Editing Homology (biology) (section)

==In different taxa==
[[File:PAX6 Phenotypes Washington etal PLoSBiol e1000247.png|thumb|''[[pax6]]'' alterations result in similar changes to eye morphology and function across a wide range of taxa.]]

Homologies provide the fundamental basis for all biological classification, although some may be highly counter-intuitive. For example, [[deep homology|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.<ref name=Brusca/><ref>{{cite book |last=Carroll |first=Sean B. |author-link=Sean B. Carroll |title=Endless Forms Most Beautiful |title-link=Endless Forms Most Beautiful (book) |date=2006 |publisher=Weidenfeld & Nicolson |isbn=978-0-297-85094-6 |pages=28, 66–69}}</ref>

{{anchor|In arthropods}}

===In arthropods===
{{further|Arthropod leg}}

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.<ref>{{cite book |last=Hall |first=Brian |title=Homology |url=https://books.google.com/books?id=vptaNfbkd8sC&pg=PA29 |date=2008 |publisher=John Wiley |isbn=978-0-470-51566-2 |page=29}}</ref> The homologies between these have been discovered by comparing [[gene]]s in [[evolutionary developmental biology]].<ref name=Brusca>{{cite book |last1=Brusca |first1=R. C. |last2=Brusca |first2=G. J. |year=1990 |title=Invertebrates |url=https://archive.org/details/invertebrates0000brus |url-access=registration |publisher=Sinauer Associates |page=[https://archive.org/details/invertebrates0000brus/page/669 669]}}</ref>

[[File:Arthropod segment Hox gene expression.svg|thumb|upright=1.3|[[Hox gene]]s in [[arthropod]] [[Segmentation (biology)|segmentation]]
]]

{| class="wikitable"
|-
! [[Somite]]<br/>(body<br/>segment)
! [[Trilobite]]<br/>([[Trilobitomorpha]])<br/>[[File:202003 Trilobite.svg|50px]]
! [[Spider]]<br/>([[Chelicerata]])<br/>[[File:202201 Common house spider.svg|50px]]
! [[Centipede]]<br/>([[Myriapoda]])<br/>[[File:Scolopendra subspinipes japonica (no background).png|80px]]
! [[Insect]]<br/>([[Hexapoda]])<br/>[[File:202101 Chrysodeixis eriosoma.svg|50px]]
! [[Shrimp]]<br/>([[Crustacea]])<br/>[[File:202112 Japanese tiger prawn.svg|50px]]
|-
| 1
| antennae
| [[chelicerae]] (jaws and fangs)
| antennae
| antennae
| 1st antennae
|-
| 2
| 1st legs
| [[pedipalps]]
| -
| -
| 2nd antennae
|-|
| 3
| 2nd legs
| 1st legs
| [[Mandible (arthropod mouthpart)|mandibles]]
| mandibles
| mandibles (jaws)
|-
| 4
| 3rd legs
| 2nd legs
| 1st [[Maxilla (arthropod mouthpart)|maxillae]]
| 1st maxillae
| 1st maxillae
|-
| 5
| 4th legs
| 3rd legs
| 2nd maxillae
| 2nd maxillae
| 2nd maxillae
|-
| 6
| 5th legs
| 4th legs
| collum (no legs)
| 1st legs
| 1st legs
|-
| 7
| 6th legs
| -
| 1st legs
| 2nd legs
| 2nd legs
|-
| 8
| 7th legs
| -
| 2nd legs
| 3rd legs
| 3rd legs
|-
| 9
| 8th legs
| -
| 3rd legs
| -
| 4th legs
|-
| 10
| 9th legs
| -
| 4th legs
| -
| 5th legs
|-
|}

Among insects, 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.<ref>{{cite journal |last1=Shing |first1=H. |last2=Erickson |first2=E. H. |title=Some ultrastructure of the honeybee (''Apis mellifera'' L.) sting |journal=Apidologie |date=1982 |volume=13 |issue=3 |pages=203–213 |url=https://hal.archives-ouvertes.fr/hal-00890568/document |doi=10.1051/apido:19820301|doi-access=free }}</ref>

{{anchor|In mammals}}

===In mammals===
{{further|Comparative anatomy}}

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.<ref>{{cite web |title=Homology: From jaws to ears — an unusual example of a homology|url=http://evolution.berkeley.edu/evolibrary/article/homology_06|publisher=UC Berkeley |access-date=15 December 2016}}</ref>

