Vertebrate

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For an explanation of similar terms, see Craniate. Vertebrate_sentence_0

Vertebrate_table_infobox_0

Vertebrate

Temporal range: Cambrian Stage 3Present, 520–0 Ma PreꞒ O S D C P T J K Pg NVertebrate_header_cell_0_0_0

Scientific classification VertebrataVertebrate_header_cell_0_1_0
Kingdom:Vertebrate_cell_0_2_0 AnimaliaVertebrate_cell_0_2_1
Superphylum:Vertebrate_cell_0_3_0 DeuterostomiaVertebrate_cell_0_3_1
Phylum:Vertebrate_cell_0_4_0 ChordataVertebrate_cell_0_4_1
Clade:Vertebrate_cell_0_5_0 OlfactoresVertebrate_cell_0_5_1
Subphylum:Vertebrate_cell_0_6_0 Vertebrata

J-B. Lamarck, 1801Vertebrate_cell_0_6_1

Simplified grouping (see text)Vertebrate_header_cell_0_7_0
SynonymsVertebrate_header_cell_0_8_0

Vertebrates (/ˈvɜːrtɪbrəts/) comprise all species of animals within the subphylum Vertebrata (/vɜːrtɪˈbreɪtə/) (chordates with backbones). Vertebrate_sentence_1

Vertebrates represent the overwhelming majority of the phylum Chordata, with currently about 69,963 species described. Vertebrate_sentence_2

Vertebrates include such groups as the following: Vertebrate_sentence_3

Vertebrate_unordered_list_0

Extant vertebrates range in size from the frog species Paedophryne amauensis, at as little as 7.7 mm (0.30 in), to the blue whale, at up to 33 m (108 ft). Vertebrate_sentence_4

Vertebrates make up less than five percent of all described animal species; the rest are invertebrates, which lack vertebral columns. Vertebrate_sentence_5

The vertebrates traditionally include the hagfish, which do not have proper vertebrae due to their loss in evolution, though their closest living relatives, the lampreys, do. Vertebrate_sentence_6

Hagfish do, however, possess a cranium. Vertebrate_sentence_7

For this reason, the vertebrate subphylum is sometimes referred to as "Craniata" when discussing morphology. Vertebrate_sentence_8

Molecular analysis since 1992 has suggested that hagfish are most closely related to lampreys, and so also are vertebrates in a monophyletic sense. Vertebrate_sentence_9

Others consider them a sister group of vertebrates in the common taxon of craniata. Vertebrate_sentence_10

The populations of vertebrates have dropped in the past 50 years. Vertebrate_sentence_11

Etymology Vertebrate_section_0

The word vertebrate derives from the Latin word vertebratus (Pliny), meaning joint of the spine. Vertebrate_sentence_12

Vertebrate is derived from the word vertebra, which refers to any of the bones or segments of the spinal column. Vertebrate_sentence_13

Anatomy and morphology Vertebrate_section_1

See also: Vertebrate anatomy Vertebrate_sentence_14

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), with a hollow tube of nervous tissue (the spinal cord) above it and the gastrointestinal tract below. Vertebrate_sentence_15

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. Vertebrate_sentence_16

The remaining part of the body continuing after the anus forms a tail with vertebrae and spinal cord, but no gut. Vertebrate_sentence_17

Vertebral column Vertebrate_section_2

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). Vertebrate_sentence_18

However, a few vertebrates have secondarily lost this anatomy, retaining the notochord into adulthood, such as the sturgeon and coelacanth. Vertebrate_sentence_19

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. Vertebrate_sentence_20

Gills Vertebrate_section_3

All basal vertebrates breathe with gills. Vertebrate_sentence_21

The gills are carried right behind the head, bordering the posterior margins of a series of openings from the pharynx to the exterior. Vertebrate_sentence_22

Each gill is supported by a cartilagenous or bony gill arch. Vertebrate_sentence_23

The bony fish have three pairs of arches, cartilaginous fish have five to seven pairs, while the primitive jawless fish have seven. Vertebrate_sentence_24

The vertebrate ancestor no doubt had more arches than this, as some of their chordate relatives have more than 50 pairs of gills. Vertebrate_sentence_25

In amphibians and some primitive bony fishes, the larvae bear external gills, branching off from the gill arches. Vertebrate_sentence_26

These are reduced in adulthood, their function taken over by the gills proper in fishes and by lungs in most amphibians. Vertebrate_sentence_27

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. Vertebrate_sentence_28

While 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. Vertebrate_sentence_29

Central nervous system Vertebrate_section_4

The central nervous system of vertebrates is based on a hollow nerve cord running along the length of the animal. Vertebrate_sentence_30

Of particular importance and unique to vertebrates is the presence of neural crest cells. Vertebrate_sentence_31

These are progenitors of stem cells, and critical to coordinating the functions of cellular components. Vertebrate_sentence_32

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. Vertebrate_sentence_33

The vertebrates are the only chordate group to exhibit cephalisation, the concentration of brain functions in the head. Vertebrate_sentence_34

