Inbiology,homology is similarity inanatomical structures orgenes betweenorganisms of differenttaxa due to sharedancestry,regardless of current functional differences.Evolutionary biology explains homologous structures as retainedheredity from acommon ancestor after having been subjected toadaptive modifications for different purposes as the result ofnatural selection.
The term was first applied to biology in a non-evolutionary context by the anatomistRichard Owen in 1843. Homology was later explained byCharles Darwin's theory of evolution in 1859, but had been observed before this fromAristotle's biology onwards, and it was explicitly analysed byPierre Belon in 1555. A common example of homologous structures is theforelimbs ofvertebrates, where thewings of bats andbirds, thearms ofprimates, the frontflippers ofwhales, and theforelegs offour-legged vertebrates likehorses andcrocodilians are all derived from the same ancestraltetrapod structure.
Indevelopmental biology, organs that developed in theembryo in the same manner and from similar origins, such as from matchingprimordia in successive segments of the same animal, areserially homologous. Examples include the legs of acentipede, themaxillary and labial palps of aninsect, and thespinous processes of successivevertebrae in a vertebrate'sbackbone. Male and femalereproductive organs are homologous if they develop from the same embryonic tissue, as do theovaries andtesticles ofmammals, includinghumans.[citation needed]
Sequence homology betweenprotein orDNA sequences is similarly defined in terms of shared ancestry. Two segments ofDNA can have shared ancestry because of either aspeciation event (orthologs) or aduplication event (paralogs). Homology among proteins or DNA is inferred from their sequence similarity. Significant similarity is strong evidence that two sequences are related bydivergent evolution from a common ancestor.Alignments of multiple sequences are used to discover the homologous regions.
Homology remains controversial inanimal behaviour, but there is suggestive evidence that, for example,dominance hierarchies are homologous across theprimates.
Homology was noticed byAristotle (c. 350 BC),[1] and was explicitly analysed byPierre Belon in his 1555Book of Birds, where he systematically compared the skeletons of birds and humans. The pattern of similarity was interpreted as part of the staticgreat chain of being through themediaeval andearly modern periods: it was not then seen as implying evolutionary change. In the GermanNaturphilosophie tradition, homology was of special interest as demonstrating unity in nature.[2][3] In 1790,Goethe stated hisfoliar theory in his essay "Metamorphosis of Plants", showing that flower parts are derived from leaves.[4] Theserial homology of limbs was described late in the 18th century. The French zoologistEtienne Geoffroy Saint-Hilaire showed in 1818 in histheorie d'analogue ("theory of homologues") that structures were shared between fishes, reptiles, birds, and mammals.[5] When Geoffroy went further and sought homologies betweenGeorges Cuvier'sembranchements, such as vertebrates and molluscs, his claims triggered the 1830Cuvier-Geoffroy debate. Geoffroy stated the principle of connections, namely that what is important is the relative position of different structures and their connections to each other.[3]EmbryologistKarl Ernst von Baer stated what are now calledvon Baer's laws in 1828, noting that related animals begin their development as similar embryos and then diverge: thus, animals in the samefamily are more closely related and diverge later than animals which are only in the sameorder and have fewer homologies. Von Baer's theory recognises that eachtaxon (such as a family) has distinctive shared features, and that embryonic development parallels the taxonomic hierarchy: not the same asrecapitulation theory.[3] The term "homology" was first used in biology by the anatomistRichard Owen in 1843 when studying the similarities of vertebratefins and limbs, defining it as the "same organ in different animals under every variety of form and function",[6] and contrasting it with the matching term "analogy" which he used to describe different structures with the same function. Owen codified 3 main criteria for determining if features were homologous: position, development, and composition. In 1859,Charles Darwin explained homologous structures as meaning that the organisms concerned shared abody plan from a common ancestor, and that taxa were branches of a singletree of life.[2][7][3]
The word homology, coined in about 1656, is derived from theGreek ὁμόλογοςhomologos from ὁμόςhomos 'same' and λόγοςlogos 'relation'.[8][9][a]
Similar biological structures or sequences in differenttaxa are homologous if they are derived from acommon ancestor. Homology thus impliesdivergent evolution. For example, manyinsects (such asdragonflies) possess two pairs of flyingwings. Inbeetles, the first pair of wings has evolved into a pair ofhard wing covers,[12] while inDipteran flies the second pair of wings has evolved into smallhalteres used for balance.[b][13]
Similarly, the forelimbs of ancestralvertebrates have evolved into the front flippers ofwhales, the wings ofbirds, the running forelegs ofdogs,deer, andhorses, the short forelegs offrogs andlizards, and the graspinghands ofprimates including humans. The same major forearm bones (humerus,radius, andulna[c]) are found in fossils oflobe-finned fish such asEusthenopteron.[14]
