Symmetry in biology

2025-10-31T04:05:24Z

150.203.2.220: /* Radial symmetry */ tiny edit - fixed name of George --> Georges Cuvier

{{Short description|Geometric symmetry in living beings}}
{{Redirect|Symmetry in nature|Symmetry in physics|Symmetry (physics)|symmetry in chemistry|Molecular symmetry|}}
{{Other uses|Symmetry (disambiguation)}}
{{Use dmy dates|date=April 2017}}
[[File:SymmetryOfLifeFormsOnEarth.jpg|thumb|250x250px|A selection of animals showing a range of possible body symmetries, including asymmetry, radial, and bilateral [[body plan]]s]]
[[File:Diagram_comparing_bilateral,_radial,_and_spherical_symmetry.jpg|thumb|right|250x250px|Illustration depicting the difference between bilateral (''[[Drosophila]]''), radial ([[actinomorphic]] flowers) and spherical ([[coccus]] bacteria) symmetry]]

'''Symmetry in biology''' refers to the symmetry observed in [[organism]]s, including plants, animals, [[fungi]], and [[bacteria]]. 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 [[gas]]es, [[nutrient]]s, and waste products) which are [[cylindrical]] 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]] whereby there is near-repetition of the pattern element, either by [[Reflection (geometry)|reflection]] or [[Rotation (geometry)|rotation]].

While [[sponge]]s and [[placozoa]]ns represent two groups of animals which do not show any symmetry (i.e. are asymmetrical), the [[body plan]]s of most [[multicellular organism]]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 [[rotational symmetry|radial]] (cylindrical) symmetry, [[reflection symmetry|bilateral]], biradial and [[circular symmetry|spherical]] symmetry.<ref name="A new paradigm for animal symmetry">{{cite journal |last1=Holló |first1=Gábor |title=A new paradigm for animal symmetry |journal=Interface Focus |date=2015 |volume=5 |issue=6 |article-number=20150032 |doi=10.1098/rsfs.2015.0032|pmid=26640644 |pmc=4633854 |doi-access=free }}</ref> While the classification of [[virus]]es as an "organism" remains controversial, viruses also contain [[icosahedral symmetry]].

The importance of symmetry is illustrated by the fact that groups of animals have traditionally been defined by this feature in [[Taxonomy (biology)|taxonomic]] groupings. The [[Radiata]], animals with radial symmetry, formed one of the four branches of [[Georges Cuvier]]'s classification of the [[animal]] [[kingdom (biology)|kingdom]].<ref>{{cite book |last1=McBirney |first1=Alexander |title=Georges Cuvier. In: The Philosophy of Zoology Before Darwin. |date=2009 |publisher=Springer, Dordrecht |pages=87–98}}</ref><ref>{{cite web |last1=Waggoner |first1=Ben M. |title=Georges Cuvier (1769–1832) |url=http://www.ucmp.berkeley.edu/history/cuvier.html |publisher=UCMP Berkeley |access-date=8 March 2018 |quote=Cuvier's insistence on the functional integration of organisms led him to classify animals into four "branches," or embranchements: Vertebrata, Articulata (arthropods and segmented worms), Mollusca (which at the time meant all other soft, bilaterally symmetrical invertebrates), and Radiata (cnidarians and echinoderms).}}</ref><ref name="Whittaker and Company">{{cite book |last1=Cuvier |first1=Georges |author1-link=Georges Cuvier |last2=Griffith |first2=Edward |last3=Pidgeon |first3=Edward |title=The Mollusca and Radiata: Arranged by the Baron Cuvier, with Supplementary Additions to Each Order |url=https://books.google.com/books?id=mskQAAAAIAAJ&pg=PA435 |year=1834 |publisher=Whittaker and Company |pages=435–}}</ref> Meanwhile, [[Bilateria]] is a taxonomic grouping still used today to represent organisms with [[embryo]]nic bilateral symmetry.

==Radial symmetry==
{{Redirect|Radial symmetry|radial symmetry in mathematics|rotational 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.

