets) in an apple
An apple is an edible fruit produced by an apple tree (''Malus domestica''). Apple fruit tree, trees are agriculture, cultivated worldwide and are the most widely grown species in the genus ''Malus''. The tree originated in Central Asia, wh ...
when cut transversely. Among animals, only the echinoderms such as sea star
Starfish or sea stars are Star polygon, star-shaped echinoderms belonging to the class (biology), class Asteroidea (). Common usage frequently finds these names being also applied to brittle star, ophiuroids, which are correctly referred to ...
s, sea urchin
Sea urchins () are spiny, globular echinoderms in the class Echinoidea. About 950 species of sea urchin live on the seabed of every ocean and inhabit every depth zone from the intertidal seashore down to . The spherical, hard shells (tests) of ...
s, and sea lilies
Crinoids are marine animals that make up the class Crinoidea. Crinoids that are attached to the sea bottom by a stalk in their adult form are commonly called sea lilies, while the unstalked forms are called feather stars or comatulids, which are ...
are pentamerous as adults, with five arms arranged around the mouth. Being bilaterian animals, however, they initially develop with mirror symmetry as larvae, then gain pentaradial symmetry later.
is found in the coral
Corals are marine invertebrates within the class Anthozoa of the phylum Cnidaria. They typically form compact colonies of many identical individual polyps. Coral species include the important reef builders that inhabit tropical oceans and sec ...
s and sea anemones
Sea anemones are a group of predatory marine invertebrates of the order Actiniaria. Because of their colourful appearance, they are named after the ''Anemone'', a terrestrial flowering plant. Sea anemones are classified in the phylum Cnidaria, ...
(class Anthozoa
Anthozoa is a subphylum of marine invertebrates which includes the sea anemones, Scleractinia, stony corals and Alcyonacea, soft corals. Adult anthozoans are almost all attached to the seabed, while their larvae can disperse as part of the plank ...
), which are divided into two groups based on their symmetry. The most common corals in the subclass Hexacorallia
Hexacorallia is a Class (biology), class of Anthozoa comprising approximately 4,300 species of aquatic organisms formed of polyp (zoology), polyps, generally with 6-fold symmetry. It includes all of the stony corals, most of which are Colony (b ...
have a hexameric body plan; their polyps have six-fold internal symmetry and a number of tentacle
In zoology, a tentacle is a flexible, mobile, and elongated organ present in some species of animals, most of them invertebrates. In animal anatomy, tentacles usually occur in one or more pairs. Anatomically, the tentacles of animals work main ...
s that is a multiple of six.
is found in corals of the subclass Octocorallia
Octocorallia (also known as Alcyonaria) is a class of Anthozoa comprising around 3,000 species of water-based organisms formed of colonial polyps with 8-fold symmetry. It includes the blue coral, soft corals, sea pens, and gorgonians (sea fan ...
. These have polyps with eight tentacles and octameric radial symmetry. The octopus
An octopus ( : octopuses or octopodes, see below for variants) is a soft-bodied, eight- limbed mollusc of the order Octopoda (, ). The order consists of some 300 species and is grouped within the class Cephalopoda with squids, cuttle ...
, however, has bilateral symmetry, despite its eight arms.
Triradial symmetry was present in Trilobozoa
''Trilobozoa'' (meaning "three-lobed animals") is a phylum of extinct mobile animals that were originally classified into the Cnidaria. The basic body plan of ''Trilobozoa'' is often a tri-radial or radial sphere-shaped form with lobes radiating ...
from the Late Ediacaran
The Ediacaran Period ( ) is a geological period that spans 96 million years from the end of the Cryogenian Period 635 million years ago (Mya), to the beginning of the Cambrian Period 538.8 Mya. It marks the end of the Proterozoic Eon, and th ...
period.
Icosahedral symmetry
Icosahedral symmetry occurs in an organism which contains 60 subunits generated by 20 faces, each an equilateral triangle
In geometry, an equilateral triangle is a triangle in which all three sides have the same length. In the familiar Euclidean geometry, an equilateral triangle is also equiangular; that is, all three internal angles are also congruent to each othe ...
