In
biology
Biology is the scientific study of life. It is a natural science with a broad scope but has several unifying themes that tie it together as a single, coherent field. For instance, all organisms are made up of cells that process hereditary i ...
, homology is similarity due to shared ancestry between a pair of structures or
genes in different
taxa
In biology, a taxon (back-formation from ''taxonomy''; plural taxa) is a group of one or more populations of an organism or organisms seen by taxonomists to form a unit. Although neither is required, a taxon is usually known by a particular nam ...
. A common example of homologous structures is the forelimbs of
vertebrates
Vertebrates () comprise all animal taxa within the subphylum Vertebrata () (chordates with backbones), including all mammals, birds, reptiles, amphibians, and fish. Vertebrates represent the overwhelming majority of the phylum Chordata, with c ...
, where the
wings of bats and
birds, the arms of
primate
Primates are a diverse order of mammals. They are divided into the strepsirrhines, which include the lemurs, galagos, and lorisids, and the haplorhines, which include the tarsiers and the simians ( monkeys and apes, the latter including ...
s, the front flippers of
whale
Whales are a widely distributed and diverse group of fully aquatic placental marine mammals. As an informal and colloquial grouping, they correspond to large members of the infraorder Cetacea, i.e. all cetaceans apart from dolphins and ...
s and the forelegs of
four-legged vertebrates like
dogs and
crocodiles are all derived from the same ancestral
tetrapod
Tetrapods (; ) are four-limbed vertebrate animals constituting the superclass Tetrapoda (). It includes extant and extinct amphibians, sauropsids ( reptiles, including dinosaurs and therefore birds) and synapsids ( pelycosaurs, extinct t ...
structure.
Evolutionary biology
Evolutionary biology is the subfield of biology that studies the evolutionary processes ( natural selection, common descent, speciation) that produced the diversity of life on Earth. It is also defined as the study of the history of life ...
explains homologous structures
adapted
In biology, adaptation has three related meanings. Firstly, it is the dynamic evolutionary process of natural selection that fits organisms to their environment, enhancing their evolutionary fitness. Secondly, it is a state reached by the po ...
to different purposes as the result of descent with modification from a
common ancestor
Common descent is a concept in evolutionary biology applicable when one species is the ancestor of two or more species later in time. All living beings are in fact descendants of a unique ancestor commonly referred to as the last universal comm ...
. The term was first applied to biology in a non-evolutionary context by the anatomist
Richard Owen in 1843. Homology was later explained by
Charles Darwin
Charles Robert Darwin ( ; 12 February 1809 – 19 April 1882) was an English naturalist, geologist, and biologist, widely known for his contributions to evolutionary biology. His proposition that all species of life have descended ...
'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 and polymath during the Classical period in Ancient Greece. Taught by Plato, he was the founder of the Peripatetic school of ph ...
onwards, and it was explicitly analysed by
Pierre Belon
Pierre Belon (1517–1564) was a French traveller, naturalist, writer and diplomat. Like many others of the Renaissance period, he studied and wrote on a range of topics including ichthyology, ornithology, botany, comparative anatomy, architectur ...
in 1555.
In
developmental biology, organs that developed in the embryo in the same manner and from similar origins, such as from matching
primordia
A primordium (; plural: primordia; synonym: anlage) in embryology, is an organ or tissue in its earliest recognizable stage of development. Cells of the primordium are called primordial cells. A primordium is the simplest set of cells capable o ...
in successive segments of the same animal, are
serially homologous Serial homology is a special type of homology (biology), 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 Ern ...
. Examples include the legs of a
centipede, the
maxillary palp
Maxillary means "related to the maxilla (upper jaw bone)". Terms containing "maxillary" include:
*Maxillary artery
*Maxillary nerve
*Maxillary prominence
*Maxillary sinus
The pyramid-shaped maxillary sinus (or antrum of Highmore) is the large ...
and
labial palp
The term ''labial'' originates from '' Labium'' (Latin for "lip"), and is the adjective that describes anything of or related to lips, such as lip-like structures. Thus, it may refer to:
* the lips
** In linguistics, a labial consonant
** In zoolog ...
of an insect, and the
spinous processes
The spinal column, a defining synapomorphy shared by nearly all vertebrates,Hagfish are believed to have secondarily lost their spinal column is a moderately flexible series of vertebrae (singular vertebra), each constituting a characteristic i ...
of successive
vertebrae
The spinal column, a defining synapomorphy shared by nearly all vertebrates, Hagfish are believed to have secondarily lost their spinal column is a moderately flexible series of vertebrae (singular vertebra), each constituting a characteristi ...
in a
vertebral 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 composition) found in all chordate ...
