Homology (biology)

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For other uses, see Homology (disambiguation). Homology (biology)_sentence_0

In biology, homology is similarity due to shared ancestry between a pair of structures or genes in different taxa. Homology (biology)_sentence_1

A common example of homologous structures is the forelimbs of vertebrates, where the wings of bats and birds, the arms of primates, the front flippers of whales and the forelegs of four-legged vertebrates like dogs and crocodiles are all derived from the same ancestral tetrapod structure. Homology (biology)_sentence_2

Evolutionary biology explains homologous structures adapted to different purposes as the result of descent with modification from a common ancestor. Homology (biology)_sentence_3

The term was first applied to biology in a non-evolutionary context by the anatomist Richard Owen in 1843. Homology (biology)_sentence_4

Homology was later explained by Charles Darwin's theory of evolution in 1859, but had been observed before this, from Aristotle onwards, and it was explicitly analysed by Pierre Belon in 1555. Homology (biology)_sentence_5

In developmental biology, organs that developed in the embryo in the same manner and from similar origins, such as from matching primordia in successive segments of the same animal, are serially homologous. Homology (biology)_sentence_6

Examples include the legs of a centipede, the maxillary palp and labial palp of an insect, and the spinous processes of successive vertebrae in a vertebral column. Homology (biology)_sentence_7

Male and female reproductive organs are homologous if they develop from the same embryonic tissue, as do the ovaries and testicles of mammals including humans. Homology (biology)_sentence_8

Sequence homology between protein or DNA sequences is similarly defined in terms of shared ancestry. Homology (biology)_sentence_9

Two segments of DNA can have shared ancestry because of either a speciation event (orthologs) or a duplication event (paralogs). Homology (biology)_sentence_10

Homology among proteins or DNA is inferred from their sequence similarity. Homology (biology)_sentence_11

Significant similarity is strong evidence that two sequences are related by divergent evolution from a common ancestor. Homology (biology)_sentence_12

Alignments of multiple sequences are used to discover the homologous regions. Homology (biology)_sentence_13

Homology remains controversial in animal behaviour, but there is suggestive evidence that, for example, dominance hierarchies are homologous across the primates. Homology (biology)_sentence_14

History Homology (biology)_section_0

Homology was noticed by Aristotle (c. 350 BC), and was explicitly analysed by Pierre Belon in his 1555 Book of Birds, where he systematically compared the skeletons of birds and humans. Homology (biology)_sentence_15

The pattern of similarity was interpreted as part of the static great chain of being through the mediaeval and early modern periods: it was not then seen as implying evolutionary change. Homology (biology)_sentence_16

In the German Naturphilosophie tradition, homology was of special interest as demonstrating unity in nature. Homology (biology)_sentence_17

In 1790, Goethe stated his foliar theory in his essay "Metamorphosis of Plants", showing that flower part are derived from leaves. Homology (biology)_sentence_18

The serial homology of limbs was described late in the 18th century. Homology (biology)_sentence_19

The French zoologist Etienne Geoffroy Saint-Hilaire showed in 1818 in his theorie d'analogue ("theory of homologues") that structures were shared between fishes, reptiles, birds, and mammals. Homology (biology)_sentence_20

When Geoffroy went further and sought homologies between Georges Cuvier's embranchements, such as vertebrates and molluscs, his claims triggered the 1830 Cuvier-Geoffroy debate. Homology (biology)_sentence_21

Geoffroy stated the principle of connections, namely that what is important is the relative position of different structures and their connections to each other. Homology (biology)_sentence_22

The Estonian embryologist Karl Ernst von Baer stated what are now called von Baer's laws in 1828, noting that related animals begin their development as similar embryos and then diverge: thus, animals in the same family are more closely related and diverge later than animals which are only in the same order and have fewer homologies. Homology (biology)_sentence_23

von Baer's theory recognises that each taxon (such as a family) has distinctive shared features, and that embryonic development parallels the taxonomic hierarchy: not the same as recapitulation theory. Homology (biology)_sentence_24