Georges Cuvier classified animals with radial symmetry in the taxon Radiata (''Zoophytes''),<ref>{{cite web |last1=Waggoner |first1=Ben M. |title=Georges Cuvier (1769–1832) |url=http://www.ucmp.berkeley.edu/history/cuvier.html |publisher=UCMP Berkeley |access-date=8 March 2018 |quote=Cuvier's insistence on the functional integration of organisms led him to classify animals into four "branches," or embranchements: Vertebrata, Articulata (arthropods and segmented worms), Mollusca (which at the time meant all other soft, bilaterally symmetrical invertebrates), and Radiata (cnidarians and echinoderms).}}</ref><ref name="Whittaker and Company"/> which is now generally accepted to be an assemblage of different animal phyla that do not share a single common ancestor (a [[polyphyly|polyphyletic]] group).<ref>{{cite book |last=Hadzi |first=J. |title=The Evolution of the Metazoa |url=https://archive.org/details/evolutionofmetaz00hadz |year=1963 |publisher=Macmillan |isbn=978-0-08-010079-1 |pages=[https://archive.org/details/evolutionofmetaz00hadz/page/56 56–57]}}</ref> 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]] and [[Echinoderm]]ata generally show radial symmetry,<ref name=IAS/> although many [[sea anemone]]s and some [[coral]]s within the Cnidaria have bilateral symmetry defined by a single structure, the [[siphonoglyph]].<ref>{{cite journal | author=Finnerty, J.R. | title=The origins of axial patterning in the metazoa: How old is bilateral symmetry? | journal=The International Journal of Developmental Biology | volume=47 | pages=523–9 | year=2003 | id=14756328 16341006 | pmid=14756328 | issue=7–8}}</ref> Radial symmetry is especially suitable for [[Sessility (zoology)|sessile]] animals such as the sea anemone, floating animals such as [[jellyfish]], and slow moving organisms such as [[starfish]]; whereas bilateral symmetry favours [[Aquatic locomotion|locomotion]] by generating a [[streamlined]] body.

Many flowers are also radially symmetric, or "[[Floral symmetry|actinomorphic]]". Roughly identical floral structures – [[petal]]s, [[sepal]]s, and [[stamen]]s – occur at regular intervals around the axis of the flower, which is often the female [[sex organ|reproductive organ]] containing the [[carpel]], [[style (botany)|style]] and [[stigma (botany)|stigma]].<ref>{{cite journal | author=Endress, P. K. | title=Evolution of Floral Symmetry | journal=Current Opinion in Plant Biology | volume=4 | issue=1 | pages=86–91 | date=February 2001 | doi=10.1016/S1369-5266(00)00140-0 | pmid=11163173| bibcode=2001COPB....4...86E }}</ref>

[[File:LiliumBulbiferumCroceumBologna.jpg|thumb|''[[Lilium bulbiferum]]'' displays hexamerism with repeated parts arranged around the axis of the flower.]]

===Subtypes of radial symmetry===
Three-fold triradial symmetry was present in [[Trilobozoa]] from the Late [[Ediacaran]] period.

Four-fold tetramerism appears in some jellyfish, such as ''[[Aurelia (cnidarian)|Aurelia]] marginalis''. This is immediately obvious when looking at the jellyfish due to the presence of four [[gonad]]s, visible through its [[transparency and translucency|translucent]] body. This radial symmetry is [[ecology|ecologically]] important in allowing the jellyfish to detect and respond to [[Stimulus (physiology)|stimuli]] (mainly food and danger) from all directions.

[[File:Sterappel dwarsdrsn.jpg|thumb|right|top|200x200px|alt=Alt text|Apple cut horizontally showing that pentamerism also occurs in fruit]]

[[Flowering plant]]s show five-fold pentamerism, in many of their flowers and fruits. This is easily seen through the arrangement of five [[carpel]]s (seed pockets) in an [[apple]] when cut [[transverse plane|transversely]]. Among animals, only the echinoderms such as [[sea star]]s, [[sea urchin]]s, and [[crinoid|sea lilies]] 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.<ref>Stewart, 2001. pp 64–65.</ref>

{{vanchor|Hexamerism}} is found in the [[coral]]s and [[sea anemone]]s (class [[Anthozoa]]), which are divided into two groups based on their symmetry. The most common corals in the subclass [[Hexacorallia]] have a hexameric body plan; their [[polyp (zoology)|polyp]]s have six-fold internal symmetry and a number of [[tentacle]]s that is a multiple of six.