, and 12 corners. Within the icosahedron
In geometry, an icosahedron ( or ) is a polyhedron with 20 faces. The name comes and . The plural can be either "icosahedra" () or "icosahedrons".
There are infinitely many non- similar shapes of icosahedra, some of them being more symmetrica ...
there is 2-fold, 3-fold and 5-fold symmetry. Many viruses, including ''canine parvovirus
Canine parvovirus (also referred to as CPV, CPV2, or parvo) is a contagious virus mainly affecting dogs. CPV is highly contagious and is spread from dog to dog by direct or indirect contact with their feces. Vaccines can prevent this infection, ...
'', show this form of symmetry due to the presence of an icosahedral viral shell. Such symmetry has evolved
Evolution is change in the heritable characteristics of biological populations over successive generations. These characteristics are the expressions of genes, which are passed on from parent to offspring during reproduction. Variation t ...
because it allows the viral particle to be built up of repetitive subunits consisting of a limited number of structural protein
Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, respo ...
s (encoded by viral genes
In biology, the word gene (from , ; "...Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredity..." meaning ''generation'' or ''birth'' or ''gender'') can have several different meanings. The Mendelian gene is a ba ...
), thereby saving space in the viral genome
In the fields of molecular biology and genetics, a genome is all the genetic information of an organism. It consists of nucleotide sequences of DNA (or RNA in RNA viruses). The nuclear genome includes protein-coding genes and non-coding ge ...
. The icosahedral symmetry can still be maintained with more than 60 subunits, but only in factors of 60. For example, the T=3 ''Tomato bushy stunt virus
Tomato bushy stunt virus (TBSV) is a virus of the tombusvirus family. It was first reported in tomatoes in 1935 and primarily affects vegetable crops, though it is not generally considered an economically significant plant pathogen. Depending up ...
'' has 60x3 protein subunits (180 copies of the same structural protein). Although these viruses are often referred to as 'spherical', they do not show true mathematical spherical symmetry.
In the early 20th century, Ernst Haeckel
Ernst Heinrich Philipp August Haeckel (; 16 February 1834 – 9 August 1919) was a German zoologist, naturalist, eugenicist, philosopher, physician, professor, marine biologist and artist. He discovered, described and named thousands of new sp ...
described (Haeckel, 1904) a number of species of Radiolaria
The Radiolaria, also called Radiozoa, are protozoa of diameter 0.1–0.2 mm that produce intricate mineral skeletons, typically with a central capsule dividing the cell (biology), cell into the inner and outer portions of endoplasm and Ecto ...
, some of whose skeletons are shaped like various regular polyhedra. Examples include ''Circoporus octahedrus'', ''Circogonia icosahedra'', ''Lithocubus geometricus'' and ''Circorrhegma dodecahedra''. The shapes of these creatures should be obvious from their names. Tetrahedral symmetry is not present in ''Callimitra agnesae''.
Spherical symmetry
Spherical symmetry is characterised by the ability to draw an endless, or great but finite, number of symmetry axes through the body. This means that spherical symmetry occurs in an organism if it is able to be cut into two identical halves through any cut that runs through the organism's center. True spherical symmetry is not found in animal body plans. Organisms which show approximate spherical symmetry include the freshwater green alga ''Volvox
''Volvox'' is a polyphyletic genus of chlorophyte green algae in the family Volvocaceae. It forms spherical colonies of up to 50,000 cells. They live in a variety of freshwater habitats, and were first reported by Antonie van Leeuwenhoek in 1700 ...
''.
Bacteria are often referred to as having a 'spherical' shape. Bacteria are categorized based on their shapes into three classes: cocci (spherical-shaped), bacillus (rod-shaped) and spirochetes (spiral-shaped) cells. In reality, this is a severe over-simplification as bacterial cells can be curved, bent, flattened, oblong spheroids and many more shapes. Due to the huge number of bacteria considered to be cocci (coccus if a single cell), it is unlikely that all of these show true spherical symmetry. It is important to distinguish between the generalized use of the word 'spherical' to describe organisms at ease, and the true meaning of spherical symmetry. The same situation is seen in the description of viruses – 'spherical' viruses do not necessarily show spherical symmetry, being usually icosahedral.