The term "homology" was first used in biology by the anatomist Richard Owen in 1843 when studying the similarities of vertebrate fins and limbs, defining it as the "same organ in different animals under every variety of form and function", and contrasting it with the matching term "analogy" which he used to describe different structures with the same function. Homology (biology)_sentence_25

Owen codified 3 main criteria for determining if features were homologous: position, development, and composition. Homology (biology)_sentence_26

In 1859, Charles Darwin explained homologous structures as meaning that the organisms concerned shared a body plan from a common ancestor, and that taxa were branches of a single tree of life. Homology (biology)_sentence_27

Definition Homology (biology)_section_1

The word homology, coined in about 1656, is derived from the Greek ὁμόλογος homologos from ὁμός homos "same" and λόγος logos "relation". Homology (biology)_sentence_28

Similar biological structures or sequences in different taxa are homologous if they are derived from a common ancestor. Homology (biology)_sentence_29

Homology thus implies divergent evolution. Homology (biology)_sentence_30

For example, many insects (such as dragonflies) possess two pairs of flying wings. Homology (biology)_sentence_31

In beetles, the first pair of wings has evolved into a pair of hard wing covers, while in Dipteran flies the second pair of wings has evolved into small halteres used for balance. Homology (biology)_sentence_32

Similarly, the forelimbs of ancestral vertebrates have evolved into the front flippers of whales, the wings of birds, the running forelegs of dogs, deer, and horses, the short forelegs of frogs and lizards, and the grasping hands of primates including humans. Homology (biology)_sentence_33

The same major forearm bones (humerus, radius, and ulna) are found in fossils of lobe-finned fish such as Eusthenopteron. Homology (biology)_sentence_34

Homology vs analogy Homology (biology)_section_2

Further information: Convergent evolution Homology (biology)_sentence_35

The opposite of homologous organs are analogous organs which do similar jobs in two taxa that were not present in their most recent common ancestor but rather evolved separately. Homology (biology)_sentence_36

For example, the wings of insects and birds evolved independently in widely separated groups, and converged functionally to support powered flight, so they are analogous. Homology (biology)_sentence_37

Similarly, the wings of a sycamore maple seed and the wings of a bird are analogous but not homologous, as they develop from quite different structures. Homology (biology)_sentence_38

A structure can be homologous at one level, but only analogous at another. Homology (biology)_sentence_39

Pterosaur, bird and bat wings are analogous as wings, but homologous as forelimbs because the organ served as a forearm (not a wing) in the last common ancestor of tetrapods, and evolved in different ways in the three groups. Homology (biology)_sentence_40

Thus, in the pterosaurs, the "wing" involves both the forelimb and the hindlimb. Homology (biology)_sentence_41

Analogy is called homoplasy in cladistics, and convergent or parallel evolution in evolutionary biology. Homology (biology)_sentence_42

In cladistics Homology (biology)_section_3

Further information: Cladistics Homology (biology)_sentence_43

Specialised terms are used in taxonomic research. Homology (biology)_sentence_44

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. Homology (biology)_sentence_45

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. Homology (biology)_sentence_46

This has been referred to as topographical correspondence. Homology (biology)_sentence_47

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. Homology (biology)_sentence_48

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. Homology (biology)_sentence_49

Thus, two Adenines at the same aligned nucleotide site are hypothesized to be homologous unless that hypothesis is subsequently contradicted by other evidence. Homology (biology)_sentence_50

Secondary homology is implied by parsimony analysis, where a character state that arises only once on a tree is taken to be homologous. Homology (biology)_sentence_51

As implied in this definition, many cladists consider secondary homology to be synonymous with synapomorphy, a shared derived character or trait state that distinguishes a clade from other organisms. Homology (biology)_sentence_52

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. Homology (biology)_sentence_53