The opposite of homologous organs are analogous organs which do similar jobs in two taxa that were notpresent in their most recent common ancestor but ratherevolved separately. For example, thewings of insects and birds evolved independently inwidely separated groups, and converged functionally to support poweredflight, so they are analogous. Similarly, the wings of asycamore maple seed and the wings of a bird are analogous but not homologous, as they develop from quite different structures.[15][16] A structure can be homologous at one level, but only analogous at another.Pterosaur,bird andbat wings are analogous as wings, but homologous as forelimbs because the organ served as a forearm (not a wing) in the last common ancestor oftetrapods, and evolved in different ways in the three groups. Thus, in the pterosaurs, the "wing" involves both the forelimb and the hindlimb.[17] Analogy is calledhomoplasy incladistics, andconvergent or parallel evolution in evolutionary biology.[18][19]
Specialised terms are used in taxonomic research. Primary homology is a researcher's initial hypothesis based on similar structure or anatomical connections, suggesting that a character state in two or more taxa share is shared due to common ancestry. Primary homology may be conceptually broken down further: we may consider all of the states of the same character as "homologous" parts of a single, unspecified, transformation series. This has been referred to as topographical correspondence. For example, in an aligned DNA sequence matrix, all of the A, G, C, T or implied gaps at a given nucleotide site are homologous in this way. Character state identity is the hypothesis that the particular condition in two or more taxa is "the same" as far as our character coding scheme is concerned. Thus, two Adenines at the same aligned nucleotide site are hypothesized to be homologous unless that hypothesis is subsequently contradicted by other evidence. Secondary homology is implied byparsimony analysis, where a character state that arises only once on a tree is taken to be homologous.[20][21] As implied in this definition, manycladists consider secondary homology to be synonymous withsynapomorphy, a shared derived character ortrait state that distinguishes aclade from other organisms.[22][23][24]
Shared ancestral character states, symplesiomorphies, represent either synapomorphies of a more inclusive group, or complementary states (often absences) that unite no natural group of organisms. For example, the presence of wings is a synapomorphy for pterygote insects, but a symplesiomorphy for holometabolous insects. Absence of wings in non-pterygote insects and other organisms is a complementary symplesiomorphy that unites no group (for example, absence of wings provides no evidence of common ancestry of silverfish, spiders and annelid worms). On the other hand, absence (or secondary loss) of wings is a synapomorphy for fleas. Patterns such as these lead many cladists to consider the concept of homology and the concept of synapomorphy to be equivalent.[25][24] Some cladists follow the pre-cladistic definition of homology of Haas and Simpson,[26] and view both synapomorphies and symplesiomorphies as homologous character states.[27]
Homologies provide the fundamental basis for all biological classification, although some may be highly counter-intuitive. For example,deep homologies like thepax6 genes that control the development of the eyes of vertebrates and arthropods were unexpected, as the organs are anatomically dissimilar and appeared to have evolved entirely independently.[28][29]
The embryonic body segments (somites) of differentarthropod taxa have diverged from a simple body plan with many similar appendages which are serially homologous, into a variety of body plans with fewer segments equipped with specialised appendages.[30] The homologies between these have been discovered by comparinggenes inevolutionary developmental biology.[28]
| Somite (body segment) | Trilobite (Trilobitomorpha)  | Spider (Chelicerata)  | Centipede (Myriapoda) .png) | Insect (Hexapoda)  | Shrimp (Crustacea)  |
|---|---|---|---|---|---|
| 1 | antennae | chelicerae (jaws and fangs) | antennae | antennae | 1st antennae |
| 2 | 1st legs | pedipalps | - | - | 2nd antennae |
| 3 | 2nd legs | 1st legs | mandibles | mandibles | mandibles (jaws) |
| 4 | 3rd legs | 2nd legs | 1stmaxillae | 1st maxillae | 1st maxillae |
| 5 | 4th legs | 3rd legs | 2nd maxillae | 2nd maxillae | 2nd maxillae |
| 6 | 5th legs | 4th legs | collum (no legs) | 1st legs | 1st legs |
| 7 | 6th legs | - | 1st legs | 2nd legs | 2nd legs |
| 8 | 7th legs | - | 2nd legs | 3rd legs | 3rd legs |
| 9 | 8th legs | - | 3rd legs | - | 4th legs |
| 10 | 9th legs | - | 4th legs | - | 5th legs |
Among insects, thestinger of the femalehoney bee is a modifiedovipositor, homologous with ovipositors in other insects such as theOrthoptera,Hemiptera, and thoseHymenoptera without stingers.[31]
The three small bones in themiddle ear of mammals including humans, themalleus,incus, andstapes, are today used to transmit sound from theeardrum to theinner ear. The malleus and incus develop in the embryo from structures that form jaw bones (the quadrate and the articular) in lizards, and in fossils of lizard-like ancestors of mammals. Both lines of evidence show that these bones are homologous, sharing a common ancestor.[32]
Among the manyhomologies in mammal reproductive systems,ovaries andtesticles are homologous.[33]
Rudimentary organs such as the humantailbone, now much reduced from their functional state, are readily understood as signs ofevolution, the explanation being that they were cut down bynatural selection from functioning organs when their functions were no longer needed, but make no sense at all if species are considered to be fixed. The tailbone is homologous to the tails of other primates.[34]
