Homology (biology)

In

biology

, homology is similarity due to shared ancestry between a pair of structures or genes in different taxa. A common example of homologous structures is the forelimbs of

vertebrates

, where the wings of bats and birds, the arms of

primate

s, the front flippers of

whale

s and the forelegs of four-legged vertebrates like dogs and

crocodile

s are all derived from the same ancestral tetrapod structure. Evolutionary biology explains homologous structures adapted to different purposes as the result of descent with modification from a common ancestor. 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

's theory of evolution in 1859, but had been observed before this, from Aristotle onwards, and it was explicitly analysed by Pierre Belon in 1555.

In developmental biology, organs that developed in the embryo in the same manner and from similar origins, such as from matching primordia in successive segments of the same animal, are serially homologous. Examples include the legs of a

centipede

, the

maxillary palp

and

labial palp

of an insect, and the

spinous processes

of successive

vertebrae

in a

vertebral column

. Male and female reproductive organs are homologous if they develop from the same embryonic tissue, as do the

ovaries

and

testicles

of mammals including humans.

Sequence homology between

protein

or

DNA sequence

s is similarly defined in terms of shared ancestry. Two segments of DNA can have shared ancestry because of either a

speciation

event (orthologs) or a duplication event (paralogs). Homology among proteins or DNA is inferred from their sequence similarity. Significant similarity is strong evidence that two sequences are related by divergent evolution from a common ancestor.

Alignments

of multiple sequences are used to discover the homologous regions.

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

History

Homology was noticed by Aristotle (c. 350 BC), and was explicitly analysed by Pierre Belon in his 1555 ''Book of Birds'', where he systematically compared the skeletons of birds and humans. The pattern of similarity was interpreted as part of the static great chain of being through the mediaeval and early modern periods: it was not then seen as implying evolutionary change.
In the German ''Naturphilosophie'' tradition, homology was of special interest as demonstrating unity in nature.
In 1790,

Goethe

stated his foliar theory in his essay "Metamorphosis of Plants", showing that flower part are derived from leaves.
The serial homology 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

stated what are now called von Baer's laws in 1828, noting that related animals begin their development as similar embryos and then diverge: thus, animals in the same family are more closely related and diverge later than animals which are only in the same order and have fewer homologies. von Baer's theory recognises that each taxon (such as a family) has distinctive shared features, and that embryonic development parallels the taxonomic hierarchy: not the same as recapitulation theory.
The term "homology" was first used in biology by the anatomist

Richard Owen

in 1843 when studying the similarities of vertebrate

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

explained homologous structures as meaning that the organisms concerned shared a body plan from a common ancestor, and that taxa were branches of a single tree of life.

Definition

The word homology, coined in about 1656, is derived from the Greek ὁμόλογος from ὁμός "same" and λόγος "relation".

Similar biological structures or sequences in different taxa are homologous if they are derived from a common ancestor. Homology thus implies divergent evolution. For example, many

insect

s (such as

dragonflies

) possess two pairs of flying wings. In

beetle

s, the first pair of wings has evolved into a pair of hard wing covers, while in

Diptera

n flies the second pair of wings has evolved into small halteres used for balance.

Similarly, the forelimbs of ancestral vertebrates have evolved into the front flippers of

whale

s, the wings of

bird

s, the running forelegs of

dog

s,

deer

, and

horse

s, the short forelegs of

frog

s and

lizard

s, and the grasping

hand

s of

primate

s including humans. The same major forearm bones (

humerus

, radius, and

ulna

) are found in fossils of lobe-finned fish 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

, 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, bird and bat wings are analogous as wings, but homologous as forelimbs because the organ served as a forearm (not a wing) in the last common ancestor of tetrapods, and evolved in different ways in the three groups. Thus, in the pterosaurs, the "wing" involves both the forelimb and the hindlimb. Analogy is called

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, 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

, 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'' 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

s) of different arthropod taxa have diverged from a simple body plan with many similar appendages which are serially homologous, into a variety of body plans with fewer segments equipped with specialised appendages. The homologies between these have been discovered by comparing

gene

s in evolutionary developmental biology.

Among insects, the

stinger

of the female

honey bee

is a modified ovipositor, homologous with ovipositors in other insects such as the

Orthoptera

,

Hemiptera

, and those

Hymenoptera

without stingers.

In mammals

The three small bones in the

middle ear

of mammals including humans, the

malleus

, incus, and stapes, are today used to transmit sound from the eardrum to the

inner ear

. 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

and

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

, the explanation being that they were cut down by natural selection from functioning organs when their functions were no longer needed, but make no sense at all if species are considered to be fixed. 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

s. Leaves are variously modified from photosynthetic structures to form the insect-trapping pitchers of

pitcher plants

, the insect-trapping jaws of

Venus flytrap

, and the spines of

cactus

es, all homologous.

Certain compound leaves of

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 leaf of European ash
File:Detail on a palm frond (8297623365).jpg, Detail of palm leaf
File:Ocotillothron02262006.JPG, Leaf petioles adapted as spines in ''

Fouquieria splendens

''
File:Musa acuminata Gran Canaria 2.JPG, The very large leaves of the banana, ''

Musa acuminata

''
File:Split Aloe.jpg, Succulent water storage leaf of ''

Aloe

''
File:Venus Flytrap showing trigger hairs.jpg, Insect-trapping leaf of

Venus flytrap

File:Nepenthes muluensis.jpg, Insect-trapping leaf of

pitcher plant

File:Onions 002.jpg, Food storage leaves in an

onion

bulb

Flower parts

The four types of flower parts, namely carpels, stamens,

petal

s, and sepals, are homologous with and derived from leaves, as

Goethe

correctly noted in 1790. The development of these parts through a pattern of

gene expression

in the growing zones (meristems) is described by the ABC model of flower development. 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

s and E for the floral whorls, complete the model. The genes are evidently ancient, as old as the

flowering plant

s themselves.

Developmental biology

Developmental biology can identify homologous structures that arose from the same tissue in

embryogenesis

. For example, adult

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

evidence: the

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

or

DNA sequence

s is defined in terms of shared ancestry. Two segments of DNA can have shared ancestry because of either a

speciation

event (orthologs) or a duplication event (paralogs). Homology among proteins or DNA is typically inferred from their sequence similarity. Significant similarity is strong evidence that two sequences are related by divergent evolution of a common ancestor.

Alignments

of multiple sequences are used to indicate which regions of each sequence are homologous.

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 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, 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 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
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External links

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