For example, the presence of wings is a synapomorphy for pterygote insects, but a symplesiomorphy for holometabolous insects. Homology (biology)_sentence_54

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). Homology (biology)_sentence_55

On the other hand, absence (or secondary loss) of wings is a synapomorphy for fleas. Homology (biology)_sentence_56

Patterns such as these lead many cladists to consider the concept of homology and the concept of synapomorphy to be equivalent. Homology (biology)_sentence_57

Some cladists follow the pre-cladistic definition of homology of Haas and Simpson, and view both synapomorphies and symplesiomorphies as homologous character states Homology (biology)_sentence_58

In different taxa Homology (biology)_section_4

Homologies provide the fundamental basis for all biological classification, although some may be highly counter-intuitive. Homology (biology)_sentence_59

For example, deep homologies like the pax6 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. Homology (biology)_sentence_60

In arthropods Homology (biology)_section_5

Further information: Arthropod leg Homology (biology)_sentence_61

The embryonic body segments (somites) of different arthropod 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. Homology (biology)_sentence_62

The homologies between these have been discovered by comparing genes in evolutionary developmental biology. Homology (biology)_sentence_63

Homology (biology)_table_general_0

Somite

(body segment)Homology (biology)_header_cell_0_0_0

Trilobite

(Trilobitomorpha)Homology (biology)_header_cell_0_0_1

Spider

(Chelicerata)Homology (biology)_header_cell_0_0_2

Centipede

(Myriapoda)Homology (biology)_header_cell_0_0_3

Insect

(Hexapoda)Homology (biology)_header_cell_0_0_4

Shrimp

(Crustacea)Homology (biology)_header_cell_0_0_5

1Homology (biology)_cell_0_1_0 antennaeHomology (biology)_cell_0_1_1 chelicerae (jaws and fangs)Homology (biology)_cell_0_1_2 antennaeHomology (biology)_cell_0_1_3 antennaeHomology (biology)_cell_0_1_4 1st antennaeHomology (biology)_cell_0_1_5
2Homology (biology)_cell_0_2_0 1st legsHomology (biology)_cell_0_2_1 pedipalpsHomology (biology)_cell_0_2_2 -Homology (biology)_cell_0_2_3 -Homology (biology)_cell_0_2_4 2nd antennaeHomology (biology)_cell_0_2_5
3Homology (biology)_cell_0_3_0 2nd legsHomology (biology)_cell_0_3_1 1st legsHomology (biology)_cell_0_3_2 mandiblesHomology (biology)_cell_0_3_3 mandiblesHomology (biology)_cell_0_3_4 mandibles (jaws)Homology (biology)_cell_0_3_5
4Homology (biology)_cell_0_4_0 3rd legsHomology (biology)_cell_0_4_1 2nd legsHomology (biology)_cell_0_4_2 1st maxillaeHomology (biology)_cell_0_4_3 1st maxillaeHomology (biology)_cell_0_4_4 1st maxillaeHomology (biology)_cell_0_4_5
5Homology (biology)_cell_0_5_0 4th legsHomology (biology)_cell_0_5_1 3rd legsHomology (biology)_cell_0_5_2 2nd maxillaeHomology (biology)_cell_0_5_3 2nd maxillaeHomology (biology)_cell_0_5_4 2nd maxillaeHomology (biology)_cell_0_5_5
6Homology (biology)_cell_0_6_0 5th legsHomology (biology)_cell_0_6_1 4th legsHomology (biology)_cell_0_6_2 collum (no legs)Homology (biology)_cell_0_6_3 1st legsHomology (biology)_cell_0_6_4 1st legsHomology (biology)_cell_0_6_5
7Homology (biology)_cell_0_7_0 6th legsHomology (biology)_cell_0_7_1 -Homology (biology)_cell_0_7_2 1st legsHomology (biology)_cell_0_7_3 2nd legsHomology (biology)_cell_0_7_4 2nd legsHomology (biology)_cell_0_7_5
8Homology (biology)_cell_0_8_0 7th legsHomology (biology)_cell_0_8_1 -Homology (biology)_cell_0_8_2 2nd legsHomology (biology)_cell_0_8_3 3rd legsHomology (biology)_cell_0_8_4 3rd legsHomology (biology)_cell_0_8_5
9Homology (biology)_cell_0_9_0 8th legsHomology (biology)_cell_0_9_1 -Homology (biology)_cell_0_9_2 3rd legsHomology (biology)_cell_0_9_3 -Homology (biology)_cell_0_9_4 4th legsHomology (biology)_cell_0_9_5
10Homology (biology)_cell_0_10_0 9th legsHomology (biology)_cell_0_10_1 -Homology (biology)_cell_0_10_2 4th legsHomology (biology)_cell_0_10_3 -Homology (biology)_cell_0_10_4 5th legsHomology (biology)_cell_0_10_5

