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vertebrate
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{{For|an explanation of similar terms|Craniate}}{{Use dmy dates|date=August 2013}}{{Automatic taxobox| name = Vertebrate! colspan=3 | Vertebrate groups! Image! Class! Estimated number ofdescribed species! Grouptotals! rowspan=6 style="background:rgb(120,120,185)" | Anamniotelackamnioticmembraneso need toreproducein water! rowspan=2 style="background:rgb(120,120,185)" | Jawless! rowspan=5 style="background:rgb(130,130,130)" | Fish! rowspan=7 style="background:rgb(120,120,185)" | Jawed! rowspan=4 style="background:rgb(130,130,130)" | Tetrapods! rowspan=3 style="background:rgb(120,120,185)" | Amniotehaveamnioticmembraneadapted toreproducingon land
Cambrian–Holocene>Present,{{fossil range|520|0|ref=JOURNAL, Peterson, Kevin J., Cotton, James A., Gehling, James G., Pisani, Davide, 27 April 2008, The Ediacaran emergence of bilaterians: congruence between the genetic and the geological fossil records, Philosophical Transactions of the Royal Society B: Biological Sciences, 363, 1496, 1435–1443, 10.1098/rstb.2007.2233, 18192191, 2614224, }}| image = Vertebrates.png| image_caption = Individual organisms from each major vertebrate group. Clockwise, starting from top left:Fire salamander (Amphibia), saltwater crocodile (Reptilia), southern cassowary (Aves), black-and-rufous giant elephant shrew (Mammalia), ocean sunfish (Osteichthyes)| taxon = VertebrataJean-Baptiste Lamarck>J-B. Lamarck, 1801JOURNAL, Nielsen, C., July 2012, The authorship of higher chordate taxa, Zoologica Scripta issue=4, 435–436, 10.1111/j.1463-6409.2012.00536.x, | subdivision_ranks = Simplified grouping (see text)| subdivision = }}Vertebrates {{IPAc-en|ˈ|v|ɜːr|t|áµ»|b|r|áµ»|t|s}} comprise all species of animals within the subphylum Vertebrata {{IPAc-en|-|eɪ}} (chordates with backbones). Vertebrates represent the overwhelming majority of the phylum Chordata, with currently about 69,276 species described.IUCN Red List of Threatened Species, Table 1: Numbers of threatened species by major groups of organisms (1996–2018),weblink Vertebrates include such groups as the following: Extant vertebrates range in size from the frog species Paedophryne amauensis, at as little as {{convert|7.7|mm|in|abbr=on}}, to the blue whale, at up to {{convert|33|m|ft|abbr=on}}. Vertebrates make up less than five percent of all described animal species; the rest are invertebrates, which lack vertebral columns.The vertebrates traditionally include the hagfish, which do not have proper vertebrae due to their loss in evolution,JOURNAL, Ota, Kinya G., Fujimoto, Satoko, Oisi, Yasuhiro, Kuratani, Shigeru, 2017-01-25, Identification of vertebra-like elements and their possible differentiation from sclerotomes in the hagfish, Nature Communications, 2, 373, 10.1038/ncomms1355, 2041-1723, 3157150, 21712821, 2011NatCo...2E.373O, though their closest living relatives, the lampreys, do.JOURNAL, Monophyly of Lampreys and Hagfishes Supported by Nuclear DNA–Coded Genes, Kuraku, December 1999, Journal of Molecular Evolution, 49, 729–35, 10.1007/PL00006595, 10594174, Hoshiyama, D, Katoh, K, Suga, H, Miyata, T, 6, 1, 1999JMolE..49..729K, Hagfish do, however, possess a cranium. For this reason, the vertebrate subphylum is sometimes referred to as "Craniata" when discussing morphology.Molecular analysis since 1992 has suggested that hagfish are most closely related to lampreys,JOURNAL, Stock, D., Whitt, G.S., Evidence from 18S ribosomal RNA sequences that lampreys and hagfish form a natural group, Science, 7 August 1992, 257, 5071, 10.1126/science.1496398, 787–9, 1496398, 1992Sci...257..787S, and so also are vertebrates in a monophyletic sense. Others consider them a sister group of vertebrates in the common taxon of craniata.JOURNAL, Nicholls, H., Mouth to Mouth, 10 September 2009, Nature, 461, 7261, 164–166, 10.1038/461164a, 19741680,

Etymology

The word vertebrate derives from the Latin word vertebratus (Pliny), meaning joint of the spine.WEB, vertebrate, Dictionary.com., Online Etymology Dictionary,weblink Vertebrate is derived from the word vertebra, which refers to any of the bones or segments of the spinal column.WEB, vertebra, Dictionary.com., Online Etymology Dictionary,weblink