. Male and female
reproductive organs are homologous if they develop from the same embryonic tissue, as do the
ovaries
The ovary is an organ in the female reproductive system that produces an ovum. When released, this travels down the fallopian tube into the uterus, where it may become fertilized by a sperm. There is an ovary () found on each side of the body. T ...
and
testicles
A testicle or testis (plural testes) is the male reproductive gland or gonad in all bilaterians, including humans. It is homologous to the female ovary. The functions of the testes are to produce both sperm and androgens, primarily testostero ...
of mammals including humans.
Sequence homology between
protein
Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, res ...
or
DNA sequences is similarly defined in terms of shared ancestry. Two segments of DNA can have shared ancestry because of either a
speciation event (
orthologs
Sequence homology is the biological homology between DNA, RNA, or protein sequences, defined in terms of shared ancestry in the evolutionary history of life. Two segments of DNA can have shared ancestry because of three phenomena: either a sp ...
) 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. Two segments of DNA can have shared ancestry because of three phenomena: either a sp ...
). 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. Divergent evolution is typically exhibited when two populations become separated by a geog ...
from a common ancestor.
Alignments of multiple sequences are used to discover the homologous regions.
Homology remains controversial in
animal behaviour
Ethology is the scientific study of animal behaviour, usually with a focus on behaviour under natural conditions, and viewing behaviour as an evolutionarily adaptive trait. Behaviourism as a term also describes the scientific and objectiv ...
, but there is suggestive evidence that, for example,
dominance hierarchies are homologous across the
primates.
History
![BelonBirdSkel](https://upload.wikimedia.org/wikipedia/commons/6/68/BelonBirdSkel.jpg)
Homology was noticed by
Aristotle
Aristotle (; grc-gre, Ἀριστοτέλης ''Aristotélēs'', ; 384–322 BC) was a Greek philosopher and polymath during the Classical period in Ancient Greece. Taught by Plato, he was the founder of the Peripatetic school of ph ...
(c. 350 BC),
and was explicitly analysed by
Pierre Belon
Pierre Belon (1517–1564) was a French traveller, naturalist, writer and diplomat. Like many others of the Renaissance period, he studied and wrote on a range of topics including ichthyology, ornithology, botany, comparative anatomy, architectur ...
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 to have been decreed by God. The chain begins with God and descends through angels, humans, animals and plants to minerals.
The great ...
through the
mediaeval and
early modern periods: it was not then seen as implying evolutionary change.
In the German ''
Naturphilosophie'' 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 poet, playwright, novelist, scientist, statesman, theatre director, and critic. His works include plays, poetry, literature, and aesthetic criticism, as well as tr ...
stated his foliar theory in his essay "Metamorphosis of Plants", showing that flower part are derived from leaves.
The serial homology Serial 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 preferre ...
of limbs was described late in the 18th century.
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. 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. 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 Karl Ernst von Baer
Karl Ernst Ritter von Baer Edler von Huthorn ( – ) was a Baltic German scientist and explorer. Baer was a naturalist, biologist, geologist, meteorologist, geographer, and is considered a, or the, founding father of embryology. He was ...
stated what are now called von Baer's laws
Von Baer's laws of embryology (or laws of development) are four rules proposed by Karl Ernst von Baer to explain the observed pattern of embryonic development in different species.
Von Baer formulated the laws in the book ''Über Entwickelungsge ...
in 1828, noting that related animals begin their development as similar embryos and then diverge: thus, animals in the same family
Family (from la, familia) is a group of people related either by consanguinity (by recognized birth) or affinity (by marriage or other relationship). The purpose of the family is to maintain the well-being of its members and of society. Idea ...
are more closely related and diverge later than animals which are only in the same order and have fewer homologies. von Baer's theory recognises that each taxon
In biology, a taxon ( back-formation from '' taxonomy''; plural taxa) is a group of one or more populations of an organism or organisms seen by taxonomists to form a unit. Although neither is required, a taxon is usually known by a particular n ...
(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's phrase "ontogeny recapitulates phylogeny"—is a historical hypothesis that the development of the embryo of an a ...
.