{{vanchor|Octamerism}} is found in corals of the subclass [[Octocorallia]]. These have polyps with eight tentacles and octameric radial symmetry. The [[octopus]], however, has bilateral symmetry, despite its eight arms.

==Icosahedral symmetry==
[[File:Gastroenteritis viruses.jpg|thumb|left|top|200x200px|alt=Alt text|[[Gastroenteritis]] viruses have icosahedral symmetry.]]
Icosahedral symmetry occurs in an organism which contains 60 subunits generated by 20 faces, each an [[equilateral triangle]], and 12 corners. Within the [[icosahedron]] there is [[rotational symmetry|2-fold, 3-fold and 5-fold symmetry]]. Many viruses, including ''[[canine parvovirus]]'', show this form of symmetry due to the presence of an icosahedral [[capsid|viral shell]]. Such symmetry has [[evolution|evolved]] because it allows the viral particle to be built up of repetitive subunits consisting of a limited number of structural [[protein]]s (encoded by viral [[gene]]s), thereby saving space in the viral [[genome]]. The icosahedral symmetry can still be maintained with more than 60 subunits, but only in multiples of 60. For example, the T=3 ''[[Tomato bushy stunt virus]]'' has 60x3 protein subunits (180 copies of the same structural protein).<ref>{{cite book |author=Alan J Cann |title=eLS|chapter=Virus Structure|doi=10.1002/9780470015902.a0000439.pub2 |year=2015 |pages=1–9|publisher=American Cancer Society|isbn=978-0-470-01590-2|chapter-url=https://doi.org/10.1002/9780470015902.a0000439.pub2}}</ref><ref>{{cite journal | title=Symmetry in virus architecture |author1=Horne, R. W.|author2=Wildy, P. | journal=Virology | date=November 1961 | volume=15 | issue=3 | pages=348–373 | doi=10.1016/0042-6822(61)90366-X| pmid=14448959 }}</ref> Although these viruses are often referred to as 'spherical', they do not show true mathematical spherical symmetry.

In the early 20th century, [[Ernst Haeckel]] described (Haeckel, 1904) a number of species of [[Radiolaria]], 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==
[[File:Mikrofoto.de-volvox-4.jpg|thumb|right|top|200x200px|alt= Alt text|''[[Volvox]]'' is a microscopic green freshwater [[alga]] with spherical symmetry. Young colonies can be seen inside the larger ones.]]

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.<ref name="A new paradigm for animal symmetry"/> Organisms which show approximate spherical symmetry include the freshwater green alga ''[[Volvox]]''.<ref name=IAS>{{cite web | url=http://www.iaszoology.com/symmetry/ | title=Symmetry | publisher=IAS | access-date=14 June 2014 | author=Chandra, Girish| date=11 October 2008 }}</ref>

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.<ref>{{cite journal |last1=Young |first1=K. D. |title=The Selective Value of Bacterial Shape |journal=Microbiology and Molecular Biology Reviews |date=2006 |volume=70 |issue=3 |pages=660–703 |doi=10.1128/MMBR.00001-06|pmid=16959965 |pmc=1594593 |doi-access=free }}</ref> 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==
{{Redirect|Bilateral symmetry|bilateral symmetry in mathematics|reflection symmetry}}
{{Main|Bilateria}}
Organisms with bilateral symmetry contain a single plane of symmetry, the [[sagittal plane]], which divides the organism into two roughly mirror image left and right halves – approximate reflectional symmetry.
[[File:20 petit paon de nuit.jpg|thumb|left|top|200x200px|alt= Alt text|The small emperor moth, ''[[Saturnia pavonia]]'', displays a [[deimatic]] pattern with bilateral symmetry.]]
[[File:Ophrys apifera (flower).jpg|thumb|right|top|200x200px|alt= Alt text|Flower of [[Ophrys apifera|bee orchid]] (''Ophrys apifera'') is bilaterally symmetrical ([[zygomorphic]]). The lip of the flower resembles the (bilaterally symmetric) abdomen of a female bee; pollination occurs when a male bee attempts to mate with it.]]