In many plants, defensive or storage structures are made by modifications of the development of primaryleaves,stems, androots. Leaves are variously modified fromphotosynthetic structures to form the insect-trapping pitchers ofpitcher plants, the insect-trapping jaws of theVenus flytrap, and the spines ofcactuses, all homologous.[35]
| Primary organs | Defensive structures | Storage structures |
|---|---|---|
| Leaves | Spines | Swollen leaves (e.g.succulents) |
| Stems | Thorns | Tubers (e.g.potato), rhizomes (e.g.ginger), fleshy stems (e.g.cacti) |
| Roots | - | Root tubers (e.g.sweet potato), taproot (e.g.carrot) |
Certaincompound leaves of flowering plants are partially homologous both to leaves and shoots, because theirdevelopment has evolved from agenetic mosaic of leaf and shoot development.[36][37]
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The four types of flower parts, namelycarpels,stamens,petals, andsepals, are homologous with and derived from leaves, asGoethe correctly noted in 1790. The development of these parts through a pattern ofgene expression in the growing zones (meristems) is described by theABC model of flower development. Each of the four types of flower parts is serially repeated in concentric whorls, controlled by a small number of genes acting in various combinations. Thus, A genes working alone result in sepal formation; A and B together produce petals; B and C together create stamens; C alone produces carpels. When none of the genes are active, leaves are formed. Two more groups of genes, D to formovules and E for the floral whorls, complete the model. The genes are evidently ancient, as old as theflowering plants themselves.[4]
Developmental biology can identify homologous structures that arose from the same tissue inembryogenesis. For example, adultsnakes have no legs, but their early embryos have limb-buds for hind legs, which are soon lost as the embryos develop. The implication that the ancestors of snakes had hind legs is confirmed byfossil evidence: theCretaceous snakePachyrhachis problematicus had hind legs complete with hip bones (ilium,pubis,ischium), thigh bone (femur), leg bones (tibia,fibula) and foot bones (calcaneum,astragalus) as in tetrapods with legs today.[38]
As with anatomical structures,sequence homology betweenprotein orDNA sequences is defined in terms of shared ancestry. Two segments of DNA can have shared ancestry because of either aspeciation event (orthologs) or aduplication event (paralogs). Homology among proteins or DNA is typically inferred from their sequence similarity. Significant similarity is strong evidence that two sequences are related by divergent evolution of a common ancestor.Alignments of multiple sequences are used to indicate which regions of each sequence are homologous.[40]
Homologous sequences are orthologous if they are descended from the same ancestral sequence separated by aspeciation event: when a species diverges into two separate species, the copies of a single gene in the two resulting species are said to beorthologous. The term "ortholog" was coined in 1970 by themolecular evolutionistWalter Fitch.[41]
Homologous sequences are paralogous if they were created by a duplication event within the genome. Forgene duplication events, if a gene in an organism is duplicated, the two copies are paralogous. They can shape the structure of whole genomes and thus explain genome evolution to a large extent. Examples include theHomeobox (Hox) genes in animals. These genes not only underwent gene duplications withinchromosomes but alsowhole genome duplications. As a result, Hox genes in most vertebrates are spread across multiple chromosomes: the HoxA–D clusters are the best studied.[42]
Some sequences are homologous, but they have diverged so much that their sequence similarity is not sufficient to establish homology. However, many proteins have retained very similar structures, andstructural alignment can be used to demonstrate their homology.[43]
It has been suggested that somebehaviours might be homologous, based either on sharing across related taxa or on common origins of the behaviour in an individual's development; however, the notion of homologous behavior remains controversial,[44] largely because behavior is more prone tomultiple realizability than other biological traits. For example, D. W. Rajecki and Randall C. Flanery, using data on humans and on nonhumanprimates, argue that patterns of behaviour indominance hierarchies are homologous across the primates.[45]
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As with morphological features or DNA, shared similarity in behavior provides evidence for common ancestry.[46] The hypothesis that a behavioral character is not homologous should be based on an incongruent distribution of that character with respect to other features that are presumed to reflect the true pattern of relationships. This is an application of Willi Hennig's[47]auxiliary principle.
{{cite book}}:|journal= ignored (help)elytra have very little similarity with typical wings, but are clearly homologous to forewings. Hence butterflies, flies, and beetles all have two pairs of dorsal appendages that are homologous among species.
For example, wing and haltere are homologous, yet widely divergent, organs that normally arise as dorsal appendages of the second thoracic (T2) and third thoracic (T3) segments, respectively.
Finally, much recent information on children'sand nonhuman primates' behavior in groups, a conjunction of hard human data and hard nonhuman primate data, lends credence to our comparison. Our conclusion is that, based on their agreement in several unusual characteristics, dominance patterns are homologous in primates. This agreement of unusual characteristics is found at several levels, including fine motor movement, gross motor movement, and behavior at the group level.
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