Anatomy and morphology

{{see also|Vertebrate anatomy}}All vertebrates are built along the basic chordate body plan: a stiff rod running through the length of the animal (vertebral column and/or notochord),WEB, Waggoner, Ben, Vertebrates: More on Morphology,weblink UCMP, 13 July 2011, with a hollow tube of nervous tissue (the spinal cord) above it and the gastrointestinal tract below.In all vertebrates, the mouth is found at, or right below, the anterior end of the animal, while the anus opens to the exterior before the end of the body. The remaining part of the body continuing after the anus forms a tail with vertebrae and spinal cord, but no gut.Romer, A.S. (1949): The Vertebrate Body. W.B. Saunders, Philadelphia. (2nd ed. 1955; 3rd ed. 1962; 4th ed. 1970)

Vertebral column

The defining characteristic of a vertebrate is the vertebral column, in which the notochord (a stiff rod of uniform composition) found in all chordates has been replaced by a segmented series of stiffer elements (vertebrae) separated by mobile joints (intervertebral discs, derived embryonically and evolutionarily from the notochord).However, a few vertebrates have secondarily lost this anatomy, retaining the notochord into adulthood, such as the sturgeonBOOK, Functional anatomy of the vertebrates: an evolutionary perspective, 2001, Harcourt College Publishers, 978-0-03-022369-3, Liem, K.F., Walker, W.F., 277, and coelacanth. Jawed vertebrates are typified by paired appendages (fins or legs, which may be secondarily lost), but this trait is not required in order for an animal to be a vertebrate.File:Naturkundemuseum Berlin - Dinosaurierhalle.jpg|upright=3.65|thumb|centre|Fossilized skeleton of Diplodocus carnegii, showing an extreme example of the backbone that characterizes the vertebrates.]]

Gills

File:Gills (esox).jpg|thumb|right|Gill arches bearing gills in a pike ]]All basal vertebrates breathe with gills. The gills are carried right behind the head, bordering the posterior margins of a series of openings from the pharynx to the exterior. Each gill is supported by a cartilagenous or bony gill arch.BOOK, Scott, T., Concise encyclopedia biology, 1996, Walter de Gruyter, 978-3-11-010661-9, 542, The bony fish have three pairs of arches, cartilaginous fish have five to seven pairs, while the primitive jawless fish have seven. The vertebrate ancestor no doubt had more arches than this, as some of their chordate relatives have more than 50 pairs of gills.In amphibians and some primitive bony fishes, the larvae bear external gills, branching off from the gill arches.JOURNAL, The American Naturalist, 1957, 91, 860, 283–301, 2458911, The Origin of the Larva and Metamorphosis in Amphibia, 10.1086/281990, Szarski, Henryk, These are reduced in adulthood, their function taken over by the gills proper in fishes and by lungs in most amphibians. Some amphibians retain the external larval gills in adulthood, the complex internal gill system as seen in fish apparently being irrevocably lost very early in the evolution of tetrapods.Clack, J. A. (2002): Gaining ground: the origin and evolution of tetrapods. Indiana University Press, Bloomington, Indiana. 369 ppWhile the more derived vertebrates lack gills, the gill arches form during fetal development, and form the basis of essential structures such as jaws, the thyroid gland, the larynx, the columella (corresponding to the stapes in mammals) and, in mammals, the malleus and incus.