Among insects, the stinger of the female honey bee is a modified ovipositor, homologous with ovipositors in other insects such as the Orthoptera, Hemiptera, and those Hymenoptera without stingers. Homology (biology)_sentence_64

In mammals Homology (biology)_section_6

Further information: Comparative anatomy Homology (biology)_sentence_65

The three small bones in the middle ear of mammals including humans, the malleus, incus, and stapes, are today used to transmit sound from the eardrum to the inner ear. Homology (biology)_sentence_66

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. Homology (biology)_sentence_67

Both lines of evidence show that these bones are homologous, sharing a common ancestor. Homology (biology)_sentence_68

Among the many homologies in mammal reproductive systems, ovaries and testicles are homologous. Homology (biology)_sentence_69

Rudimentary organs such as the human tailbone, now much reduced from their functional state, are readily understood as signs of evolution, the explanation being that they were cut down by natural selection from functioning organs when their functions were no longer needed, but make no sense at all if species are considered to be fixed. Homology (biology)_sentence_70

The tailbone is homologous to the tails of other primates. Homology (biology)_sentence_71

In plants Homology (biology)_section_7

Leaves, stems, and roots Homology (biology)_section_8

In many plants, defensive or storage structures are made by modifications of the development of primary leaves, stems, and roots. Homology (biology)_sentence_72

Leaves are variously modified from photosynthetic structures to form the insect-trapping pitchers of pitcher plants, the insect-trapping jaws of Venus flytrap, and the spines of cactuses, all homologous. Homology (biology)_sentence_73

Homology (biology)_table_general_1

Primary organsHomology (biology)_header_cell_1_0_0 Defensive structuresHomology (biology)_header_cell_1_0_1 Storage structuresHomology (biology)_header_cell_1_0_2
LeavesHomology (biology)_cell_1_1_0 SpinesHomology (biology)_cell_1_1_1 Swollen leaves (e.g. succulents)Homology (biology)_cell_1_1_2
StemsHomology (biology)_cell_1_2_0 ThornsHomology (biology)_cell_1_2_1 Tubers (e.g. potato), rhizomes (e.g. ginger), fleshy stems (e.g. cacti)Homology (biology)_cell_1_2_2
RootsHomology (biology)_cell_1_3_0 -Homology (biology)_cell_1_3_1 Root tubers (e.g. sweet potato), taproot (e.g. carrot)Homology (biology)_cell_1_3_2

Certain compound leaves of flowering plants are partially homologous both to leaves and shoots, because their development has evolved from a genetic mosaic of leaf and shoot development. Homology (biology)_sentence_74

Homology (biology)_unordered_list_0

  • Homology (biology)_item_0_0
  • Homology (biology)_item_0_1
  • Homology (biology)_item_0_2
  • Homology (biology)_item_0_3
  • Homology (biology)_item_0_4
  • Homology (biology)_item_0_5
  • Homology (biology)_item_0_6
  • Homology (biology)_item_0_7

Flower parts Homology (biology)_section_9

Further information: ABC model of flower development Homology (biology)_sentence_75