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.<ref>{{cite web |last=Valentine |first=James W. |title=Bilateria |url=http://www.accessscience.com/abstract.aspx?id=802620&referURL=http%3a%2f%2fwww.accessscience.com%2fcontent.aspx%3fid%3d802620 |publisher=AccessScience |access-date=29 May 2013 |archive-url=https://web.archive.org/web/20080118213208/http://www.accessscience.com/abstract.aspx?id=802620&referURL=http%3A%2F%2Fwww.accessscience.com%2Fcontent.aspx%3Fid%3D802620 |archive-date=18 January 2008 }}</ref>

The bilateral symmetry of bilaterians is a complex trait which develops due to the [[gene expression|expression]] of many [[gene]]s. The bilateria have two axes of [[polarity in embryogenesis|polarity]]. The first is an [[anterior]]–[[posterior (anatomy)|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 [[Anatomical terms of location|dorsal]]–[[ventral]] (DV) axis which runs [[perpendicular]] to the AP axis.<ref>{{cite journal |last1=Finnerty |first1=John R |title=Evolution & Development |journal=IJDB |date=2003 |volume=47 |pages=465–705 |url=http://www.ijdb.ehu.es/web/paper/14756328}}</ref><ref name="A new paradigm for animal symmetry"/> During development the AP axis is always specified before the DV axis,<ref>{{cite journal |last1=Freeman |first1=Gary |title=The rise of bilaterians |journal=Historical Biology |date=2009 |volume=21 |issue=1–2 |pages=99–114 |doi=10.1080/08912960903295843|bibcode=2009HBio...21...99F |s2cid=83841216 }}</ref> 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.<ref>{{cite journal |last1=Finnerty |first1=John R. |title=Did internal transport, rather than directed locomotion, favor the evolution of bilateral symmetry in animals? |journal=BioEssays |date=2005 |volume=27 |issue=11 |pages=1174–1180 |doi=10.1002/bies.20299|pmid=16237677 }}</ref> 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 (physics)|drag]].

In addition to animals, the flowers of some plants also show bilateral symmetry. Such plants are referred to as [[zygomorphic]] and include the orchid (''[[Orchidaceae]]'') and pea (''[[Fabaceae]]'') families, and most of the figwort family (''[[Scrophulariaceae]]'').<ref>{{cite web | url=http://botany.csdl.tamu.edu/FLORA/301Manhart/Dicots/Asteridae/Scr/Scr.html | title=SCROPHULARIACEAE – Figwort or Snapdragon Family | publisher=Texas A&M University Bioinformatics Working Group | access-date=14 June 2014}}</ref><ref>[http://www.factmonster.com/ce6/sci/A0847482.html Symmetry, biological], from ''[[The Columbia Electronic Encyclopedia]]'' (2007).</ref> 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.<ref name=MartindaleHenry>{{cite journal | title=The Development of Radial and Biradial Symmetry: The Evolution of Bilaterality1 |author1=Martindale, Mark Q. |author2=Henry, Jonathan Q. | journal=American Zoology | year=1998 | volume=38 | issue=4 | pages=672–684 | doi=10.1093/icb/38.4.672| doi-access=free }}</ref>

The animal group with the most obvious biradial symmetry is the [[ctenophora|ctenophores]]. In ctenophores the two planes of symmetry are (1) the plane of the tentacles and (2) the plane of the pharynx.<ref name="A new paradigm for animal symmetry"/> In addition to this group, evidence for biradial symmetry has even been found in the 'perfectly radial' freshwater polyp '' [[Hydra (genus)|Hydra]]'' (a cnidarian). Biradial symmetry, especially when considering both internal and external features, is more common than originally accounted for.<ref>{{cite journal |last1=Watanabe |first1=Hiroshi |last2=Schmidt |first2=Heiko A. |last3=Kuhn |first3=Anne |last4=Höger |first4=Stefanie K. |last5=Kocagöz |first5=Yigit |last6=Laumann-Lipp |first6=Nico |last7=Özbek |first7=Suat |last8=Holstein |first8=Thomas W. |title=Nodal signalling determines biradial asymmetry in Hydra |journal=Nature |date=24 August 2014 |volume=515 |issue=7525 |pages=112–115 |doi=10.1038/nature13666|pmid=25156256 |bibcode=2014Natur.515..112W |s2cid=4467701 }}</ref>