The term "homology" was first used in biology by the anatomist 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 or thrust, or provide the ability to steer or stabilize motion while traveling in water, air, or other fluids. 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, geologist, and biologist, widely known for his contributions to evolutionary biology. His proposition that all species of life have descended ...
explained homologous structures as meaning that the organisms concerned shared a body plan
A body plan, ( ), or ground plan is a set of morphological features common to many members of a phylum of animals. The vertebrates share one body plan, while invertebrates have many.
This term, usually applied to animals, envisages a "blueprin ...
from a common ancestor, and that taxa were branches of a single tree of life
The tree of life is a fundamental archetype in many of the world's mythological, religious, and philosophical traditions. It is closely related to the concept of the sacred tree.Giovino, Mariana (2007). ''The Assyrian Sacred Tree: A Hist ...
.
Definition
The word homology, coined in about 1656, is derived from the Greek
Greek may refer to:
Greece
Anything of, from, or related to Greece, a country in Southern Europe:
*Greeks, an ethnic group.
*Greek language, a branch of the Indo-European language family.
**Proto-Greek language, the assumed last common ancestor ...
ὁμόλογος from ὁμός "same" and λόγος "relation".
Similar biological structures or sequences in different taxa
In biology, a taxon (back-formation from ''taxonomy''; plural taxa) is a group of one or more populations of an organism or organisms seen by taxonomists to form a unit. Although neither is required, a taxon is usually known by a particular nam ...
are homologous if they are derived from a common ancestor
Common descent is a concept in evolutionary biology applicable when one species is the ancestor of two or more species later in time. All living beings are in fact descendants of a unique ancestor commonly referred to as the last universal comm ...
. 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. Divergent evolution is typically exhibited when two populations become separated by a geog ...
. For example, many insect
Insects (from Latin ') are pancrustacean hexapod invertebrates of the class Insecta. They are the largest group within the arthropod phylum. Insects have a chitinous exoskeleton, a three-part body ( head, thorax and abdomen), three ...
s (such as dragonflies
A dragonfly is a flying insect belonging to the infraorder Anisoptera below the order Odonata. About 3,000 extant species of true dragonfly are known. Most are tropical, with fewer species in temperate regions. Loss of wetland habitat threa ...
) possess two pairs of flying wings
A wing is a type of fin that produces lift while moving through air or some other fluid. Accordingly, wings have streamlined cross-sections that are subject to aerodynamic forces and act as airfoils. A wing's aerodynamic efficiency is expre ...
. In beetle
Beetles are insects that form the order Coleoptera (), in the superorder Endopterygota. Their front pair of wings are hardened into wing-cases, elytra, distinguishing them from most other insects. The Coleoptera, with about 400,000 describ ...
s, 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
''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 orders of flying insects that provide infor ...
used for balance.
Similarly, the forelimbs of ancestral vertebrate
Vertebrates () comprise all animal taxa within the subphylum Vertebrata () (chordates with backbones), including all mammals, birds, reptiles, amphibians, and fish. Vertebrates represent the overwhelming majority of the phylum Chordata, with c ...
s have evolved into the front flippers of whale
Whales are a widely distributed and diverse group of fully aquatic placental marine mammals. As an informal and colloquial grouping, they correspond to large members of the infraorder Cetacea, i.e. all cetaceans apart from dolphins and ...
s, the wings of bird
Birds are a group of warm-blooded vertebrates constituting the class Aves (), characterised by feathers, toothless beaked jaws, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a strong yet lightweig ...
s, the running forelegs of dog
The dog (''Canis familiaris'' or ''Canis lupus familiaris'') is a domesticated descendant of the wolf. Also called the domestic dog, it is derived from the extinct Pleistocene wolf, and the modern wolf is the dog's nearest living relative. Do ...
s, deer
Deer or true deer are hoofed ruminant mammals forming the family Cervidae. The two main groups of deer are the Cervinae, including the muntjac, the elk (wapiti), the red deer, and the fallow deer; and the Capreolinae, including the re ...
, and horse
The horse (''Equus ferus caballus'') is a domesticated, one-toed, hoofed mammal. It belongs to the taxonomic family Equidae and is one of two extant subspecies of ''Equus ferus''. The horse has evolved over the past 45 to 55 million yea ...
s, the short forelegs of frog
A frog is any member of a diverse and largely carnivorous group of short-bodied, tailless amphibians composing the order Anura (ανοὐρά, literally ''without tail'' in Ancient Greek). The oldest fossil "proto-frog" ''Triadobatrachus'' is ...
s and lizards, and the grasping hands of primate
Primates are a diverse order of mammals. They are divided into the strepsirrhines, which include the lemurs, galagos, and lorisids, and the haplorhines, which include the tarsiers and the simians ( monkeys and apes, the latter including ...
s including humans. The same major forearm bones ( humerus, radius
In classical geometry, a radius ( : radii) of a circle or sphere is any of the line segments from its center to its perimeter, and in more modern usage, it is also their length. The name comes from the latin ''radius'', meaning ray but also the ...