Bilateral symmetry
Organisms with bilateral symmetry contain a single plane of symmetry, the sagittal plane
The sagittal plane (; also known as the longitudinal plane) is an anatomical plane that divides the body into right and left sections. It is perpendicular to the transverse and coronal planes. The plane may be in the center of the body and divid ...
, which divides the organism into two roughly mirror image left and right halves – approximate reflectional symmetry.
Animals with bilateral symmetry are classified into a large group called the bilateria which contains 99% of all animals (comprising over 32 phyla and 1 million described species). All bilaterians have some asymmetrical features; for example, the human heart and liver are positioned asymmetrically despite the body having external bilateral symmetry.
The bilateral symmetry of bilaterians is a complex trait which develops due to the expression
Expression may refer to:
Linguistics
* Expression (linguistics), a word, phrase, or sentence
* Fixed expression, a form of words with a specific meaning
* Idiom, a type of fixed expression
* Metaphorical expression, a particular word, phrase, o ...
of many genes
In biology, the word gene (from , ; "...Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredity..." meaning ''generation'' or ''birth'' or ''gender'') can have several different meanings. The Mendelian gene is a ba ...
. The bilateria have two axes of polarity
Polarity may refer to:
Science
*Electrical polarity, direction of electrical current
*Polarity (mutual inductance), the relationship between components such as transformer windings
* Polarity (projective geometry), in mathematics, a duality of ord ...
. The first is an anterior
Standard anatomical terms of location are used to unambiguously describe the anatomy of animals, including humans. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position prov ...
- posterior (AP) axis which can be visualised as an imaginary axis running from the head or mouth to the tail or other end of an organism. The second is the dorsal
Dorsal (from Latin ''dorsum'' ‘back’) may refer to:
* Dorsal (anatomy), an anatomical term of location referring to the back or upper side of an organism or parts of an organism
* Dorsal, positioned on top of an aircraft's fuselage
* Dorsal co ...
-ventral
Standard anatomical terms of location are used to unambiguously describe the anatomy of animals, including humans. The terms, typically derived from Latin or Greek language, Greek roots, describe something in its standard anatomical position. Th ...
(DV) axis which runs perpendicular
In elementary geometry, two geometric objects are perpendicular if they intersect at a right angle (90 degrees or π/2 radians). The condition of perpendicularity may be represented graphically using the ''perpendicular symbol'', ⟂. It can ...
to the AP axis. During development the AP axis is always specified before the DV axis
which is known as the ''second embryonic axis''.
The AP axis is essential in defining the polarity of bilateria and allowing the development of a front and back to give the organism direction. The front end encounters the environment before the rest of the body so sensory organs such as eyes tend to be clustered there. This is also the site where a mouth develops since it is the first part of the body to encounter food. Therefore, a distinct head, with sense organs connected to a central nervous system, tends to develop. This pattern of development (with a distinct head and tail) is called cephalization
Cephalization is an evolutionary trend in which, over many generations, the mouth, sense organs, and nerve ganglia become concentrated at the front end of an animal, producing a head region. This is associated with movement and bilateral symmetry, ...
. It is also argued that the development of an AP axis is important in locomotion – bilateral symmetry gives the body an intrinsic direction and allows streamlining to reduce drag.
In addition to animals, the flowers of some plants also show bilateral symmetry. Such plants are referred to as zygomorphic
Floral symmetry describes whether, and how, a flower, in particular its perianth, can be divided into two or more identical or mirror-image parts.
Uncommonly, flowers may have no axis of symmetry at all, typically because their parts are spirall ...
and include the orchid (''Orchidaceae
Orchids are plants that belong to the family Orchidaceae (), a diverse and widespread group of flowering plants with blooms that are often colourful and fragrant.
Along with the Asteraceae, they are one of the two largest families of flowering ...