Central nervous system

The central nervous system of vertebrates is based on a hollow nerve cord running along the length of the animal. Of particular importance and unique to vertebrates is the presence of neural crest cells. These are progenitors of stem cells, and critical to coordinating the functions of cellular components.Teng, L.; Labosky, P. A. (2006). "Neural crest stem cells" In: Jean-Pierre Saint-Jeannet, Neural Crest Induction and Differentiation, pp. 206-212, Springer Science & Business Media. {{ISBN|9780387469546}}. Neural crest cells migrate through the body from the nerve cord during development, and initiate the formation of neural ganglia and structures such as the jaws and skull.JOURNAL, Gans, C., Northcutt, R. G., 1983, Neural crest and the origin of vertebrates: a new head, Science, 220, 4594, 268–273, 10.1126/science.220.4594.268, 17732898, 1983Sci...220..268G, JOURNAL, Bronner, M. E., LeDouarin, N. M., 1 June 2012, Evolution and development of the neural crest: An overview, Developmental Biology, 366, 1, 2–9, 10.1016/j.ydbio.2011.12.042, 3351559, 22230617, Dupin, E.; Creuzet, S.; Le Douarin, N.M. (2007) "The Contribution of the Neural Crest to the Vertebrate Body".In: Jean-Pierre Saint-Jeannet, Neural Crest Induction and Differentiation, pp. 96–119, Springer Science & Business Media. {{ISBN|9780387469546}}. {{doi|10.1007/978-0-387-46954-6_6}}. Full textThe vertebrates are the only chordate group to exhibit cephalisation, the concentration of brain functions in the head. A slight swelling of the anterior end of the nerve cord is found in the lancelet, a chordate, though it lacks the eyes and other complex sense organs comparable to those of vertebrates. Other chordates do not show any trends towards cephalisation.A peripheral nervous system branches out from the nerve cord to innervate the various systems. The front end of the nerve tube is expanded by a thickening of the walls and expansion of the central canal of spinal cord into three primary brain vesicles: The prosencephalon (forebrain), mesencephalon (midbrain) and rhombencephalon (hindbrain), further differentiated in the various vertebrate groups.Hildebrand, M.; Gonslow, G. (2001): Analysis of Vertebrate Structure. 5th edition. John Wiley & Sons, Inc. New York Two laterally placed eyes form around outgrowths from the midbrain, except in hagfish, though this may be a secondary loss.WEB,weblink Keeping an eye on evolution, 3 December 2007, 4 December 2007, PhysOrg.com, Hyperotreti - Hagfishes The forebrain is well developed and subdivided in most tetrapods, while the midbrain dominates in many fish and some salamanders. Vesicles of the forebrain are usually paired, giving rise to hemispheres like the cerebral hemispheres in mammals.The resulting anatomy of the central nervous system, with a single hollow nerve cord topped by a series of (often paired) vesicles, is unique to vertebrates. All invertebrates with well-developed brains, such as insects, spiders and squids, have a ventral rather than dorsal system of ganglions, with a split brain stem running on each side of the mouth or gut.

Evolutionary history

{{see also|Evolution of fish|Evolution of tetrapods}}

First vertebrates

File:Haikouichthys cropped.jpg|thumb|right|The early vertebrate HaikouichthysHaikouichthysVertebrates originated about 525 million years ago during the Cambrian explosion, which saw the rise in organism diversity. The earliest known vertebrate is believed to be the Myllokunmingia.JOURNAL, Lower Cambrian vertebrates from south China, Shu, 4 November 1999, Nature, 402, 42–46, 10.1038/46965, Luo, H-L., Conway Morris, S., Zhang, X-L., Hu, S-X., Chen, L., Han, J., Zhu, M., Li, Y., 6757, 1999Natur.402...42S, L-Z., 1, Chen, Another early vertebrate is Haikouichthys ercaicunensis. Unlike the other fauna that dominated the Cambrian, these groups had the basic vertebrate body plan: a notochord, rudimentary vertebrae, and a well-defined head and tail.WEB, Waggoner, B., Vertebrates: Fossil Record,weblink UCMP, 15 July 2011, All of these early vertebrates lacked jaws in the common sense and relied on filter feeding close to the seabed.BOOK, The Complete Guide to Prehistoric Life, 2005, Firefly Books, Tim Haines, T., Chambers, P., A vertebrate group of uncertain phylogeny, small-eel-like conodonts, are known from microfossils of their paired tooth segments from the late Cambrian to the end of the Triassic.JOURNAL, 10.1111/j.1469-185X.1999.tb00045.x, Donoghue, P. C. J., Forey, P. L., Aldridge, R. J., May 2000, Conodont affinity and chordate phylogeny, Biological Reviews, 75, 2, 191–251, 10881388,

From fish to amphibians

File:Acanthostega BW.jpg|thumb|Acanthostega, a fish-like early labyrinthodont.]]The first jawed vertebrates appeared in the latest Ordovician and became common in the Devonian, often known as the "Age of Fishes".BOOK, Encyclopædia Britannica: a new survey of universal knowledge, Volume 17, 1954, Encyclopædia Britannica, 107, The two groups of bony fishes, the actinopterygii and sarcopterygii, evolved and became common.BOOK, Biology, 2004, Cengage Learning, 978-0-534-49276-2, Berg, L.R., Solomon, E.P., Martin, D.W., 599, The Devonian also saw the demise of virtually all jawless fishes, save for lampreys and hagfish, as well as the Placodermi, a group of armoured fish that dominated the entirety of that period since the late Silurian. The Devonian also saw the rise of the first labyrinthodonts, which was a transitional form between fishes and amphibians.