==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]]. This involves changes in the [[frequency (statistics)|frequency]] 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 [[gene expression|expression]] of ''CYCLOIDEA'' genes. Evidence for the role of the ''CYCLOIDEA'' gene family comes from [[mutation]]s in these genes which cause a reversion to radial symmetry. The ''CYCLOIDEA'' genes encode [[transcription factor]]s, proteins which control the expression of other genes. This allows their expression to influence developmental pathways relating to symmetry.<ref>{{cite journal |last1=Cubas |first1=Pilar |last2=Vincent |first2=Coral |last3=Coen |first3=Enrico |title=An epigenetic mutation responsible for natural variation in floral symmetry |journal=Nature |date=1999 |volume=401 |issue=6749 |pages=157–161 |doi=10.1038/43657|pmid=10490023 |bibcode=1999Natur.401..157C |s2cid=205033495 }}</ref><ref>{{cite journal |last1=Citerne |first1=H |title=Diversity of cycloidea-like Genes in Gesneriaceae in Relation to Floral Symmetry |journal=Annals of Botany |date=2000 |volume=86 |issue=1 |pages=167–176 |doi=10.1006/anbo.2000.1178|doi-access=free |bibcode=2000AnBot..86..167C }}</ref> For example, in ''[[Antirrhinum majus]]'', ''CYCLOIDEA'' is expressed during early development in the dorsal domain of the flower [[meristem]] 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.<ref>{{cite journal |last1=Hileman |first1=Lena C |last2=Cubas |first2=Pilar |title=An expanded evolutionary role for flower symmetry genes |journal=Journal of Biology |date=2009 |volume=8 |issue=10 |page=90 |doi=10.1186/jbiol193|pmid=19895716 |pmc=2790833 |doi-access=free }}</ref>

===Evolution of symmetry in animals===
[[File:Tribrachiidae.JPG|thumb|200px|The [[Ediacaran]] phylum [[Trilobozoa]] possess a wide variety of body shapes, mostly tri-radial symmetry, although their most famous member, ''[[Tribrachidium]]'', possesses a [[triskelion]] body shape.<ref name="Ivantsov_2021">{{cite journal
| author = Ivantsov, A. Yu.
| author2 = Zakrevskaya, M. A.
| year = 2021
| title = Trilobozoa, Precambrian Tri-Radial Organisms.
| journal = Paleontological Journal
| volume = 55
| issue = 7
| pages = 727–741
| url = https://www.researchgate.net/publication/356726510
| doi = 10.1134/S0031030121070066
| bibcode = 2021PalJ...55..727I
| s2cid = 245330736
}}</ref>]]
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. Additionally, female [[barn swallow]]s, a species where adults have long tail streamers, prefer to mate with males that have the most symmetrical tails.<ref>[[John Maynard Smith|Maynard Smith, John]]; Harper, David (2003). ''Animal Signals''. Oxford University Press. pp. 63–65.</ref>

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]]. [[Cnidaria]]ns, 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 [[ctenophora|ctenophores]]. Ctenophores show biradial symmetry leading to the suggestion that they represent an intermediate step in the evolution of bilateral symmetry from radial symmetry.<ref>{{cite journal |last1=Martindale |first1=Mark Q. |last2=Henry |first2=Jonathon Q |title=The Development of Radial and Biradial Symmetry: The Evolution of Bilaterality |journal=American Zoologist |date=1998 |volume=38 |issue=4 |pages=672–684 |doi=10.1093/icb/38.4.672|doi-access=free }}</ref>