Among the many [[List of related male and female reproductive organs|homologies in mammal reproductive systems]], [[ovaries]] and [[testicle]]s are homologous.<ref name="Hyde2010">{{cite book |last1=Hyde |first1=Janet Shibley |author-link1=Janet Shibley Hyde |title=Understanding Human Sexuality |last2=DeLamater |first2=John D. |author-link2=John DeLamater |date=June 2010 |publisher=[[McGraw-Hill]] |isbn=978-0-07-338282-1 |edition=11th |place=New York |page=103 |chapter=Chapter 5 |chapter-url=http://highered.mcgraw-hill.com/sites/dl/free/0072986360/238525/hyd86360_ch05.pdf}}</ref>

Rudimentary organs such as the human [[coccyx|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.<ref>{{cite book |last=Larson |first=Edward J. |author-link=Edward Larson |title=Evolution: The Remarkable History of Scientific Theory |publisher=Modern Library |year=2004 |isbn=978-0-679-64288-6 |url-access=registration |url=https://archive.org/details/evolutionremarka00lars |page=112}}</ref>

{{anchor|In plants}}

===In plants===
====Leaves, stems and roots====
In many plants, defensive or storage structures are made by modifications of the development of primary [[leaves]], [[plant stem|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 the [[Venus flytrap]], and the spines of [[cacti]], all homologous.<ref>{{cite web |title=Homology: Leave it to the plants |url=http://evolution.berkeley.edu/evolibrary/article/homology_01 |publisher=University of California at Berkeley |access-date=7 May 2017}}</ref>

{| class="wikitable"
|-
! Primary organs
! Defensive structures
! Storage structures
|-
| Leaves
| [[Thorns, spines, and prickles|Spines]]
| Swollen leaves (e.g. [[succulents]])
|-
| Stems
| [[Thorns, spines, and prickles|Thorns]]
| Tubers (e.g. [[potato]]), rhizomes (e.g. [[ginger]]), fleshy stems (e.g. [[cacti]])
|-
| Roots
| -
| Root tubers (e.g. [[sweet potato]]), taproot (e.g. [[carrot]])
|}

Certain [[compound leaf|compound leaves]] of flowering plants are partially homologous both to leaves and shoots, because their [[evolutionary developmental biology|development has evolved]] from a [[mosaic (genetics)|genetic mosaic]] of leaf and shoot development.<ref>{{cite journal |last=Sattler, R. |title=Homology — a continuing challenge |journal=Systematic Botany |volume=9 |pages=382–394 |year=1984 |doi=10.2307/2418787 |issue=4 |jstor=2418787}}</ref><ref>{{cite book |last=Sattler, R. |chapter=Homology, homeosis, and process morphology in plants |editor=Hall, Brian Keith |title=Homology: the hierarchical basis of comparative biology |publisher=Academic Press |year=1994 |pages=423–75 |isbn=978-0-12-319583-8 }}</ref>

<gallery mode="packed">
File:EurAshLeaf.jpg|One [[pinnate]] leaf of [[European ash]]
File:Detail on a palm frond (8297623365).jpg|Detail of [[palm (plant)|palm]] leaf
File:Ocotillothron02262006.JPG|Leaf [[petiole (botany)|petioles]] adapted as [[Thorns, spines, and prickles|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 organ|storage]] leaf of ''[[Aloe]]''
File:Venus Flytrap showing trigger hairs.jpg|Insect-trapping leaf of a [[Venus flytrap]]
File:Nepenthes muluensis.jpg|Insect-trapping leaf of [[pitcher plant]]
File:Onions 002.jpg|Food storage leaves in an [[onion]] [[bulb]]
</gallery>

====Flower parts====
[[File:ABC flower developement.svg|thumb|The [[ABC model of flower development]]. Class A genes affect [[sepal]]s and [[petal]]s, class B genes affect [[petal]]s and [[stamen]]s, class C genes affect stamens and [[carpel]]s. In two specific whorls of the floral [[meristem]], each class of organ identity genes is switched on.]]
{{further|ABC model of flower development}}

The four types of flower parts, namely [[carpel]]s, [[stamen]]s, [[petal]]s and [[sepal]]s, 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 ([[meristem]]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]]s and E for the floral whorls, complete the model. The genes are evidently ancient, as old as the [[flowering plant]]s themselves.<ref name="Dornelas"/>