Mesozoic vertebrates

Amniotes branched from labyrinthodonts in the subsequent Carboniferous period. The Parareptilia and synapsid amniotes were common during the late Paleozoic, while diapsids became dominant during the Mesozoic. In the sea, the bony fishes became dominant; the birds, a derived form of dinosaurs, evolved in the Jurassic.BOOK, Cloudsley-Thompson, J. L., Ecology and behaviour of Mesozoic reptiles, 2005, Springer, 9783540224211, 6, The demise of the non-avian dinosaurs at the end of the Cretaceous allowed for the expansion of mammals, which had evolved from the therapsids, a group of synapsid amniotes, during the late Triassic Period.

After the Mesozoic

The Cenozoic world has seen great diversification of bony fishes, frogs, birds and mammals.Over half of all living vertebrate species (about 32,000 species) are fish (non-tetrapod craniates), a diverse set of lineages that inhabit all the world's aquatic ecosystems, from snow minnows (Cypriniformes) in Himalayan lakes at elevations over {{convert|4,600|m|ft|abbr=off}} to flatfishes (order Pleuronectiformes) in the Challenger Deep, the deepest ocean trench at about {{convert|11,000|m|ft|abbr=off}}. Fishes of myriad varieties are the main predators in most of the world's water bodies, both freshwater and marine. The rest of the vertebrate species are tetrapods, a single lineage that includes amphibians (with roughly 7,000 species); mammals (with approximately 5,500 species); and reptiles and birds (with about 20,000 species divided evenly between the two classes). Tetrapods comprise the dominant megafauna of most terrestrial environments and also include many partially or fully aquatic groups (e.g., sea snakes, penguins, cetaceans).

Classification

There are several ways of classifying animals. Evolutionary systematics relies on anatomy, physiology and evolutionary history, which is determined through similarities in anatomy and, if possible, the genetics of organisms. Phylogenetic classification is based solely on phylogeny.BOOK, Australian water bugs: their biology and identification (Hemiptera-Heteroptera, Gerromorpha & Nepomorpha), 2004, Apollo Books, 978-87-88757-78-1, Andersen, N.M., Weir, T.A., 38, Evolutionary systematics gives an overview; phylogenetic systematics gives detail. The two systems are thus complementary rather than opposed.Hildebran, M.; Gonslow, G. (2001): Analysis of Vertebrate Structure. 5th edition. John Wiley & Sons, Inc. New York, page 33: Comment: The problem of naming sister groups

Traditional classification

(File:Spindle diagram.jpg|thumb|right|Traditional spindle diagram of the evolution of the vertebrates at class level)Conventional classification has living vertebrates grouped into seven classes based on traditional interpretations of gross anatomical and physiological traits. This classification is the one most commonly encountered in school textbooks, overviews, non-specialist, and popular works. The extant vertebrates are: In addition to these, there are two classes of extinct armoured fishes, the Placodermi and the Acanthodii.Other ways of classifying the vertebrates have been devised, particularly with emphasis on the phylogeny of early amphibians and reptiles. An example based on Janvier (1981, 1997), Shu et al. (2003), and Benton (2004)BOOK, Benton, M.J., Michael Benton, Vertebrate Palaeontology, Blackwell Publishing, 1 November 2004, Third, 33, 455 pp,weblink 978-0632056378, is given here: Class Actinopterygii (ray-finned bony fishes) Class Sarcopterygii (lobe-finned fishes, including the tetrapods) Class Amphibia (amphibians, some ancestral to the amniotes)- now a paraphyletic group Class Synapsida (mammals and the extinct mammal-like reptiles) Class Sauropsida (reptiles and birds)While this traditional classification is orderly, most of the groups are paraphyletic, i.e. do not contain all descendants of the class's common ancestor. For instance, descendants of the first reptiles include modern reptiles, as well as mammals and birds. Most of the classes listed are not "complete" (and are therefore paraphyletic) taxa, meaning they do not include all the descendants of the first representative of the group. For example, the agnathans have given rise to the jawed vertebrates; the bony fishes have given rise to the land vertebrates; the traditional "amphibians" have given rise to the reptiles (traditionally including the synapsids, or mammal-like "reptiles"), which in turn have given rise to the mammals and birds. Most scientists working with vertebrates use a classification based purely on phylogeny{{Citation needed|date=October 2016}}, organized by their known evolutionary history and sometimes disregarding the conventional interpretations of their anatomy and physiology.

