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In
biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechanisms, Development ...

biology
, homology is similarity due to shared ancestry between a pair of structures or
genes In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechani ...
in different
taxa In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechani ...
. A common example of homologous structures is the forelimbs of
vertebrates Vertebrates () comprise all species of animal Animals (also called Metazoa) are multicellular A multicellular organism is an organism In biology, an organism () is any organic, life, living system that functions as an indiv ...

vertebrates
, where the wings of bats and
birds Birds are a group of warm-blooded vertebrate Vertebrates () comprise all species of animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biology), biological kingdom Animalia. With ...
, the arms of
primate A primate ( ) (from Latin , from 'prime, first rank') is a eutherian mammal constituting the Taxonomy (biology), taxonomic order (biology), order Primates (). Primates arose 85–55 million years ago first from small Terrestrial animal, ...

primate
s, the front flippers of
whale Whales are a widely distributed and diverse group of fully aquatic Aquatic means relating to water Water (chemical formula H2O) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is ...

whale
s and the forelegs of four-legged vertebrates like
dogs The dog or domestic dog (''Canis familiaris'' or ''Canis lupus familiaris'') is a domesticated Domestication is a sustained multi-generational relationship in which one group of organisms assumes a significant degree of influence over the ...
and
crocodile Crocodiles (family In , family (from la, familia) is a of people related either by (by recognized birth) or (by marriage or other relationship). The purpose of families is to maintain the well-being of its members and of society. I ...

crocodile
s are all derived from the same ancestral
tetrapod Tetrapods (; ) are four-limbed animals constituting the superclass Tetrapoda (). It includes extant Extant is the opposite of the word extinct Extinction is the termination of a kind of organism or of a group of kinds (taxon), usually a sp ...
structure.
Evolutionary biology Evolutionary biology is the subfield of biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interacti ...
explains homologous structures
adapted In biology, adaptation has three related meanings. Firstly, it is the dynamic evolutionary process that fits organisms to their environment, enhancing their Fitness (biology), evolutionary fitness. Secondly, it is a state reached by the popula ...
to different purposes as the result of descent with modification from a
common ancestor Common descent is a concept in evolutionary biology Evolutionary biology is the subfield of biology that studies the evolution, evolutionary processes (natural selection, common descent, speciation) that produced the Biodiversity, diversity ...
. The term was first applied to biology in a non-evolutionary context by the anatomist
Richard Owen Sir Richard Owen (20 July 1804 – 18 December 1892) was an English biologist, comparative anatomist and paleontologist. Despite being a controversial figure, Owen is generally considered to have been an outstanding naturalist with a remark ...

Richard Owen
in 1843. Homology was later explained by
Charles Darwin Charles Robert Darwin (; ; 12 February 1809 – 19 April 1882) was an English naturalist Natural history is a domain of inquiry involving organism In biology, an organism () is any organic, life, living system that fu ...

Charles Darwin
's theory of evolution in 1859, but had been observed before this, from
Aristotle Aristotle (; grc-gre, Ἀριστοτέλης ''Aristotélēs'', ; 384–322 BC) was a Greek philosopher A philosopher is someone who practices philosophy Philosophy (from , ) is the study of general and fundamental questio ...
onwards, and it was explicitly analysed by
Pierre Belon Pierre Belon (1517–1564) was a French traveler, naturalist Natural history is a domain of inquiry involving organism In biology, an organism () is any organic, life, living system that functions as an individual entity. All organ ...
in 1555. In
developmental biology Developmental biology is the study of the process by which animal Animals (also called Metazoa) are multicellular A multicellular organism is an organism In biology, an organism () is any organic, life, living system that fu ...
, organs that developed in the embryo in the same manner and from similar origins, such as from matching
primordia Image:Root primordia.JPG, 250px, Root primordia (brown spots) as seen on the butt of a freshly cut pineapple crown intended for vegetative reproduction. A primordium (; plural: primordia; synonym: anlage) in embryology, is an Organ (anatomy), orga ...
in successive segments of the same animal, are serially homologous. Examples include the legs of a
centipede Centipedes (from the New Latin New Latin (also called Neo-Latin or modern Latin) is the revival of Latin Latin (, or , ) is a classical language belonging to the Italic branch of the Indo-European languages. Latin was originally s ...

centipede
, the
maxillary palp
maxillary palp
and
labial palp
labial palp
of an insect, and the
spinous processes In the vertebrate spinal column, each vertebra is an irregular bone with a complex structure composed of bone and some hyaline cartilage, the proportions of which vary according to the segment of the vertebral column, backbone and the species of ...

spinous processes
of successive
vertebrae In the vertebrate spinal column The vertebral column, also known as the backbone or spine, is part of the axial skeleton. The vertebral column is the defining characteristic of a vertebrate in which the notochord (a flexible rod of uniform c ...

vertebrae
in a
vertebral column The vertebral column, also known as the backbone or spine, is part of the axial skeleton Axial may refer to: * one of the describing relationships in an animal body * and submarine volcano off Oregon, USA * , a ghost town * In geometry: :* ...