The four types of flower parts, namely carpels, stamens, petals, and sepals, are homologous with and derived from leaves, as Goethe correctly noted in 1790. Homology (biology)_sentence_76

The development of these parts through a pattern of gene expression in the growing zones (meristems) is described by the ABC model of flower development. Homology (biology)_sentence_77

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. Homology (biology)_sentence_78

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. Homology (biology)_sentence_79

When none of the genes are active, leaves are formed. Homology (biology)_sentence_80

Two more groups of genes, D to form ovules and E for the floral whorls, complete the model. Homology (biology)_sentence_81

The genes are evidently ancient, as old as the flowering plants themselves. Homology (biology)_sentence_82

Developmental biology Homology (biology)_section_10

Developmental biology can identify homologous structures that arose from the same tissue in embryogenesis. Homology (biology)_sentence_83

For example, adult snakes have no legs, but their early embryos have limb-buds for hind legs, which are soon lost as the embryos develop. Homology (biology)_sentence_84

The implication that the ancestors of snakes had hind legs is confirmed by fossil evidence: the Cretaceous snake Pachyrhachis 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. Homology (biology)_sentence_85

Sequence homology Homology (biology)_section_11

Main article: Sequence homology Homology (biology)_sentence_86

Further information: Deep homology and Evolutionary developmental biology Homology (biology)_sentence_87

As with anatomical structures, sequence homology between protein or DNA sequences is defined in terms of shared ancestry. Homology (biology)_sentence_88

Two segments of DNA can have shared ancestry because of either a speciation event (orthologs) or a duplication event (paralogs). Homology (biology)_sentence_89

Homology among proteins or DNA is typically inferred from their sequence similarity. Homology (biology)_sentence_90

Significant similarity is strong evidence that two sequences are related by divergent evolution of a common ancestor. Homology (biology)_sentence_91

Alignments of multiple sequences are used to indicate which regions of each sequence are homologous. Homology (biology)_sentence_92

Homologous sequences are orthologous if they are descended from the same ancestral sequence separated by a speciation event: when a species diverges into two separate species, the copies of a single gene in the two resulting species are said to be orthologous. Homology (biology)_sentence_93

The term "ortholog" was coined in 1970 by the molecular evolutionist Walter Fitch. Homology (biology)_sentence_94

Homologous sequences are paralogous if they were created by a duplication event within the genome. Homology (biology)_sentence_95

For gene duplication events, if a gene in an organism is duplicated to occupy two different positions in the same genome, then the two copies are paralogous. Homology (biology)_sentence_96

Paralogous genes often belong to the same species. Homology (biology)_sentence_97

They can shape the structure of whole genomes and thus explain genome evolution to a large extent. Homology (biology)_sentence_98

Examples include the Homeobox (Hox) genes in animals. Homology (biology)_sentence_99

These genes not only underwent gene duplications within chromosomes but also whole genome duplications. Homology (biology)_sentence_100

As a result, Hox genes in most vertebrates are spread across multiple chromosomes: the HoxA–D clusters are the best studied. Homology (biology)_sentence_101

In behaviour Homology (biology)_section_12

Main article: Homology (psychology) Homology (biology)_sentence_102

It has been suggested that some behaviours 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, largely because behavior is more prone to multiple realizability than other biological traits. Homology (biology)_sentence_103

For example, D. W. Rajecki and Randall C. Flanery, using data on humans and on nonhuman primates, argue that patterns of behaviour in dominance hierarchies are homologous across the primates. Homology (biology)_sentence_104

As with morphological features or DNA, shared similarity in behavior provides evidence for common ancestry. Homology (biology)_sentence_105

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. Homology (biology)_sentence_106

This is an application of Willi Hennig's auxiliary principle. Homology (biology)_sentence_107


Credits to the contents of this page go to the authors of the corresponding Wikipedia page: en.wikipedia.org/wiki/Homology (biology).