Phylogenetic relationships

In phylogenetic taxonomy, the relationships between animals are not typically divided into ranks, but illustrated as a nested "family tree" known as a phylogenetic tree. The one below is based on studies compiled by Philippe Janvier and others for the Tree of Life Web Project and Delsuc et al.Janvier, P. 1997. Vertebrata. Animals with backbones. Version 1 January 1997 (under construction).weblink in The Tree of Life Web Project,weblink Delsuc F, Philippe H, Tsagkogeorga G, Simion P, Tilak MK, Turon X, López-Legentil S, Piette J, Lemaire P, Douzery EJ, April 2018, A phylogenomic framework and timescale for comparative studies of tunicates, BMC Biology, 16, 1, 39, 10.1186/s12915-018-0499-2, 5899321, 29653534, {{clade| style=font-size:85%;line-height:70%|label1=Vertebrata/|1={{clade
|label1=Agnatha/
|sublabel1=Cyclostomes
|1={{clade
Hyperoartia (lampreys)(File:Nejonöga, Iduns kokbok.jpg>70 px)
|2=Myxini
}}
|2=?†Euconodonta
|label3=unnamed
|3={{clade
|1=†Pteraspidomorphi(File:Astraspis desiderata.png|50 px)
|2=?†Thelodonti(File:Sphenonectris turnernae white background.jpg|50 px)
|label3=unnamed
|3={{clade
|1=?†Anaspida(File:Pharyngolepis2.png|50 px)
|label2=unnamed
|2={{clade
|1=†Galeaspida
|label2=unnamed
|2={{clade
|1=?†Pituriaspida
|2=†Osteostraci
|label3=Gnathostomata
|3={{clade|state1=double
|1=†Placodermi (armoured fishes)(File:Dunkleosteus intermedius.jpg|70 px)
|label2= unnamed
|2={{clade
|1=Acanthodians, incl. Chondrichthyes (cartilaginous fishes)(File:Acanthodes BW.jpg|70px)(File:Carcharodon carcharias drawing.jpg|70 px)
|label2=Euteleostomi
|2={{clade
|1=Actinopterygii (ray-finned fishes)(File:Cyprinus carpio3.jpg|70px)
|label2=Sarcopterygii (lobe-finned fish)
|2={{clade
|1=?†Onychodontiformes(File:OnychodusDB15 flipped.jpg|70 px)
|2=Actinistia (coelacanths)(File:Coelacanth flipped.png|70 px)
|label3= unnamed
|3={{clade
|1=†Porolepiformes(File:Reconstruction of Porolepis sp flipped.jpg|70 px)
|2=Dipnoi (lungfishes)(File:Barramunda coloured.jpg|70 px)
}}
|label4= unnamed
|4={{clade
|1=†Rhizodontimorpha(File:Gooloogongia loomesi reconstruction.jpg|70 px)
|2={{clade
|1=†Tristichopteridae(File:Eusthenodon DB15 flipped.jpg|70 px)
|2=Tetrapoda(File:Deutschlands Amphibien und Reptilien (Salamandra salamdra).jpg|70 px)
}}
}}
}}
}}
}}
}}
}}
}}
}}
}}
}}
|sublabel1=Craniata}}

Number of extant species

The number of described vertebrate species are split evenly between tetrapods and fish. The following table lists the number of described extant species for each vertebrate class as estimated in the IUCN Red List of Threatened Species, 2014.3.The World Conservation Union. 2014. IUCN Red List of Threatened Species, 2014.3. Summary Statistics for Globally Threatened Species. Table 1: Numbers of threatened species by major groups of organisms (1996–2014).{| class="wikitable"
140px) Myxini(hagfish) 32,900
140px) Hyperoartia(lamprey)
140px) cartilaginousfish
140px) ray-finnedfish|
140px) lobe-finnedfish
140px) amphibians 7,302 33,278
140px) reptiles 10,711
140px) birds 10,425
140px) mammals 5,513
Total described species 66,178
The IUCN estimates that 1,305,075 extant invertebrate species have been described, which means that less than 5% of the described animal species in the world are vertebrates.

Vertebrate species databases

The following databases maintain (more or less) up-to-date lists of vertebrate species:

Reproductive systems

Nearly all vertebrates undergo sexual reproduction. They produce haploid gametes by meiosis. The smaller, motile gametes are spermatozoa and the larger, non-motile gametes are ova. These fuse by the process of fertilisation to form diploid zygotes, which develop into new individuals.