vertebral column
. Male and female
reproductive organ A sex organ (or reproductive organ) is any part of an animal or plant that is involved in sexual reproduction Sexual reproduction is a type of reproduction Reproduction (or procreation or breeding) is the biological process Biological ...
s are homologous if they develop from the same embryonic tissue, as do the
ovaries The ovary is an organ found in the female reproductive system 300px, 1. Labia_majora.html"_;"title="Vulva: 2. Labia_majora">Vulva: 2. Labia_majora; 3. Labia_minora; 4. Vulval_vestibule.html" "title="Labia_minora.html ...

ovaries
and
testicles Testicle or testis (plural testes) is the male Male (♂) is the sex of an organism that produces the gamete known as sperm. A male gamete can fuse with a larger female gamete, or ovum, in the process of fertilization. A male cannot sexual r ...

testicles
of mammals including humans.
Sequence homology Sequence homology is the biological homology between DNA Deoxyribonucleic acid (; DNA) is a molecule File:Pentacene on Ni(111) STM.jpg, A scanning tunneling microscopy image of pentacene molecules, which consist of linear chains of f ...
between
protein Proteins are large biomolecule , showing alpha helices, represented by ribbons. This poten was the first to have its suckture solved by X-ray crystallography by Max Perutz and Sir John Cowdery Kendrew in 1958, for which they received a No ...

protein
or
DNA sequence DNA sequencing is the process of determining the nucleic acid sequence A nucleic acid sequence is a succession of bases signified by a series of a set of five different letters that indicate the order of nucleotides Nucleotides are organic ...

DNA sequence
s is similarly defined in terms of shared ancestry. Two segments of DNA can have shared ancestry because of either a
speciation Speciation is the evolutionary process by which populations evolve to become distinct species. The biologist Orator F. Cook coined the term in 1906 for cladogenesis, the splitting of lineages, as opposed to anagenesis, phyletic evolution within ...

speciation
event (
orthologs Sequence homology is the biological homology between DNA Deoxyribonucleic acid (; DNA) is a molecule File:Pentacene on Ni(111) STM.jpg, A scanning tunneling microscopy image of pentacene molecules, which consist of linear chains of f ...
) or a duplication event (
paralogs Sequence homology is the biological homology between DNA, RNA, or protein sequences, defined in terms of shared ancestry in the evolutionary history of life The history of life on Earth Earth is the third planet from the Sun and the ...
). Homology among proteins or DNA is inferred from their sequence similarity. Significant similarity is strong evidence that two sequences are related by
divergent evolution Divergent evolution or divergent selection is the accumulation of differences between closely related populations within a species, leading to speciation Speciation is the evolution Evolution is change in the Heredity, heritable Phenoty ...
from a common ancestor.
Alignments
Alignments
of multiple sequences are used to discover the homologous regions. Homology remains controversial in
animal behaviour Ethology is the scientific method, scientific and objective study of animal behaviour, usually with a focus on behaviour under natural conditions, and viewing behaviour as an evolutionarily Adaptive behavior (ecology), adaptive trait. Behav ...
, but there is suggestive evidence that, for example,
dominance hierarchies Dominance hierarchy is a type of social hierarchy A hierarchy (from the Greek: , from , 'president of sacred rites') is an arrangement of items (objects, names, values, categories, etc.) in which the items are represented as being "above", "be ...
are homologous across the
primates A primate ( ) (from Latin , from 'prime, first rank') is a eutherian mammal constituting the Taxonomy (biology), taxonomic order (biology), order Primates (). Primates arose 85–55 million years ago first from small Terrestrial animal, ...
.


History

Homology was noticed by
Aristotle Aristotle (; grc-gre, Ἀριστοτέλης ''Aristotélēs'', ; 384–322 BC) was a Greek philosopher A philosopher is someone who practices philosophy Philosophy (from , ) is the study of general and fundamental questio ...
(c. 350 BC), and was explicitly analysed by
Pierre Belon Pierre Belon (1517–1564) was a French traveler, naturalist Natural history is a domain of inquiry involving organism In biology, an organism () is any organic, life, living system that functions as an individual entity. All organ ...
in his 1555 ''Book of Birds'', where he systematically compared the skeletons of birds and humans. The pattern of similarity was interpreted as part of the static
great chain of being The Great Chain of Being is a hierarchical structure of all matter and life, thought by medieval Christianity Christianity is an Abrahamic religions, Abrahamic Monotheism, monotheistic religion based on the Life of Jesus in the New Testament, ...
through the
mediaeval In the history of Europe, the Middle Ages or medieval period lasted approximately from the 5th to the late 15th centuries, similarly to the Post-classical, Post-classical period of global history. It began with the fall of the Western Rom ...
and
early modern The early modern period of modern history Human history, or world history, is the narrative of 's past. It is understood through , , , and , and since the , from and s. Humanity's written history was preceded by its , beginning with ...
periods: it was not then seen as implying evolutionary change. In the German ''
Naturphilosophie ''Naturphilosophie'' (German for "nature-philosophy") is a term used in English-language philosophy to identify a current in the philosophy, philosophical tradition of German idealism, as applied to the study of nature in the earlier 19th century ...
'' tradition, homology was of special interest as demonstrating unity in nature. In 1790,
Goethe Johann Wolfgang von Goethe (28 August 1749 – 22 March 1832) was a German German(s) may refer to: Common uses * of or related to Germany * Germans, Germanic ethnic group, citizens of Germany or people of German ancestry * For citizens of G ...

