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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 am ...
. 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), o ...
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 infi ...
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, ...
whereby there is near-repetition of the pattern element, either by reflection 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 th ...
and
placozoans The Placozoa are a 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'', ''Hoilungia hongkongensis'', an ...
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 "bluepri ...
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 ...
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), a line * Radius, adjective form of * Radial distance, a directional coordinate in a polar coordinate system * Radial set * A bearing from ...
(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 l ...
, 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 ...
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 ...
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 groupings. The Radiata, 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 naturalist and zoologist, sometimes referred to as the "founding father of paleontology". Cuvier was a major figure in na ...
's classification of the
animal Animals are multicellular, eukaryotic organisms in the biological kingdom Animalia. With few exceptions, animals consume organic material, breathe oxygen, are able to move, can reproduce sexually, and go through an ontogenetic stage ...
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 sperm ...
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 conver ...
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 ...
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 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 ...
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 ...
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 umbrel ...
, 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 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 leaves that surround the reproductive parts of flowers. They are often brightly colored or unusually shaped to attract pollinators. All of the petals of a flower are collectively known as the ''corolla''. Petals are usually ...
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 ...
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 filam ...
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 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 s ...
, 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 ...
body. This radial symmetry is
ecologically Ecology () is the study of the relationships between living organisms, including humans, and their physical environment. Ecology considers organisms at the individual, population, community, ecosystem, and biosphere level. Ecology overlaps w ...
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 (economi ...
(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 language, Greek words ('container, vessel') and ('seed'), and refers to ...
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 trees are cultivated worldwide and are the most widely grown species in the genus ''Malus''. The tree originated in Central Asia, where its wild ancestor ...
when cut transversely. Among animals, only the echinoderms such as
sea star 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 ...
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) ...
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 ar ...
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 ...
s and sea anemones (class
Anthozoa Anthozoa is a subphylum of marine invertebrates which includes the sea anemones, stony corals and soft corals. Adult anthozoans are almost all attached to the seabed, while their larvae can disperse as part of the plankton. The basic unit of ...
), which are divided into two groups based on their symmetry. The most common corals in the subclass
Hexacorallia Hexacorallia is a class of Anthozoa comprising approximately 4,300 species of aquatic organisms formed of polyps, generally with 6-fold symmetry. It includes all of the stony corals, most of which are colonial and reef-forming, as well as al ...
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 fa ...
. 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, cuttlef ...
, 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 t ...
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 oth ...
, 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 proteins (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 ...
), 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 ...
. 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 ...
'' 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 ...
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 into the inner and outer portions of endoplasm and ectoplasm. The ela ...
, 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 17 ...
''. 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 divi ...
, 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 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 ...
. The bilateria have two axes of polarity. 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 pr ...
- 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-
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 roots, describe something in its standard anatomical position. This position prov ...
(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. 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 (''
Fabaceae The Fabaceae or Leguminosae,International Code of Nomenc ...
'') 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 Sc ...
''). 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"), and ...
. 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. Char ...
. 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 e ...
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 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, mitos ...
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 fun ...
, 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 rev ...
'', ''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 who ...
.
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"), and ...
. 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 p ...
. 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 showing asymmetrical upper and lower beak File:Pseudopleuronectes americanus.jpg, A winter flounder, 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 and its helical shell File:Chicoreus palmarosae.jpg, ''
Chicoreus palmarosae ''Chicoreus palmarosae'' (rose-branch murex) is a species of predatory sea snail, a marine gastropod mollusk in the family Muricidae, the murex snails. Description The size of an adult shell varies between 65 mm and 130 mm. It is mo ...
'', 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. Th ...
in the form of a helical shell File:Orange slug.jpg, A red slug, clearly showing the
pneumostome The pneumostome or breathing pore is a respiratory opening of the external body anatomy of an air-breathing land slug or land snail. It is a part of the respiratory system of gastropods. It is an opening in the right side of the mantle of a ...
File:Caribou (PSF).jpg, alt=Illustration of adult caribou in profile., Male
caribou Reindeer (in North American English, known as caribou if wild and ''reindeer'' if domesticated) are deer in the genus ''Rangifer''. For the last few decades, reindeer were assigned to one species, ''Rangifer tarandus'', with about 10 subsp ...
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 sperm ...
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 ( 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 Left-right asymmetry, (LR asymmetry) is the process in early embryonic development that breaks the normal symmetry in the bilateral embryo. In vertebrates, left-right asymmetry is established early in development at a structure called the left-r ...
page. Plants also show asymmetry. For example the direction of helical growth in ''
Arabidopsis ''Arabidopsis'' (rockcress) is a genus in the family Brassicaceae. They are small flowering plants related to cabbage and mustard. This genus is of great interest since it contains thale cress (''Arabidopsis thaliana''), one of the model orga ...
'', 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 The standard anatomical position, or standard anatomical model, is the scientifically agreed upon reference position for anatomical location terms. Standard anatomical positions are used to standardise the position of appendages of animals with ...
*
Anatomical terms of motion Motion, the process of movement, is described using specific anatomical terms. Motion includes movement of organs, joints, limbs, and specific sections of the body. The terminology used describes this motion according to its direction relativ ...
*
Anatomical terms of muscle Anatomical terminology is used to uniquely describe aspects of skeletal muscle, cardiac muscle, and smooth muscle such as their actions, structure, size, and location. Types There are three types of muscle tissue in the body: skeletal, smooth, a ...
*
Anatomical terms of bone Many anatomical terms descriptive of bone are defined in anatomical terminology, and are often derived from Greek and Latin. Bone in the human body is categorized into long bone, short bone, flat bone, irregular bone and sesamoid bone. Types of ...
*
Anatomical terms of neuroanatomy This article describes anatomical terminology that is used to describe the central and peripheral nervous systems - including the brain, brainstem, spinal cord, and nerves. Anatomical terminology in neuroanatomy Neuroanatomy, like other aspe ...
*
Glossary of botanical terms This glossary of botanical terms is a list of definitions of terms and concepts relevant to botany and plants in general. Terms of plant morphology are included here as well as at the more specific Glossary of plant morphology and Glossary o ...
*
Glossary of plant morphology This page provides a glossary of plant morphology. Botanists and other biologists who study plant morphology use a number of different terms to classify and identify plant organs and parts that can be observed using no more than a handheld magnify ...
*
Glossary of leaf morphology The following is a list of terms which are used to describe leaf morphology in the description and taxonomy of plants. Leaves may be simple (a single leaf blade or lamina) or compound (with several leaflets). The edge of the leaf may be regular ...
*
Glossary of entomology terms This glossary of entomology describes terms used in the formal study of insect species by entomologists. A–C A synthetic chlorinated hydrocarbon insecticide, toxic to vertebrates. Though i ...
*
Plant morphology Phytomorphology is the study of the morphology (biology), physical form and external structure of plants.Raven, P. H., R. F. Evert, & S. E. Eichhorn. ''Biology of Plants'', 7th ed., page 9. (New York: W. H. Freeman, 2005). . This is usually cons ...