Inbreeding

During sexual reproduction, mating with a close relative (inbreeding) often leads to inbreeding depression. Inbreeding depression is considered to be largely due to expression of deleterious recessive mutations.JOURNAL, Charlesworth, D., Willis, J.H., The genetics of inbreeding depression, Nat. Rev. Genet., 10, 11, 783–796, November 2009, 19834483, 10.1038/nrg2664, The effects of inbreeding have been studied in many vertebrate species.In several species of fish, inbreeding was found to decrease reproductive success.JOURNAL, Gallardo, J.A., Neira, R., Environmental dependence of inbreeding depression in cultured Coho salmon (Oncorhynchus kisutch): aggressiveness, dominance and intraspecific competition, Heredity (Edinb), 95, 6, 449–56, July 2005, 16189545, 10.1038/sj.hdy.6800741, JOURNAL, Ala-Honkola, O., Uddström, A., Pauli, B.D., Lindström, K., Strong inbreeding depression in male mating behaviour in a poeciliid fish, J. Evol. Biol., 22, 7, 1396–1406, 2009, 19486236, 10.1111/j.1420-9101.2009.01765.x, JOURNAL, Bickley, L.K., Brown, A.R., Hosken, D.J., Hamilton, P.B., Le Page, G., Paull, G.C., Owen, S.F., Tyler, C.R., Interactive effects of inbreeding and endocrine disruption on reproduction in a model laboratory fish, Evol Appl, 6, 2, 279–289, February 2013, 23798977, 3689353, 10.1111/j.1752-4571.2012.00288.x, Inbreeding was observed to increase juvenile mortality in 11 small animal species.JOURNAL, Ralls, K., Ballou, J., Effect of inbreeding on juvenile mortality in some small mammal species, Lab. Anim., 16, 2, 159–66, 1982, 7043080, 10.1258/002367782781110151, A common breeding practice for pet dogs is mating between close relatives (e.g. between half- and full siblings).JOURNAL, Leroy, G., Genetic diversity, inbreeding and breeding practices in dogs: results from pedigree analyses, Vet. J., 189, 2, 177–182, August 2011, 21737321, 10.1016/j.tvjl.2011.06.016, This practice generally has a negative effect on measures of reproductive success, including decreased litter size and puppy survival.JOURNAL, van der Beek, S., Nielen, A.L., Schukken, Y.H., Brascamp, E.W., Evaluation of genetic, common-litter, and within-litter effects on preweaning mortality in a birth cohort of puppies, Am. J. Vet. Res., 60, 9, 1106–10, 1999, 10490080, JOURNAL, Gresky, C., Hamann, H., Distl, O., [Influence of inbreeding on litter size and the proportion of stillborn puppies in dachshunds], German, Berl. Munch. Tierarztl. Wochenschr., 118, 3–4, 134–9, 2005, 15803761, JOURNAL, Leroy, G., Phocas, F., Hedan, B., Verrier, E., Rognon, X., Inbreeding impact on litter size and survival in selected canine breeds, Vet. J., 203, 1, 74–8, 2015, 25475165, 10.1016/j.tvjl.2014.11.008,weblink Incestuous matings in birds result in severe fitness costs due to inbreeding depression (e.g. reduction in hatchability of eggs and reduced progeny survival).JOURNAL, Keller, L.F., Grant, P.R., Grant, B.R., Petren, K., Environmental conditions affect the magnitude of inbreeding depression in survival of Darwin's finches, Evolution, 56, 6, 1229–39, 2002, 12144022, 10.1111/j.0014-3820.2002.tb01434.x, JOURNAL, Hemmings, N.L., Slate, J., Birkhead, T.R., Inbreeding causes early death in a passerine bird, Nat Commun, 3, 863, 2012, 22643890, 10.1038/ncomms1870, 2012NatCo...3E.863H, JOURNAL, Kingma, S.A., Hall, M.L., Peters, A., 2013, Breeding synchronization facilitates extrapair mating for inbreeding avoidance, Behavioral Ecology, 24, 6, 1390–1397, 10.1093/beheco/art078,

Inbreeding avoidance

As a result of the negative fitness consequences of inbreeding, vertebrate species have evolved mechanisms to avoid inbreeding. Numerous inbreeding avoidance mechanisms operating prior to mating have been described.Toads and many other amphibians display breeding site fidelity. Individuals that return to natal ponds to breed will likely encounter siblings as potential mates. Although incest is possible, Bufo americanus siblings rarely mate.JOURNAL, Waldman, B., Rice, J.E., Honeycutt, R.L., 1992, Kin recognition and incest avoidance in toads, Am. Zool., 32, 18–30, 10.1093/icb/32.1.18, These toads likely recognize and actively avoid close kins as mates. Advertisement vocalizations by males appear to serve as cues by which females recognize their kin.Inbreeding avoidance mechanisms can also operate subsequent to copulation. In guppies, a post-copulatory mechanism of inbreeding avoidance occurs based on competition between sperm of rival males for achieving fertilization.JOURNAL, Fitzpatrick, J.L., Evans, J.P., Postcopulatory inbreeding avoidance in guppies, J. Evol. Biol., 27, 12, 2585–94, 2014, 25387854, 10.1111/jeb.12545, In competitions between sperm from an unrelated male and from a full sibling male, a significant bias in paternity towards the unrelated male was observed.When female sand lizards mate with two or more males, sperm competition within the female's reproductive tract may occur. Active selection of sperm by females appears to occur in a manner that enhances female fitness.JOURNAL, Olsson, M., Shine, R., Madsen, T., Gullberg, A. Tegelström H, Sperm choice by females, Trends Ecol. Evol., 12, 11, 445–6, 1997, 21238151, 10.1016/s0169-5347(97)85751-5, On the basis of this selective process, the sperm of males that are more distantly related to the female are preferentially used for fertilization, rather than the sperm of close relatives. This preference may enhance the fitness of progeny by reducing inbreeding depression.