Goethe
stated his foliar theory in his essay "Metamorphosis of Plants", showing that flower part are derived from leaves. The
serial homologySerial homology is a special type of homology, defined by Owen as "representative or repetitive relation in the segments of the same organism."R:Webster 1913 in Webster’s Revised Unabridged Dictionary, G. & C. Merriam, 1913 Ernst Haeckel preferred ...
of limbs was described late in the 18th century. The French zoologist showed in 1818 in his ''theorie d'analogue'' ("theory of homologues") that structures were shared between fishes, reptiles, birds, and mammals. When Geoffroy went further and sought homologies between
Georges Cuvier Jean Léopold Nicolas Frédéric, Baron Cuvier (; 23 August 1769 – 13 May 1832), known as Georges Cuvier, was a French naturalist Natural history is a domain of inquiry involving organism In biology, an organism () is any orga ...

Georges Cuvier
's '' embranchements'', such as vertebrates and molluscs, his claims triggered the 1830 Cuvier-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. The Estonian
embryologist Embryology (from Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is approximately 10.7 mi ...
Karl Ernst von Baer Karl Ernst Ritter von Baer Edler von Huthorn ( – ) was a Baltic German scientist and explorer. Baer is also known in Russia as Karl Maksímovich Ber (russian: Карл Макси́мович Бэр). Baer was a naturalist Natural history ...

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 In human society A society is a Social group, group of individuals involved in persistent Social relation, social interaction, or a large social group sharing the same spatial or social territory, typically subject to the same Politic ...
are more closely related and diverge later than animals which are only in the same
order Order, ORDER or Orders may refer to: * Orderliness Orderliness is a quality that is characterized by a person’s interest in keeping their surroundings and themselves well organized, and is associated with other qualities such as cleanliness a ...
and have fewer homologies. von Baer's theory recognises that each
taxon In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechani ...
(such as a family) has distinctive shared features, and that embryonic development parallels the taxonomic hierarchy: not the same as
recapitulation theory The theory of recapitulation, also called the biogenetic law or embryological parallelism—often expressed using Ernst Haeckel Ernst Heinrich Philipp August Haeckel (; 16 February 1834 – 9 August 1919) was a German zoologist Zoology ()T ...
. The term "homology" was first used in biology by the anatomist
Richard Owen Sir Richard Owen (20 July 1804 – 18 December 1892) was an English biologist, comparative anatomist and paleontologist. Despite being a controversial figure, Owen is generally considered to have been an outstanding naturalist with a remark ...

Richard Owen
in 1843 when studying the similarities of vertebrate
fin A fin is a thin component or appendage attached to a larger body or structure. Fins typically function as foils that produce lift Lift or LIFT may refer to: Physical devices * Elevator, or lift, a device used for raising and lowering people o ...

fin
s 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. Owen codified 3 main criteria for determining if features were homologous: position, development, and composition. In 1859,
Charles Darwin Charles Robert Darwin (; ; 12 February 1809 – 19 April 1882) was an English naturalist Natural history is a domain of inquiry involving organism In biology, an organism () is any organic, life, living system that fu ...

Charles Darwin
explained homologous structures as meaning that the organisms concerned shared a
body plan A body plan, ''Bauplan'' (German plural ''Baupläne''), or ground plan is a set of morphological features common to many members of a phylum In biology, a phylum (; plural The plural (sometimes list of glossing abbreviations, abbreviated ) ...
from a common ancestor, and that taxa were branches of a single
tree of life #REDIRECT Tree of life#REDIRECT Tree of life The tree of life is a fundamental widespread mytheme or archetype in many of the world's mythology, mythologies, religion, religious and philosophy, philosophical traditions. It is closely related ...
.


Definition

The word homology, coined in about 1656, is derived from the
Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is approximately 10.7 million as of ...
ὁμόλογος from ὁμός "same" and λόγος "relation". Similar biological structures or sequences in different
taxa In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechani ...
are homologous if they are derived from a
common ancestor Common descent is a concept in evolutionary biology Evolutionary biology is the subfield of biology that studies the evolution, evolutionary processes (natural selection, common descent, speciation) that produced the Biodiversity, diversity ...
. Homology thus implies
divergent evolution Divergent evolution or divergent selection is the accumulation of differences between closely related populations within a species, leading to speciation Speciation is the evolution Evolution is change in the Heredity, heritable Phenoty ...
. For example, many
insect Insects (from Latin Latin (, or , ) is a classical language A classical language is a language A language is a structured system of communication Communication (from Latin ''communicare'', meaning "to share" or "to be in ...

insect
s (such as
dragonflies A dragonfly is a flying insect The Pterygota (Ancient Greek Ancient Greek includes the forms of the Greek language Greek ( el, label=Modern Greek Modern Greek (, , or , ''Kiní Neoellinikí Glóssa''), generally referr ...

dragonflies
) possess two pairs of flying
wings A wing is a type of fin A fin is a thin component or appendage attached to a larger body or structure. Fins typically function as foils that produce lift or thrust Thrust is a reaction (physics), reaction force (physics), force describ ...
. In
beetle Beetles are a group of insect Insects (from Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken in the area around Rome, known a ...

beetle
s, the first pair of wings has evolved into a pair of hard wing covers, while in
Diptera Flies are insect Insects (from Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken in the area around Rome, known as Latium. Thro ...