Terms of orientation

* Handedness *
Laterality The term laterality refers to the preference most humans show for one side of their body over the other. Examples include left-handedness/right-handedness and left/right- footedness; it may also refer to the primary use of the left or right he ...
*
Proper right and proper left Proper right and proper left are conceptual terms used to unambiguously convey relative direction when describing an image or other object. The "proper right" hand of a figure is the hand that would be regarded by that figure as its right hand. ...
* Reflection symmetry *
Sinistral and dextral Sinistral and dextral, in some scientific fields, are the two types of chirality ("handedness") or relative direction. The terms are derived from the Latin words for "left" (''sinister'') and "right" (''dexter''). Other disciplines use different ...
*
Direction (disambiguation) Direction may refer to: * Relative direction, for instance left, right, forward, backwards, up, and down ** Anatomical terms of location for those used in anatomy ** List of ship directions *Cardinal direction Mathematics and science * Direct ...
* Symmetry (disambiguation)


References


Citations


Sources

* Ball, Philip (2009). ''Shapes''. Oxford University Press. * Stewart, Ian (2007). ''What Shape is a
Snowflake A snowflake is a single ice crystal that has achieved a sufficient size, and may have amalgamated with others, which falls through the Earth's atmosphere as snow.Knight, C.; Knight, N. (1973). Snow crystals. Scientific American, vol. 228, n ...
? Magical Numbers in Nature''. Weidenfeld and Nicolson. * Thompson, D'Arcy (1942). ''On Growth and Form''. Cambridge University Press. * 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