, and ulna
The ulna (''pl''. ulnae or ulnas) is a long bone found in the forearm that stretches from the elbow to the smallest finger, and when in anatomical position, is found on the medial side of the forearm. That is, the ulna is on the same side of t ...
) are found in fossils of lobe-finned fish
Sarcopterygii (; ) — sometimes considered synonymous with Crossopterygii () — is a taxon (traditionally a class or subclass) of the bony fishes known as the lobe-finned fishes. The group Tetrapoda, a mostly terrestrial superclass includ ...
such as '' Eusthenopteron''.
Homology vs. analogy
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. For example, the wings of insects 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 moves through a space without contacting any planetary surface, either within an atmosphere (i.e. air flight or aviation) or through the vacuum of outer space (i.e. spaceflight). This can be a ...
, so they are analogous. 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. A structure can be homologous at one level, but only analogous at another. Pterosaur
Pterosaurs (; from Greek ''pteron'' and ''sauros'', meaning "wing lizard") is an extinct clade of flying reptiles in the order, Pterosauria. They existed during most of the Mesozoic: from the Late Triassic to the end of the Cretaceous (228 ...
, bird
Birds are a group of warm-blooded vertebrates constituting the class Aves (), characterised by feathers, toothless beaked jaws, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a strong yet lightweig ...
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 vertebrate animals constituting the superclass Tetrapoda (). It includes extant and extinct amphibians, sauropsids ( reptiles, including dinosaurs and therefore birds) and synapsids ( pelycosaurs, extinct t ...
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 and phylogenetics, is the term used to describe a feature that has been gained or lost independently in separate lineages over the course of evolution. This is different from homology, which is the term used to characterize ...
in cladistics
Cladistics (; ) is an approach to biological classification in which organisms are categorized in groups (" clades") based on hypotheses of most recent common ancestry. The evidence for hypothesized relationships is typically shared derived cha ...
, 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 (or miminizes the cost of differentially weighted character-state changes) is preferred. ...
, where a character state that arises only once on a tree is taken to be homologous. As implied in this definition, many cladists consider secondary homology to be synonymous with synapomorphy
In phylogenetics, an apomorphy (or derived trait) is a novel character or character state that has evolved from its ancestral form (or plesiomorphy). A synapomorphy is an apomorphy shared by two or more taxa and is therefore hypothesized to hav ...
, a shared derived character or trait state that distinguishes a 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 that in humans is encoded by the ''PAX6'' gene.
Function
PAX6 is a member of the Pax gene family which is responsible for carrying the geneti ...
'' 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 that form in the embryonic stage of somitogenesis, along the head-to-tail axis in segmented animals. In vertebrates, somites subdivide ...
s) of different arthropod
Arthropods (, (gen. ποδός)) are invertebrate animals with an exoskeleton, a segmented body, and paired jointed appendages. Arthropods form the phylum Arthropoda. They are distinguished by their jointed limbs and cuticle made of chiti ...
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, the word gene (from , ; "... Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredity..." meaning ''generation'' or ''birth'' or ''gender'') can have several different meanings. The Mendelian gene is a b ...
s in evolutionary developmental biology.
Among insects, the stinger
A stinger (or sting) is a sharp organ found in various animals (typically insects and other arthropods) capable of injecting venom, usually by piercing the epidermis of another animal.
An insect sting is complicated by its introduction of ve ...
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 Afro-Eurasia. After bees spread naturally throughout Africa and Eurasia, humans became responsible for the current cosm ...
is a modified ovipositor, homologous with ovipositors in other insects such as the Orthoptera, Hemiptera, and those Hymenoptera without stingers.
In mammals
The three small bones in the middle ear
The middle ear is the portion of the ear medial to the eardrum, and distal to the oval window of the cochlea (of the inner ear).
The mammalian middle ear contains three ossicles, which transfer the vibrations of the eardrum into waves in the ...
of mammals including humans, the malleus
The malleus, or hammer, is a hammer-shaped small bone or ossicle of the middle ear. It connects with the incus, and is attached to the inner surface of the eardrum. The word is Latin for 'hammer' or 'mallet'. It transmits the sound vibrations f ...