Diptera
n flies the second pair of wings has evolved into small
halteres ''Halteres'' (; singular ''halter'' or ''haltere'') (from grc, ἁλτῆρες, weights held in the hands to give an impetus in leaping) are a pair of small club-shaped organs on the body of two Order (biology), orders of flying insects th ...
used for balance. Similarly, the forelimbs of ancestral
vertebrate Vertebrates () comprise all species of animal Animals (also called Metazoa) are multicellular A multicellular organism is an organism In biology, an organism () is any organic, life, living system that functions as an indiv ...
s have evolved into the front flippers of
whale Whales are a widely distributed and diverse group of fully aquatic Aquatic means relating to water Water (chemical formula H2O) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is ...

whale
s, the wings of
bird Birds are a group of warm-blooded vertebrate Vertebrates () comprise all species of animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biology), biological kingdom Animalia. With ...

bird
s, the running forelegs of
dog The dog or domestic dog (''Canis familiaris'' or ''Canis lupus familiaris'') is a domesticated Domestication is a sustained multi-generational relationship in which one group of organisms assumes a significant degree of influence over the ...

dog
s,
deer Deer or true deer are hoof A hoof ( or ), plural hooves ( or ) or hoofs , is the tip of a toe Toes are the digits (fingers) of the foot of a tetrapod. Animal Animals (also called Metazoa) are multicellular eukaryotic organis ...

deer
, and
horse The horse (''Equus ferus caballus'') is a domesticated Domestication is a sustained multi-generational relationship in which one group of organisms assumes a significant degree of influence over the reproduction and care of another group to ...

horse
s, the short forelegs of
frog A frog is any member of a diverse and largely carnivorous A carnivore , meaning "meat Meat is animal flesh that is eaten as food. Humans have hunted and killed animals for meat since prehistoric times. The advent of civilization all ...

frog
s and
lizard Lizards (suborder In biological classification, the order ( la, wikt:ordo#Latin, ordo) is # a taxonomic rank used in the classification of organisms and recognized by the nomenclature codes. The well-known ranks in descending order are: lif ...

lizard
s, and the grasping
hand A hand is a prehensile, multi- fingered appendage located at the end of the forearm The forearm is the region of the upper limb between the Elbow-joint, elbow and the wrist. The term forearm is used in anatomy to distinguish it from the arm, ...

hand
s of
primate A primate ( ) (from Latin , from 'prime, first rank') is a eutherian mammal constituting the Taxonomy (biology), taxonomic order (biology), order Primates (). Primates arose 85–55 million years ago first from small Terrestrial animal, ...

primate
s including humans. The same major forearm bones (
humerus The humerus (; ) is a long bone in the arm that runs from the shoulder to the elbow. It connects the scapula and the two bones of the lower arm, the radius (bone), radius and ulna, and consists of three sections. The humeral upper extremity of h ...

humerus
,
radius In classical geometry Geometry (from the grc, γεωμετρία; ' "earth", ' "measurement") is, with , one of the oldest branches of . It is concerned with properties of space that are related with distance, shape, size, and relative ...
, and
ulna The ulna (''pl''. ulnae or ulnas) is a long bone The long bones are those that are longer than they are wide. They are one of five types of bone A bone is a rigid tissue Tissue may refer to: Biology * Tissue (biology), an ensemble of si ...

ulna
) are found in fossils of
lobe-finned fish Sarcopterygii (; from Greek language, Greek: , flesh, and , fin)—sometimes considered synonymous with Crossopterygii ("fringe-finned fish", from Greek , fringe)—is a clade (traditionally a class (biology), class or subclass) of the Osteicht ...
such as ''
Eusthenopteron ''Eusthenopteron'' is a genus Genus /ˈdʒiː.nəs/ (plural genera /ˈdʒen.ər.ə/) is a taxonomic rank In biological classification In biology, taxonomy () is the scientific study of naming, defining (Circumscription (taxonomy), ...
''.