'') and pea ('''') families, and most of the figwort family (''Scrophulariaceae
The Scrophulariaceae are a family of flowering plants, commonly known as the figwort family. The plants are annual and perennial herbs, as well as shrubs. Flowers have bilateral (zygomorphic) or rarely radial (actinomorphic) symmetry. The Scr ...
''). The leaves of plants also commonly show approximate bilateral symmetry.
Biradial symmetry
Biradial symmetry is found in organisms which show morphological features (internal or external) of both bilateral and radial symmetry. Unlike radially symmetrical organisms which can be divided equally along many planes, biradial organisms can only be cut equally along two planes. This could represent an intermediate stage in the evolution of bilateral symmetry from a radially symmetric ancestor.
The animal group with the most obvious biradial symmetry is the ctenophores
Ctenophora (; ctenophore ; ) comprise a phylum of marine invertebrates, commonly known as comb jellies, that inhabit sea waters worldwide. They are notable for the groups of cilia they use for swimming (commonly referred to as "combs"), a ...
. In ctenophores the two planes of symmetry are (1) the plane of the tentacles and (2) the plane of the pharynx. In addition to this group, evidence for biradial symmetry has even been found in the 'perfectly radial' freshwater polyp '' Hydra'' (a cnidarian). Biradial symmetry, especially when considering both internal and external features, is more common than originally accounted for.
Evolution of symmetry
Like all the traits of organisms, symmetry (or indeed asymmetry) evolves due to an advantage to the organism – a process of natural selection
Natural selection is the differential survival and reproduction of individuals due to differences in phenotype. It is a key mechanism of evolution, the change in the heritable traits characteristic of a population over generations. Charle ...
. This involves changes in the frequency
Frequency is the number of occurrences of a repeating event per unit of time. It is also occasionally referred to as ''temporal frequency'' for clarity, and is distinct from ''angular frequency''. Frequency is measured in hertz (Hz) which is eq ...
of symmetry-related genes throughout time.
Evolution of symmetry in plants
Early flowering plants had radially symmetric flowers but since then many plants have evolved bilaterally symmetrical flowers. The evolution of bilateral symmetry is due to the expression
Expression may refer to:
Linguistics
* Expression (linguistics), a word, phrase, or sentence
* Fixed expression, a form of words with a specific meaning
* Idiom, a type of fixed expression
* Metaphorical expression, a particular word, phrase, o ...
of ''CYCLOIDEA'' genes. Evidence for the role of the ''CYCLOIDEA'' gene family comes from mutations
In biology, a mutation is an alteration in the nucleic acid sequence of the genome of an organism, virus, or extrachromosomal DNA. Viral genomes contain either DNA or RNA. Mutations result from errors during DNA or viral replication, mi ...
in these genes which cause a reversion to radial symmetry. The ''CYCLOIDEA'' genes encode transcription factors
In molecular biology, a transcription factor (TF) (or sequence-specific DNA-binding factor) is a protein that controls the rate of transcription of genetic information from DNA to messenger RNA, by binding to a specific DNA sequence. The func ...
, proteins which control the expression of other genes. This allows their expression to influence developmental pathways relating to symmetry. For example, in ''Antirrhinum majus
''Antirrhinum majus'', the common snapdragon (often - especially in horticulture - simply "snapdragon"), is a species of flowering plant belonging to the genus ''Antirrhinum''. The plant was placed in the family Plantaginaceae following a revis ...
'', ''CYCLOIDEA'' is expressed during early development in the dorsal domain of the flower meristem
The meristem is a type of tissue found in plants. It consists of undifferentiated cells (meristematic cells) capable of cell division. Cells in the meristem can develop into all the other tissues and organs that occur in plants. These cells conti ...
and continues to be expressed later on in the dorsal petals to control their size and shape. It is believed that the evolution of specialized pollinators may play a part in the transition of radially symmetrical flowers to bilaterally symmetrical flowers.