, incus, 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, which allows the foo ...
, are today used to transmit sound from the eardrum to the 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 in the female reproductive system that produces an ovum. When released, this travels down the fallopian tube into the uterus, where it may become fertilized by a sperm. There is an ovary () found on each side of the body. T ...
and testicle
A testicle or testis (plural testes) is the male reproductive gland or gonad in all bilaterians, including humans. It is homologous to the female ovary. The functions of the testes are to produce both sperm and androgens, primarily testoste ...
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 characteristics of biological populations over successive generations. These characteristics are the expressions of genes, which are passed on from parent to offspring during reproduction. Variation ...
, 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. It is a key mechanism of evolution, the change in the heritable traits characteristic of a population over generations. Cha ...
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, stems, and root
In vascular plants, the roots are the organs of a plant that are modified to provide anchorage for the plant and take in water and nutrients into the plant body, which allows plants to grow taller and faster. They are most often below the su ...
s. Leaves are variously modified from photosynthetic structures to form the insect-trapping pitchers of pitcher plants, the insect-trapping jaws of Venus flytrap
The Venus flytrap (''Dionaea muscipula'') is a carnivorous plant native to subtropical wetlands on the East Coast of the United States in North Carolina and South Carolina. It catches its prey—chiefly insects and arachnids—with a trapping ...
, and the spines of cactus
A cactus (, or less commonly, cactus) is a member of the plant family Cactaceae, a family comprising about 127 genera with some 1750 known species of the order Caryophyllales. The word ''cactus'' derives, through Latin, from the Ancient Gree ...
es, all homologous.
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.
File:EurAshLeaf.jpg, One pinnate
Pinnation (also called pennation) is the arrangement of feather-like or multi-divided features arising from both sides of a common axis. Pinnation occurs in biological morphology, in crystals, such as some forms of ice or metal crystals, and in ...
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 species in the olive family Oleaceae. It is native throughout mainland Europe east to the Caucasus and Albo ...
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
* Palm plants, of family Arecaceae
**List of Arecaceae genera
* Several other plants known as "palm"
Palm or Palms may also refer to:
Music
* Palm (ba ...
leaf
File:Ocotillothron02262006.JPG, Leaf petioles adapted as spines in ''Fouquieria splendens
''Fouquieria splendens'' (commonly known as ocotillo (), but also referred to as buggywhip, coachwhip, candlewood, slimwood, desert coral, Jacob's staff, Jacob cactus, and vine cactus) is a plant indigenous to the Sonoran Desert and Chihuahua ...
''
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 balbisia ...
''
File:Split Aloe.jpg, Succulent water storage leaf of '' 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 wetlands on the East Coast of the United States in North Carolina and South Carolina. It catches its prey—chiefly insects and arachnids—with a trapping ...
File:Nepenthes muluensis.jpg, Insect-trapping leaf of pitcher plant
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 digestive liquid. The traps of what are considered to be "true" pitcher p ...
File:Onions 002.jpg, Food storage leaves in an onion
An onion (''Allium cepa'' L., from Latin ''cepa'' meaning "onion"), also known as the bulb onion or common onion, is a vegetable that is the most widely cultivated species of the genus ''Allium''. The shallot is a botanical variety of the onio ...
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 that produce ovules and ultimately develop into the fruit and seeds. The gynoecium is the innermost whorl of a flower; it consists of (one or more) '' pistils' ...
s, stamens, petals, and sepal
A sepal () is a part of the flower of angiosperms (flowering plants). Usually green, sepals typically function as protection for the flower in bud, and often as support for the petals when in bloom., p. 106 The term ''sepalum'' was coine ...
s, are homologous with and derived from leaves, as Goethe
Johann Wolfgang von Goethe (28 August 1749 – 22 March 1832) was a German poet, playwright, novelist, scientist, statesman, theatre director, and critic. His works include plays, poetry, literature, and aesthetic criticism, as well as tr ...
correctly noted in 1790. The development of these parts through a pattern of gene expression in the growing zones (meristem
The meristem is a type of tissue found in plants. It consists of undifferentiated cells (meristematic cells) capable of cell division. Cells in the meristem can develop into all the other tissues and organs that occur in plants. These cells conti ...
s) is described by the ABC 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 form ovule
In seed plants, the ovule is the structure that gives rise to and contains the female reproductive cells. It consists of three parts: the '' integument'', forming its outer layer, the ''nucellus'' (or remnant of the megasporangium), and the ...
s and E for the floral whorls, complete the model. The genes are evidently ancient, as old as the flowering plant
Flowering plants are plants that bear flowers and fruits, and form the clade Angiospermae (), commonly called angiosperms. The term "angiosperm" is derived from the Greek words ('container, vessel') and ('seed'), and refers to those plants th ...
s themselves.