Homology vs. analogy

The opposite of homologous organs are analogous organs which do similar jobs in two taxa that were not but rather evolved separately. For example, the and birds evolved independently in widely separated groups, and converged functionally to support powered
flight Flight or flying is the process by which an object Object may refer to: General meanings * Object (philosophy), a thing, being, or concept ** Entity, something that is tangible and within the grasp of the senses ** Object (abstract), an o ...

flight
, so they are analogous. Similarly, the wings of a
sycamore maple ''Acer pseudoplatanus'', known as the sycamore in the United Kingdom and the sycamore maple in the United States, is a flowering plant The flowering plants, also known as Angiospermae (), or Magnoliophyta (), are the most diverse group of Em ...
seed and the wings of a bird are analogous but not homologous, as they develop from quite different structures. A structure can be homologous at one level, but only analogous at another.
Pterosaur Pterosaurs (; from Greek ''pteron'' and ''sauros'', meaning "wing lizard") were flying reptiles of the extinct clade or Order (biology), order Pterosauria. They existed during most of the Mesozoic: from the late Triassic to the end of the Cretac ...
,
bird Birds are a group of warm-blooded vertebrate Vertebrates () comprise all species of animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biology), biological kingdom Animalia. With ...
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
tetrapod Tetrapods (; ) are four-limbed animals constituting the superclass Tetrapoda (). It includes extant Extant is the opposite of the word extinct Extinction is the termination of a kind of organism or of a group of kinds (taxon), usually a sp ...
s, and evolved in different ways in the three groups. Thus, in the pterosaurs, the "wing" involves both the forelimb and the hindlimb. Analogy is called
homoplasy Homoplasy, in biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological ...

homoplasy
in cladistics, and convergent or parallel evolution in evolutionary biology.


In cladistics

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 by
parsimony analysis In phylogenetics, maximum parsimony is an optimality criterion under which the phylogenetic tree that minimizes the total number of character-state changes is to be preferred. Under the maximum-parsimony criterion, the optimal tree will minimize t ...
, where a character state that arises only once on a tree is taken to be homologous. As implied in this definition, many consider secondary homology to be synonymous with
synapomorphy In phylogenetics, an apomorphy (or derived trait) is a novel Phenotypic trait, character or character state that has evolution, evolved from its ancestral form (or Plesiomorphy and symplesiomorphy, plesiomorphy). A synapomorphy is an apomorphy sh ...

synapomorphy
, a shared derived character or
trait Trait may refer to: * Phenotypic trait in biology, which involve genes and characteristics of organisms * Trait (computer programming), a model for structuring object-oriented programs (a template class in the C++ programming language) * Trait the ...
state that distinguishes a
clade A clade (), also known as a monophyletic group or natural group, is a group of organisms that are monophyly, monophyletic – that is, composed of a common ancestor and all its lineage (evolution), lineal descendants - on a phylogenetic tree. R ...

clade
from other organisms. 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. Some cladists follow the pre-cladistic definition of homology of Haas and Simpson, and view both synapomorphies and symplesiomorphies as homologous character states


In different taxa

Homologies provide the fundamental basis for all biological classification, although some may be highly counter-intuitive. For example, deep homologies like the ''
pax6 Paired box protein Pax-6, also known as aniridia type II protein (AN2) or oculorhombin, is a protein Proteins are large biomolecule , showing alpha helices, represented by ribbons. This poten was the first to have its suckture solved by ...
'' 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.


In arthropods

The embryonic body segments (
somite The somites (outdated term: primitive segments) are a set of bilaterally paired blocks of paraxial mesoderm Paraxial mesoderm, also known as presomitic or somitic mesoderm is the area of mesoderm In all bilaterian animals, the mesoderm is one o ...

somite
s) of different
arthropod An arthropod (, (gen. ποδός)) is an invertebrate animal having an exoskeleton, a Segmentation (biology), segmented body, and paired jointed appendages. Arthropods form the phylum Euarthropoda,Reference showing that Euarthropoda is a phylum: ...
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. The homologies between these have been discovered by comparing
gene In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mecha ...

gene
s in
evolutionary developmental biology Evolutionary developmental biology (informally, evo-devo) is a field of biological research Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processe ...
. Among insects, the
stinger A stinger (or sting) is a sharp organ Organ may refer to: Biology * Organ (anatomy) An organ is a group of Tissue (biology), tissues with similar functions. Plant life and animal life rely on many organs that co-exist in organ systems. ...

stinger
of the female
honey bee A honey bee (also spelled honeybee) is a eusocial flying insect within the genus ''Apis'' of the bee clade, all native to Eurasia. They are known for their construction of wiktionary:perennial, perennial Colony (biology), colonial nests from B ...

honey bee
is a modified
ovipositor The ovipositor is a tube-like organ (anatomy), organ used by some animals, especially insects, for the laying of Egg (biology), eggs. In insects, an ovipositor consists of a maximum of three pairs of appendages. The details and morphology of t ...
, homologous with ovipositors in other insects such as the
Orthoptera Orthoptera () is an order (biology), order of insects that comprises the grasshoppers, locusts, and cricket (insect), crickets, including closely related insects, such as the Tettigoniidae, bush crickets or katydids and wētā. The order is subdi ...

Orthoptera
,
Hemiptera Hemiptera (; ) is an order Order or ORDER or Orders may refer to: * Orderliness Orderliness is associated with other qualities such as cleanliness Cleanliness is both the abstract state of being clean and free from germs, dirt, trash, or wast ...

Hemiptera
, and those
Hymenoptera Hymenoptera is a large order Order or ORDER or Orders may refer to: * Orderliness Orderliness is associated with other qualities such as cleanliness Cleanliness is both the abstract state of being clean and free from germs, dirt, trash, or wa ...