Evolution of symmetry in animals
Symmetry is often selected for in the evolution of animals. This is unsurprising since asymmetry is often an indication of unfitness – either defects during development or injuries throughout a lifetime. This is most apparent during mating during which females of some species select males with highly symmetrical features. For example, facial symmetry
Facial symmetry is one specific measure of bodily symmetry. Along with traits such as averageness and youthfulness it influences judgments of aesthetic traits of physical attractiveness and beauty. For instance, in mate selection, people have b ...
influences human judgements of human attractiveness.[Grammer, K.; Thornhill, R. (1994). Human (Homo sapiens) facial attractiveness and sexual selection: the role of symmetry and averageness. Journal of comparative psychology (Washington, D.C. : 1983), 108(3), 233–42.] Additionally, female barn swallow
The barn swallow (''Hirundo rustica'') is the most widespread species of swallow in the world. In fact, it appears to have the largest natural distribution of any of the world's passerines, ranging over 251 million square kilometres globally. ...
s, a species where adults have long tail streamers, prefer to mate with males that have the most symmetrical tails.
While symmetry is known to be under selection, the evolutionary history of different types of symmetry in animals is an area of extensive debate. Traditionally it has been suggested that bilateral animals evolved from a radial ancestor
An ancestor, also known as a forefather, fore-elder or a forebear, is a parent or (recursively) the parent of an antecedent (i.e., a grandparent, great-grandparent, great-great-grandparent and so forth). ''Ancestor'' is "any person from whom ...
. Cnidarians
Cnidaria () is a phylum under kingdom Animalia containing over 11,000 species of aquatic animals found both in freshwater and marine environments, predominantly the latter.
Their distinguishing feature is cnidocytes, specialized cells that th ...
, a phylum containing animals with radial symmetry, are the most closely related group to the bilaterians. Cnidarians are one of two groups of early animals considered to have defined structure, the second being the ctenophores
Ctenophora (; ctenophore ; ) comprise a phylum of marine invertebrates, commonly known as comb jellies, that inhabit sea waters worldwide. They are notable for the groups of cilia they use for swimming (commonly referred to as "combs"), a ...
. Ctenophores show biradial symmetry leading to the suggestion that they represent an intermediate step in the evolution of bilateral symmetry from radial symmetry.
Interpretations based only on morphology are not sufficient to explain the evolution of symmetry. Two different explanations are proposed for the different symmetries in cnidarians and bilateria. The first suggestion is that an ancestral animal had no symmetry (was asymmetric) before cnidarians and bilaterians separated into different evolutionary lineages. Radial symmetry could have then evolved in cnidarians and bilateral symmetry in bilaterians. Alternatively, the second suggestion is that an ancestor of cnidarians and bilaterians had bilateral symmetry before the cnidarians evolved and became different by having radial symmetry. Both potential explanations are being explored and evidence continues to fuel the debate.
Asymmetry
Although asymmetry is typically associated with being unfit, some species have evolved to be asymmetrical as an important adaptation
In biology, adaptation has three related meanings. Firstly, it is the dynamic evolutionary process of natural selection that fits organisms to their environment, enhancing their evolutionary fitness. Secondly, it is a state reached by the po ...
. Many members of the phylum Porifera (sponges) have no symmetry, though some are radially symmetric.
File:Fi Kreuzschnabel m Kopf.jpg, Head of a male crossbill
The crossbill is a genus, ''Loxia'', of birds in the finch family (Fringillidae), with six species. These birds are characterised by the mandibles with crossed tips, which gives the group its English name. Adult males tend to be red or orange in ...
showing asymmetrical upper and lower beak
File:Pseudopleuronectes americanus.jpg, A winter flounder
The winter flounder (''Pseudopleuronectes americanus''), also known as the black back, is a right-eyed ("dextral") flatfish of the family Pleuronectidae. It is native to coastal waters of the western north Atlantic coast, from Labrador, Canada to ...