Developmental biology
Developmental biology can identify homologous structures that arose from the same tissue in embryogenesis. For example, adult snake
Snakes are elongated, limbless, carnivorous reptiles of the suborder Serpentes . Like all other squamates, snakes are ectothermic, amniote vertebrates covered in overlapping scales. Many species of snakes have skulls with several more j ...
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 , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved ...
evidence: the Cretaceous
The Cretaceous ( ) is a geological period that lasted from about 145 to 66 million years ago (Mya). It is the third and final period of the Mesozoic Era, as well as the longest. At around 79 million years, it is the longest geological period of ...
snake ''Pachyrhachis problematicus
''Pachyrhachis'' (from el, παχύς , 'thick' and el, ῥάχῐς , 'spine') is an extinct genus of snake with well developed hind legs known from fossils discovered in Ein Yabrud, near Ramallah, in the central West Bank. It is a relatively ...
'' had hind legs complete with hip bones ( ilium, pubis, ischium), thigh bone (femur
The femur (; ), or thigh bone, is the proximal bone of the hindlimb in tetrapod vertebrates. The head of the femur articulates with the acetabulum in the pelvic bone forming the hip joint, while the distal part of the femur articulates wit ...
), leg bones (tibia
The tibia (; ), also known as the shinbone or shankbone, is the larger, stronger, and anterior (frontal) of the two bones in the leg below the knee in vertebrates (the other being the fibula, behind and to the outside of the tibia); it connects ...
, fibula
The fibula or calf bone is a leg bone on the lateral side of the tibia, to which it is connected above and below. It is the smaller of the two bones and, in proportion to its length, the most slender of all the long bones. Its upper extremity i ...
) and foot bones (calcaneum
In humans and many other primates, the calcaneus (; from the Latin ''calcaneus'' or ''calcaneum'', meaning heel) or heel bone is a bone of the tarsus of the foot which constitutes the heel. In some other animals, it is the point of the hock.
...
, astragalus
''Astragalus'' is a large genus of over 3,000 species of herbs and small shrubs, belonging to the legume family Fabaceae and the subfamily Faboideae. It is the largest genus of plants in terms of described species. The genus is native to tempe ...
) as in tetrapods with legs today.
Sequence homology
As with anatomical structures, sequence homology between protein
Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, res ...
or DNA sequences is defined in terms of shared ancestry. Two segments of DNA can have shared ancestry because of either a speciation event (orthologs
Sequence homology is the biological homology between DNA, RNA, or protein sequences, defined in terms of shared ancestry in the evolutionary history of life. Two segments of DNA can have shared ancestry because of three phenomena: either a sp ...
) 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. Two segments of DNA can have shared ancestry because of three phenomena: either a sp ...
). 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.
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''. The term "ortholog" was coined in 1970 by the molecular evolution
Molecular evolution is the process of change in the sequence composition of cellular molecules such as DNA, RNA, and proteins across generations. The field of molecular evolution uses principles of evolutionary biology and population genetics ...
ist 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, the two copies are paralogous. They can shape the structure of whole genomes and thus explain genome evolution to a large extent. Examples include the Homeobox ( Hox) genes in animals. These genes not only underwent gene duplications within chromosome
A chromosome is a long DNA molecule with part or all of the genetic material of an organism. In most chromosomes the very long thin DNA fibers are coated with packaging proteins; in eukaryotic cells the most important of these proteins are ...
s but also whole genome duplications. As a result, Hox genes in most vertebrates are spread across multiple chromosomes: the HoxA–D clusters are the best studied.
Structural homology. 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 which can be used to demonstrate their homology.
In behaviour
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
Multiple realizability, in the philosophy of mind, is the thesis that the same mental property, state, or event can be implemented by different physical properties, states, or events. Philosophers of mind have used multiple realizability to argue ...
than other biological traits. 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.
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] auxiliary principle.
Notes
References
Further reading
* Brigandt, Ingo (2011
"Essay: Homology."
In: ''The Embryo Project Encyclopedia''.
http://embryo.asu.edu/handle/10776/1754
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External links
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{{DEFAULTSORT:Homology (Biology)
Evolutionary biology concepts
Phylogenetics
Comparative anatomy