Hymenoptera
without stingers.


In mammals

The three small bones in the
middle ear The middle ear is the portion of the ear The ear is the organ of hearing and, in mammals, balance. In mammals, the ear is usually described as having three parts—the outer ear The outer ear, external ear, or auris externa is t ...

middle ear
of mammals including humans, the
malleus The malleus, or hammer, is a hammer-shaped small bone or ossicle The ossicles (also called auditory ossicles) are three bone A bone is a Stiffness, rigid tissue (anatomy), tissue that constitutes part of the vertebrate skeleton in animals. ...

malleus
,
incus The ''incus'' or anvil is a bone in the middle ear. The anvil-shaped small bone is one of three ossicles in the middle ear. The ''incus'' receives vibrations from the ''malleus'', to which it is connected laterally, and transmits these to the ''s ...
, and
stapes The ''stapes'' or stirrup is a bone in the middle ear of humans and other animals which is involved in the conduction of sound vibrations to the inner ear. This bone is connected to the oval window by its Annular ligament of stapes, annular ligam ...
, are today used to transmit sound from the
eardrum In the anatomy Anatomy (Greek ''anatomē'', 'dissection') is the branch of biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, ...
to the
inner ear The inner ear (internal ear, auris interna) is the innermost part of the vertebrate ear The ear is the organ of hearing and, in mammals, balance. In mammals, the ear is usually described as having three parts—the outer ear T ...

inner 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. Among the many homologies in mammal reproductive systems,
ovaries The ovary is an organ found in the female reproductive system 300px, 1. Labia_majora.html"_;"title="Vulva: 2. Labia_majora">Vulva: 2. Labia_majora; 3. Labia_minora; 4. Vulval_vestibule.html" "title="Labia_minora.html ...

ovaries
and
testicle Testicle or testis (plural testes) is the male Male (symbol: ♂) is the sex of an organism that produces the gamete (sex cell) known as sperm, which fuses with the larger female gamete, or ovum, in the process of fertilization. A male or ...

testicle
s are homologous. Rudimentary organs such as the human tailbone, now much reduced from their functional state, are readily understood as signs of
evolution Evolution is change in the heritable Heredity, also called inheritance or biological inheritance, is the passing on of Phenotypic trait, traits from parents to their offspring; either through asexual reproduction or sexual reproduction, ...

evolution
, the explanation being that they were cut down by
natural selection Natural selection is the differential survival and reproduction of individuals due to differences in phenotype right , Here the relation between genotype and phenotype is illustrated, using a Punnett square, for the character of peta ...
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.


In plants


Leaves, stems, and roots

In many plants, defensive or storage structures are made by modifications of the development of primary
leaves A leaf (plural leaves) is the principal lateral appendage of the vascular plant plant stem, stem, usually borne above ground and specialized for photosynthesis. The leaves, stem, flower and fruit together form the shoot system. Leaves are ...

leaves
, , and
root In vascular plant Vascular plants (from Latin ''vasculum'': duct), also known as Tracheophyta (the tracheophytes , from Greek τραχεῖα ἀρτηρία ''trācheia artēria'' 'windpipe' + φυτά ''phutá'' 'plants'), form a large grou ...

root
s. Leaves are variously modified from
photosynthetic Photosynthesis is a process used by plants and other organisms to convert Conversion or convert may refer to: Arts, entertainment, and media * Conversion (Doctor Who audio), "Conversion" (''Doctor Who'' audio), an episode of the audio drama ' ...
structures to form the insect-trapping pitchers of
pitcher plants 250px, Scanning electron micrograph of a pitcher's inner surface Pitcher plants are several different carnivorous plants which have modified leaves known as pitfall traps—a prey-trapping mechanism featuring a deep cavity filled with digesti ...

pitcher plants
, the insect-trapping jaws of
Venus flytrap The Venus flytrap (''Dionaea muscipula'') is a carnivorous plant native to subtropical The subtropical zones or subtropics are geographic and climate zones located to the north and south of the tropical zone. Geographically part of the ...

Venus flytrap
, and the spines of
cactus A cactus (plural cacti, cactuses, or less commonly, cactus) is a member of the plant Plants are predominantly photosynthetic Photosynthesis is a process used by plants and other organisms to Energy transformation, convert light en ...

cactus
es, all homologous. Certain compound leaves of
flower A flower, sometimes known as a bloom or blossom Image:Cerisier du Japon Prunus serrulata.jpg, Cherry blossoms in Paris in full bloom. In botany, blossoms are the flowers of stone fruit fruit tree, trees (genus ''Prunus'') and of some other plan ...