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 [[lineage (evolution)|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]]. Many members of the phylum Porifera (sponges) have no symmetry, though some are radially symmetric.<ref>{{cite web | author=Myers, Phil | title=Porifera Sponges | url=http://animaldiversity.ummz.umich.edu/accounts/Porifera/ | publisher=University of Michigan (Animal Diversity Web) | date=2001 | access-date=14 June 2014}}</ref>

{| class="wikitable"
|-
! Group/Species
! Asymmetrical Feature
! Adaptive Benefit
|-
| Some [[owl]]s<ref>{{cite journal |last1=Norberg |first1=R |title=Skull asymmetry, ear structure and function, and auditory localization in Tengmalm's owl, (Linné) |journal=Philosophical Transactions of the Royal Society of London. B, Biological Sciences |date=1997 |volume=282 |pages=325–410 |doi=10.1098/rstb.1978.0014}}</ref>
| Size and positioning of ears
| Allows the owl to more precisely determine the location of [[prey]]
|-
| [[Flatfish]]<ref>{{cite journal |last1=Friedman |first1=Matt |title=The evolutionary origin of flatfish asymmetry |journal=Nature |date=2008 |volume=454 |issue=7201 |pages=209–212 |doi=10.1038/nature07108|pmid=18615083 |bibcode=2008Natur.454..209F |s2cid=4311712 }}</ref>
| Both eyes on the same side of their head
| Rest and swim on one side (to blend in with sand floor of the ocean)
|-
| The scale-eating [[cichlid]] ''[[Perissodus microlepis]]''<ref name=Lee2012>{{cite journal | author=Lee, H. J.; Kusche, H.; Meyer, A. | year=2012 | title=Handed Foraging Behavior in Scale-Eating Cichlid Fish: Its Potential Role in Shaping Morphological Asymmetry | journal=PLOS ONE | volume=7 | issue=9 | article-number=e44670 | doi=10.1371/journal.pone.0044670 | pmid=22970282 | pmc=3435272| bibcode=2012PLoSO...744670L | doi-access=free }}</ref>
| Mouth and jaw asymmetry
| More effective at removing scales from their prey
|-
| Humans<ref name="Zaidel 2001 pp. 1321–1329">{{cite book | last=Zaidel | first=E. | title=International Encyclopedia of the Social & Behavioral Sciences | chapter=Brain Asymmetry | publisher=Elsevier | year=2001 | isbn=978-0-08-043076-8 | doi=10.1016/b0-08-043076-7/03548-8 | pages=1321–1329}}</ref><ref name="Betts2013">{{cite book|last1=Betts|first1=J. Gordon|title=Anatomy & physiology|date=2013|isbn=978-1-938168-13-0|url=http://cnx.org/content/m46676/latest/?collection=col11496/latest|access-date=11 August 2014|pages=787–846|publisher=OpenStax College, Rice University }}</ref><ref name="Holder1997">{{cite web |url=http://www.scientificamerican.com/article.cfm?id=why-are-more-people-right |title=Why are more people right-handed? |access-date=14 April 2008 |work=Scientific American |year=1997 |author=Holder, M. K.}}</ref>
| [[Handedness]] and internal asymmetry of organs e.g. left lung is smaller than the right
| Handedness is an adaptation reflecting the asymmetries of the human brain.
|-
| All [[vertebrate]]s
| Internal asymmetry of [[heart]] and [[gastrointestinal tract|bowels]]
| Internal asymmetry is thought to be caused by a developmental [[axial twist]].<ref name="Lussanet2012">{{cite journal | first1=M. H. E. | last1=de Lussanet | first2=J. W. M. | last2=Osse | year=2012 | title=An ancestral axial twist explains the contralateral forebrain and the optic chiasm in vertebrates | journal=Animal Biology | volume=62 | issue=2 | pages=193–216 | doi=10.1163/157075611X617102 | arxiv=1003.1872 | s2cid=7399128}}</ref>
|}