, a type of flatfish, with both eyes on the same side of its head
File:Diogenes pugilator.jpg, Hermit crabs have different sized claws
File:Grapevinesnail 01a.jpg, A Roman snail
''Helix pomatia'', common names the Roman snail, Burgundy snail, or escargot, is a species of large, edible, air-breathing land snail, a pulmonate gastropod terrestrial mollusc in the family Helicidae.MolluscaBase eds. (2021). MolluscaBase. ...
and its helical shell
File:Chicoreus palmarosae.jpg, ''Chicoreus palmarosae
''Chicoreus palmarosae'' (rose-branch murex) is a species of predatory sea snail, a marine (ocean), marine gastropod mollusk in the family Muricidae, the murex snails.
Description
The size of an adult shell varies between 65 mm and 130&nbs ...
'', a sea snail, illustrating asymmetry, which is seen in all gastropods
The gastropods (), commonly known as snails and slugs, belong to a large taxonomic class of invertebrates within the phylum Mollusca called Gastropoda ().
This class comprises snails and slugs from saltwater, from freshwater, and from land. Ther ...
in the form of a helical shell
File:Orange slug.jpg, A red slug, clearly showing the pneumostome
File:Caribou (PSF).jpg, alt=Illustration of adult caribou in profile., Male Reindeer, caribou usually possess one brow tine flattened into a shovel shape
Symmetry breaking
The presence of these asymmetrical features requires a process of symmetry breaking during development, both in plants and animals. Symmetry breaking occurs at several different levels in order to generate the anatomical asymmetry which we observe. These levels include asymmetric gene expression, protein expression, and activity of cells.
For example, left-right asymmetry in mammals has been investigated extensively in the embryo
An embryo is an initial stage of development of a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male spe ...
s of mice. Such studies have led to support for the nodal flow hypothesis. In a region of the embryo referred to as the node there are small hair-like structures (Cilium, monocilia) which all rotate together in a particular direction. This creates a unidirectional flow of signalling molecules causing these signals to accumulate on one side of the embryo and not the other. This results in the activation of different developmental pathways on each side, and subsequent asymmetry.
Much of the investigation of the genetic basis of symmetry breaking has been done on chick embryos. In chick embryos the left side expresses genes called ''NODAL'' and ''LEFTY2'' which activate ''PITX2'' to signal the development of left side structures. Whereas, the right side does not express ''PITX2'' and consequently develops right side structures. A more complete pathway is shown in the image at the side of the page.
For more information about symmetry breaking in animals please refer to the left-right asymmetry (biology), left-right asymmetry page.
Plants also show asymmetry. For example the direction of helical growth in ''Arabidopsis'', the most commonly studied model plant, shows left-handedness. Interestingly, the genes involved in this asymmetry are similar (closely related) to those in animal asymmetry – both ''LEFTY1'' and ''LEFTY2'' play a role. In the same way as animals, symmetry breaking in plants can occur at a molecular (genes/proteins), subcellular, cellular, tissue and organ level.
See also
Biological structures
* Standard anatomical position
* Anatomical terms of motion
* Anatomical terms of muscle
* Anatomical terms of bone
* Anatomical terms of neuroanatomy
* Glossary of botanical terms
* Glossary of plant morphology
* Glossary of leaf morphology
* Glossary of entomology terms
* Plant morphology
Terms of orientation
* Handedness
* Laterality
* Proper right and proper left
* Reflection symmetry
* Sinistral and dextral
* Direction (disambiguation)
* Symmetry (disambiguation)
References
Citations
Sources
* Philip Ball, Ball, Philip (2009). ''Shapes''. Oxford University Press.
* Ian Stewart (mathematician), Stewart, Ian (2007). ''What Shape is a Snowflake? Magical Numbers in Nature''. Weidenfeld and Nicolson.
* D'Arcy Wentworth Thompson, Thompson, D'Arcy (1942). ''On Growth and Form''. Cambridge University Press.
*Ernst Haeckel, Haeckel, Ernst, E. (1904). ''Kunstformen der Natur''. Available as Haeckel, E. (1998); ''Art forms in nature'', Prestel USA. .
{{DEFAULTSORT:Symmetry In Biology
Symmetry
Developmental biology
Animal anatomy
Evolutionary biology
pt:Simetria#Simetria na biologia