Outcrossing

Mating with unrelated or distantly related members of the same species is generally thought to provide the advantage of masking deleterious recessive mutations in progenyJOURNAL, Bernstein, H., Byerly, H.C., Hopf, F.A., Michod, R.E., Genetic damage, mutation, and the evolution of sex, Science, 229, 4719, 1277–81, 1985, 3898363, 10.1126/science.3898363, 1985Sci...229.1277B, (and see Heterosis). Vertebrates have evolved numerous diverse mechanisms for avoiding close inbreeding and promoting outcrossingJOURNAL, Pusey, A., Wolf, M., Inbreeding avoidance in animals, Trends Ecol. Evol., 11, 5, 201–6, 1996, 21237809, 10.1016/0169-5347(96)10028-8, (and see Inbreeding avoidance).Outcrossing as a way of avoiding inbreeding depression, has been especially well studied in birds. For instance, inbreeding depression occurs in the great tit when the offspring are produced as a result of a mating between close relatives. In natural populations of the great tit (Parus major), inbreeding is avoided by dispersal of individuals from their birthplace, which reduces the chance of mating with a close relative.JOURNAL, Szulkin, M., Sheldon, B.C., Dispersal as a means of inbreeding avoidance in a wild bird population, Proc. Biol. Sci., 275, 1635, 703–11, 2008, 18211876, 2596843, 10.1098/rspb.2007.0989, The purple-crowned fairywren females paired with related males may undertake extra-pair matings that can reduce the negative effects of inbreeding, despite ecological and demographic constraints. Southern pied babblers (Turdoides bicolor) appear to avoid inbreeding in two ways. The first is through dispersal, and the second is by avoiding familiar group members as mates.JOURNAL, Nelson-Flower, M.J., Hockey, P.A., O'Ryan, C., Ridley, A.R., Inbreeding avoidance mechanisms: dispersal dynamics in cooperatively breeding southern pied babblers, J Anim Ecol, 81, 4, 876–83, 2012, 22471769, 10.1111/j.1365-2656.2012.01983.x, Although both males and females disperse locally, they move outside the range where genetically related individuals are likely to be encountered. Within their group, individuals only acquire breeding positions when the opposite-sex breeder is unrelated.Cooperative breeding in birds typically occurs when offspring, usually males, delay dispersal from their natal group in order to remain with the family to help rear younger kin.JOURNAL, Riehl, C., Stern, C.A., How cooperatively breeding birds identify relatives and avoid incest: New insights into dispersal and kin recognition, BioEssays, 37, 12, 1303–8, 2015, 26577076, 10.1002/bies.201500120, Female offspring rarely stay at home, dispersing over distances that allow them to breed independently, or to join unrelated groups.