flower
ing plants are partially homologous both to leaves and shoots, because their development has evolved from a genetic mosaic of leaf and shoot development. File:EurAshLeaf.jpg, One
pinnate Pinnation (also called pennation) is the arrangement of feather Feathers are epidermis (zoology), epidermal growths that form a distinctive outer covering, or plumage, on dinosaurs, both Bird, avian (bird) and some non-avian (non-bird) and ...
leaf of
European ash ''Fraxinus excelsior'', known as the ash, or European ash or common ash to distinguish it from other types of ash, is a flowering plant Flowering plants include multiple members of the clade Angiospermae (), commonly called angiosperms. The ...
File:Detail on a palm frond (8297623365).jpg, Detail of
palm Palm most commonly refers to: * Palm of the hand, the central region of the front of the hand and a subdivision of the cubit * Palm trees, of family Arecaceae **List of Arecaceae genera * Several Arecaceae#Other plants, other plants known as Palm P ...
leaf File:Ocotillothron02262006.JPG, Leaf petioles adapted as spines in ''
Fouquieria splendens ''Fouquieria splendens'' (commonly known as ocotillo , but also referred to as coachwhip, candlewood, slimwood, desert coral, Jacob's staff, Jacob cactus, and vine cactus) is a plant Plants are mainly multicellular organisms, predominant ...

Fouquieria splendens
'' File:Musa acuminata Gran Canaria 2.JPG, The very large leaves of the banana, ''
Musa acuminata ''Musa acuminata'' is a species of banana native to Southern Asia, its range comprising the Indian Subcontinent and Southeast Asia. Many of the modern edible dessert bananas are from this species, although some are hybrids with ''Musa balbisiana ...

Musa acuminata
'' File:Split Aloe.jpg, Succulent water storage leaf of ''
Aloe ''Aloe'' ( , ), also written ''Aloë'', is a genus Genus /ˈdʒiː.nəs/ (plural genera /ˈdʒen.ər.ə/) is a taxonomic rank In biological classification In biology, taxonomy () is the scientific study of naming, defining (Circum ...

Aloe
'' File:Venus Flytrap showing trigger hairs.jpg, Insect-trapping leaf of
Venus flytrap The Venus flytrap (''Dionaea muscipula'') is a carnivorous plant native to subtropical The subtropical zones or subtropics are geographic and climate zones located to the north and south of the tropical zone. Geographically part of the ...

Venus flytrap
File:Nepenthes muluensis.jpg, Insect-trapping leaf of
pitcher plant 250px, Scanning electron micrograph of a pitcher's inner surface Pitcher plants are several different carnivorous plants which have modified leaves known as pitfall traps—a prey-trapping mechanism featuring a deep cavity filled with digesti ...

pitcher plant
File:Onions 002.jpg, Food storage leaves in an
onion The onion (''Allium cepa'' L., from Latin ''cepa'' "onion"), also known as the bulb onion or common onion, is a vegetable Vegetables are parts of plants that are consumed by humans or other animals as food. The original meaning is still co ...

onion
bulb In botany Botany, also called , plant biology or phytology, is the science of plant life and a branch of biology. A botanist, plant scientist or phytologist is a scientist who specialises in this field. The term "botany" comes from the A ...

bulb


Flower parts

The four types of flower parts, namely
carpel Gynoecium (; ) is most commonly used as a collective term for the parts of a flower A flower, sometimes known as a bloom or blossom Image:Cerisier du Japon Prunus serrulata.jpg, Cherry blossoms in Paris in full bloom. In botany, blossoms ...
s,
stamen The stamen (plural The plural (sometimes abbreviated An abbreviation (from Latin ''brevis'', meaning ''short'') is a shortened form of a word or phrase, by any method. It may consist of a group of letters, or words taken from the full ve ...
s,
petal Petals are modified leaves A leaf (plural leaves) is the principal lateral appendage of the vascular plant plant stem, stem, usually borne above ground and specialized for photosynthesis. The leaves, stem, flower and fruit together fo ...

petal
s, and
sepal A sepal ( or ) is a part of the flower A flower, sometimes known as a bloom or blossom Cherry blossoms in Paris in full bloom. In botany, blossoms are the flowers of stone fruit fruit tree, trees (genus ''Prunus'') and of some other p ...
s, are homologous with and derived from leaves, as
Goethe Johann Wolfgang von Goethe (28 August 1749 – 22 March 1832) was a German German(s) may refer to: Common uses * of or related to Germany * Germans, Germanic ethnic group, citizens of Germany or people of German ancestry * For citizens of G ...

Goethe
correctly noted in 1790. The development of these parts through a pattern of
gene expression Gene expression is the process by which information from a gene is used in the synthesis of a functional gene product that enables it to produce end products, protein or non-coding RNA, and ultimately affect a phenotype, as the final effect. The ...

gene expression
in the growing zones (
meristem The meristem is a type of tissue Tissue may refer to: Biology * Tissue (biology), an ensemble of similar cells that together carry out a specific function * ''Triphosa haesitata'', a species of geometer moth found in North America * ''Triphosa du ...
s) is described by the
ABC model of flower development The ABC model of flower development is a scientific model Scientific modelling is a scientific activity, the aim of which is to make a particular part or feature of the world easier to understand, define, quantify, visualize, or simulate ...
. 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 form
ovule In seed plant A seed is an embryonic ''Embryonic'' is the twelfth studio album by experimental rock band the Flaming Lips released on October 13, 2009, on Warner Bros. Records, Warner Bros. The band's first double album, it was released ...

ovule
s and E for the floral whorls, complete the model. The genes are evidently ancient, as old as the
flowering plant Flowering plants include multiple members of the clade Angiospermae (), commonly called angiosperms. The term "angiosperm" is derived from the Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greec ...

flowering plant
s themselves.