{{Further|List of animals featuring external asymmetry}}

<gallery>
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]]'', a sea snail, illustrating asymmetry, which is seen in all [[gastropod]]s 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<ref>{{Cite journal|last=Goss|first=Richard J.|date=1990-06-01|title=Interactions between asymmetric brow tines in caribou and reindeer antlers|url=https://www.nrcresearchpress.com/doi/abs/10.1139/z90-165|journal=Canadian Journal of Zoology|volume=68|issue=6|pages=1115–1119|doi=10.1139/z90-165|bibcode=1990CaJZ...68.1115G |issn=0008-4301}}</ref>
File:Stegosaurus stenops Life Reconstruction.png|A [[life restoration]] of [[Stegosaurus|''Stegosaurus stenops'']] with its asymmetrical plates.
</gallery>

===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]]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]]) that 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.<ref>{{cite journal |last1=Hirokawa |first1=Nobutaka |last2=Tanaka |first2=Yosuke |last3=Okada |first3=Yasushi |last4=Takeda |first4=Sen |title=Nodal Flow and the Generation of Left-Right Asymmetry |journal=Cell |date=2006 |volume=125 |issue=1 |pages=33–45 |doi=10.1016/j.cell.2006.03.002|pmid=16615888 |s2cid=18007532 |doi-access=free }}</ref><ref>{{cite journal |last1=Nonaka |first1=Shigenori |last2=Shiratori |first2=Hidetaka |last3=Saijoh |first3=Yukio |last4=Hamada |first4=Hiroshi |title=Determination of left–right patterning of the mouse embryo by artificial nodal flow |journal=Nature |date=2002 |volume=418 |issue=6893 |pages=96–99 |doi=10.1038/nature00849|pmid=12097914 |bibcode=2002Natur.418...96N |s2cid=4373455 }}</ref>

[[File:Asymmetrical_signalling_pathways_in_a_chick_embryo.png|thumb|right|200x200px|Schematic diagram of signalling pathways on the left and right side of a chick embryo, ultimately leading to the development of 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]]'' that activate ''[[PITX2]]'' to signal the development of left side structures. Whereas, the right side does not express ''PITX2'' and consequently develops right side structures.<ref>{{cite journal |last1=Raya |first1=Angel |last2=Izpisua Belmonte |first2=Juan Carlos |title=Unveiling the establishment of left–right asymmetry in the chick embryo |journal=Mechanisms of Development |date=2004 |volume=121 |issue=9 |pages=1043–1054 |doi=10.1016/j.mod.2004.05.005|pmid=15296970 |s2cid=15417027 }}</ref><ref>{{cite journal |last1=Gros |first1=J. |last2=Feistel |first2=K. |last3=Viebahn |first3=C. |last4=Blum |first4=M. |last5=Tabin |first5=C. J. |title=Cell Movements at Hensen's Node Establish Left/Right Asymmetric Gene Expression in the Chick |journal=Science |date=2009 |volume=324 |issue=5929 |pages=941–944 |doi=10.1126/science.1172478|pmid=19359542 |pmc=2993078 |bibcode=2009Sci...324..941G }}</ref> 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]] 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.<ref>{{cite journal |last1=Muñoz-Nortes |first1=Tamara |last2=Wilson-Sánchez |first2=David |last3=Candela |first3=Héctor |last4=Micol |first4=José Luis |title=Symmetry, asymmetry, and the cell cycle in plants: known knowns and some known unknowns |journal=Journal of Experimental Botany |date=2014 |volume=65 |issue=10 |pages=2645–2655 |doi=10.1093/jxb/ert476|pmid=24474806 |doi-access=free }}</ref>

===Fluctuating asymmetry===
{{excerpt|Fluctuating asymmetry}}

==See also==
===Biological structures===
* [[Standard anatomical position]]
* [[Anatomical terms of motion]]
* [[Anatomical terms of muscle]]
* [[Anatomical terms of bone]]
* [[Anatomical terms of neuroanatomy]]
* [[Floral symmetry]]
* [[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===
{{Reflist|28em}}

===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 US. {{ISBN|3-7913-1990-6}}.

{{Patterns in nature}}
{{laterality}}
{{Authority control}}

{{DEFAULTSORT:Symmetry In Biology}}
[[Category:Symmetry]]
[[Category:Developmental biology]]
[[Category:Animal anatomy]]
[[Category:Evolutionary biology]]

[[pt:Simetria#Simetria na biologia]]

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Symmetry in biology