Parthenogenesis

Parthenogenesis is a natural form of reproduction in which growth and development of embryos occur without fertilization.Reproduction in squamate reptiles is ordinarily sexual, with males having a ZZ pair of sex determining chromosomes, and females a ZW pair. However, various species, including the Colombian Rainbow boa (Epicrates maurus), Agkistrodon contortrix (copperhead snake) and Agkistrodon piscivorus (cotton mouth snake) can also reproduce by facultative parthenogenesis -that is, they are capable of switching from a sexual mode of reproduction to an asexual mode- resulting in production of WW female progeny.JOURNAL, Booth, W., Smith, C.F., Eskridge, P.H., Hoss, S.K., Mendelson, J.R., Schuett, G.W., Facultative parthenogenesis discovered in wild vertebrates, Biol. Lett., 8, 6, 983–5, 2012, 22977071, 3497136, 10.1098/rsbl.2012.0666, JOURNAL, Booth, W., Million, L., Reynolds, R.G., Burghardt, G.M., Vargo, E.L., Schal, C., Tzika, A.C., Schuett, G.W., Consecutive virgin births in the new world boid snake, the Colombian rainbow Boa, Epicrates maurus, J. Hered., 102, 6, 759–63, 2011, 21868391, 10.1093/jhered/esr080, The WW females are likely produced by terminal automixis.Mole salamanders are an ancient (2.4-3.8 million year-old) unisexual vertebrate lineage.JOURNAL, Bogart, J.P., Bi, K., Fu, J., Noble, D.W., Niedzwiecki, J., Unisexual salamanders (genus Ambystoma) present a new reproductive mode for eukaryotes, Genome, 50, 2, 119–36, February 2007, 17546077, 10.1139/g06-152, In the polyploid unisexual mole salamander females, a premeiotic endomitotic event doubles the number of chromosomes. As a result, the mature eggs produced subsequent to the two meiotic divisions have the same ploidy as the somatic cells of the female salamander. Synapsis and recombination during meiotic prophase I in these unisexual females is thought to ordinarily occur between identical sister chromosomes and occasionally between homologous chromosomes. Thus little, if any, genetic variation is produced. Recombination between homeologous chromosomes occurs only rarely, if at all.JOURNAL, Bi, K, Bogart, J.P., Probing the meiotic mechanism of intergenomic exchanges by genomic in situ hybridization on lampbrush chromosomes of unisexual Ambystoma (Amphibia: Caudata), Chromosome Res., 18, 3, 371–82, April 2010, 20358399, 10.1007/s10577-010-9121-3, Since production of genetic variation is weak, at best, it is unlikely to provide a benefit sufficient to account for the long-term maintenance of meiosis in these organisms. However, meiosis may have been maintained during evolution by the efficient recombinational repair of DNA damages that meiosis provides, an advantage that could be realized at each generation.Bernstein, H.; Bernstein, C.; Michod, R.W. (2011). Meiosis as an Evolutionary Adaptation for DNA Repair. Chapter 19 pages 357-382 in "DNA Repair" (Inna Kruman editor). InTech Open Publisher. {{DOI|10.5772/25117}} {{ISBN|978-953-307-697-3}}weblink

Self-fertilization

The mangrove killifish (Kryptolebias marmoratus) produces both eggs and sperm by meiosis and routinely reproduces by self-fertilisation. The capacity for selfing in these fishes has apparently persisted for at least several hundred thousand years.JOURNAL, Tatarenkov, A., Lima, S.M., Taylor, D.S., Avise, J.C., Long-term retention of self-fertilization in a fish clade, Proc. Natl. Acad. Sci. U.S.A., 106, 34, 14456–9, 25 August 2009, 19706532, 2732792, 10.1073/pnas.0907852106, 2009PNAS..10614456T, Each individual hermaphrodite normally fertilizes itself when an egg and sperm that it has produced by an internal organ unite inside the fish's body.JOURNAL, Gonadal morphology in the self-fertilizing mangrove killifish, Kryptolebias marmoratus, 10.1007/s10228-006-0362-2, 53, 4, Ichthyological Research, 427–430, 2006, Sakakura, Yoshitaka, Soyano, Kiyoshi, Noakes, David L.G., Hagiwara, Atsushi, 10069/35713, In nature, this mode of reproduction can yield highly homozygous lines composed of individuals so genetically uniform as to be, in effect, identical to one another.JOURNAL, Avise, J.C., Tatarenkov, A., Allard's argument versus Baker's contention for the adaptive significance of selfing in a hermaphroditic fish, Proc. Natl. Acad. Sci. U.S.A., 109, 46, 18862–7, 13 November 2012, 23112206, 3503157, 10.1073/pnas.1217202109, 2012PNAS..10918862A, JOURNAL, Earley, R.L., Hanninen, A.F., Fuller, A., Garcia, M.J., Lee, E.A., Phenotypic plasticity and integration in the mangrove rivulus (Kryptolebias marmoratus): a prospectus, Integr. Comp. Biol., 52, 6, 814–27, 2012, 22990587, 3501102, 10.1093/icb/ics118, Although inbreeding, especially in the extreme form of self-fertilization, is ordinarily regarded as detrimental because it leads to expression of deleterious recessive alleles, self-fertilization does provide the benefit of fertilization assurance (reproductive assurance) at each generation.

See also

References

{{Reflist|30em}}

Bibliography

  • BOOK, Kardong, Kenneth V., Kenneth Kardong, Vertebrates: Comparative Anatomy, Function, Evolution, second, McGraw-Hill, 1998, USA, 747 pp,


, 978-0-697-28654-3,
  • {{ITIS |id=331030 |taxon=Vertebrata |accessdate=6 August 2007}}

External links

{{Wikispecies|Vertebrata}} {{Animalia}}{{Chordata}}{{Taxonbar|from=Q25241}}{{Authority control}}

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