Developmental biology

Developmental biology Developmental biology is the study of the process by which animal Animals (also called Metazoa) are multicellular A multicellular organism is an organism In biology, an organism () is any organic, life, living system that fu ...
can identify homologous structures that arose from the same tissue in
embryogenesis An embryo is the early stage of development of a multicellular organism A multicellular organism is an organism In biology, an organism () is any organic, life, living system that functions as an individual entity. All organisms ar ...

embryogenesis
. For example, adult
snake Snakes are elongated, limbless, carnivore, carnivorous reptiles of the suborder Serpentes . Like all other Squamata, squamates, snakes are ectothermic, amniote vertebrates covered in overlapping Scale (zoology), scales. Many species of snakes ...

snake
s 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 by
fossil A fossil (from Classical Latin Classical Latin is the form of Latin language Latin (, or , ) is a classical language A classical language is a language A language is a structured system of communication used by humans, inc ...

fossil
evidence: the
Cretaceous The Cretaceous ( ) is a geological period A geological period is one of the several subdivisions of geologic time enabling cross-referencing of rocks and geologic events from place to place. These periods form elements of a hierarchy of division ...

Cretaceous
snake '' Pachyrhachis problematicus'' had hind legs complete with hip bones (ilium (bone), ilium, pubis (bone), pubis, ischium), thigh bone (femur), leg bones (tibia, fibula) and foot bones (calcaneum, astragalus) as in tetrapods with legs today.


Sequence homology

As with anatomical structures, sequence homology between
protein Proteins are large biomolecule , showing alpha helices, represented by ribbons. This poten was the first to have its suckture solved by X-ray crystallography by Max Perutz and Sir John Cowdery Kendrew in 1958, for which they received a No ...

protein
or
DNA sequence DNA sequencing is the process of determining the nucleic acid sequence A nucleic acid sequence is a succession of bases signified by a series of a set of five different letters that indicate the order of nucleotides Nucleotides are organic ...

DNA sequence
s is defined in terms of shared ancestry. Two segments of DNA can have shared ancestry because of either a
speciation Speciation is the evolutionary process by which populations evolve to become distinct species. The biologist Orator F. Cook coined the term in 1906 for cladogenesis, the splitting of lineages, as opposed to anagenesis, phyletic evolution within ...

speciation
event (
orthologs Sequence homology is the biological homology between DNA Deoxyribonucleic acid (; DNA) is a molecule File:Pentacene on Ni(111) STM.jpg, A scanning tunneling microscopy image of pentacene molecules, which consist of linear chains of f ...
) or a duplication event (
paralogs Sequence homology is the biological homology between DNA, RNA, or protein sequences, defined in terms of shared ancestry in the evolutionary history of life The history of life on Earth Earth is the third planet from the Sun and the ...
). 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. of multiple sequences are used to indicate which regions of each sequence are homologous. Homologous sequences are orthologous if they are descended from the same ancestral sequence separated by a
speciation Speciation is the evolutionary process by which populations evolve to become distinct species. The biologist Orator F. Cook coined the term in 1906 for cladogenesis, the splitting of lineages, as opposed to anagenesis, phyletic evolution within ...

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''. The term "ortholog" was coined in 1970 by the molecular evolutionist Walter M. Fitch, Walter Fitch. Homologous sequences are paralogous if they were created by a duplication event within the genome. 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. Paralogous genes often belong to the same species. They can shape the structure of whole genomes and thus explain genome evolution to a large extent. Examples include the Homeobox (Hox gene, Hox) genes in animals. These genes not only underwent gene duplications within chromosomes but also Genome evolution, whole genome duplications. As a result, Hox genes in most vertebrates are spread across multiple chromosomes: the HoxA–D clusters are the best studied.


In behaviour

It has been suggested that some Ethology, 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. For example, D. W. Rajecki and Randall C. Flanery, using data on humans and on nonhuman
primates A primate ( ) (from Latin , from 'prime, first rank') is a eutherian mammal constituting the Taxonomy (biology), taxonomic order (biology), order Primates (). Primates arose 85–55 million years ago first from small Terrestrial animal, ...
, argue that patterns of behaviour in
dominance hierarchies Dominance hierarchy is a type of social hierarchy A hierarchy (from the Greek: , from , 'president of sacred rites') is an arrangement of items (objects, names, values, categories, etc.) in which the items are represented as being "above", "be ...
are homologous across the primates. As with morphological features or DNA, shared similarity in behavior provides evidence for common ancestry. 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 Hennig, W. 1966. ''Phylogenetic Systematics''. University of Illinois Press Willi Hennig, auxiliary principle.


Notes


References


Further reading

* Brigandt, Ingo (2011
"Essay: Homology."
In: ''The Embryo Project Encyclopedia''.
http://embryo.asu.edu/handle/10776/1754
* * * * * * * * * * *


External links

* {{DEFAULTSORT:Homology (Biology) Evolutionary biology concepts Phylogenetics Comparative anatomy