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. Vertebrate_sentence_35

Other chordates do not show any trends towards cephalisation. Vertebrate_sentence_36

A peripheral nervous system branches out from the nerve cord to innervate the various systems. Vertebrate_sentence_37

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. Vertebrate_sentence_38

Two laterally placed eyes form around outgrowths from the midbrain, except in hagfish, though this may be a secondary loss. Vertebrate_sentence_39

The forebrain is well-developed and subdivided in most tetrapods, while the midbrain dominates in many fish and some salamanders. Vertebrate_sentence_40

Vesicles of the forebrain are usually paired, giving rise to hemispheres like the cerebral hemispheres in mammals. Vertebrate_sentence_41

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. Vertebrate_sentence_42

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. Vertebrate_sentence_43

Evolutionary history Vertebrate_section_5

See also: Evolution of fish and Evolution of tetrapods Vertebrate_sentence_44

First vertebrates Vertebrate_section_6

Vertebrates originated about 525 million years ago during the Cambrian explosion, which saw rise in organism diversity. Vertebrate_sentence_45

The earliest known vertebrate is believed to be the mylinofulale. Vertebrate_sentence_46

Another early vertebrate is Haikouichthys ercaicunensis. Vertebrate_sentence_47

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. Vertebrate_sentence_48

All of these early vertebrates lacked jaws in the common sense and relied on filter feeding close to the seabed. Vertebrate_sentence_49

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. Vertebrate_sentence_50

From fish to amphibians Vertebrate_section_7

The first jawed vertebrates may have appeared in the late Ordovician and became common in the Devonian, often known as the "Age of Fishes". Vertebrate_sentence_51

The two groups of bony fishes, the actinopterygii and sarcopterygii, evolved and became common. Vertebrate_sentence_52

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. Vertebrate_sentence_53

The Devonian also saw the rise of the first labyrinthodonts, which was a transitional form between fishes and amphibians. Vertebrate_sentence_54

Mesozoic vertebrates Vertebrate_section_8

Amniotes branched from labyrinthodonts in the subsequent Carboniferous period. Vertebrate_sentence_55

The Parareptilia and synapsid amniotes were common during the late Paleozoic, while diapsids became dominant during the Mesozoic. Vertebrate_sentence_56

In the sea, the bony fishes became dominant. Vertebrate_sentence_57

Birds, a derived form of dinosaur, evolved in the Jurassic. Vertebrate_sentence_58

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. Vertebrate_sentence_59

After the Mesozoic Vertebrate_section_9

The Cenozoic world has seen great diversification of bony fishes, amphibians, reptiles, birds and mammals. Vertebrate_sentence_60

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 4,600 metres (15,100 feet) to flatfishes (order Pleuronectiformes) in the Challenger Deep, the deepest ocean trench at about 11,000 metres (36,000 feet). Vertebrate_sentence_61

Fishes of myriad varieties are the main predators in most of the world's water bodies, both freshwater and marine. Vertebrate_sentence_62

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). Vertebrate_sentence_63

Tetrapods comprise the dominant megafauna of most terrestrial environments and also include many partially or fully aquatic groups (e.g., sea snakes, penguins, cetaceans). Vertebrate_sentence_64

Classification Vertebrate_section_10

There are several ways of classifying animals. Vertebrate_sentence_65

Evolutionary systematics relies on anatomy, physiology and evolutionary history, which is determined through similarities in anatomy and, if possible, the genetics of organisms. Vertebrate_sentence_66

Phylogenetic classification is based solely on phylogeny. Vertebrate_sentence_67

Evolutionary systematics gives an overview; phylogenetic systematics gives detail. Vertebrate_sentence_68

The two systems are thus complementary rather than opposed. Vertebrate_sentence_69

Traditional classification Vertebrate_section_11

Conventional classification has living vertebrates grouped into seven classes based on traditional interpretations of gross anatomical and physiological traits. Vertebrate_sentence_70

This classification is the one most commonly encountered in school textbooks, overviews, non-specialist, and popular works. Vertebrate_sentence_71

The extant vertebrates are: Vertebrate_sentence_72

Vertebrate_unordered_list_1

  • Subphylum VertebrataVertebrate_item_1_4
    • Class Agnatha (jawless fishes)Vertebrate_item_1_5
    • Class Chondrichthyes (cartilaginous fishes)Vertebrate_item_1_6
    • Class Osteichthyes (bony fishes)Vertebrate_item_1_7
    • Class Amphibia (amphibians)Vertebrate_item_1_8
    • Class Reptilia (reptiles)Vertebrate_item_1_9
    • Class Aves (birds)Vertebrate_item_1_10
    • Class Mammalia (mammals)Vertebrate_item_1_11

In addition to these, there are two classes of extinct armoured fishes, the Placodermi and the Acanthodii. Vertebrate_sentence_73

Other ways of classifying the vertebrates have been devised, particularly with emphasis on the phylogeny of early amphibians and reptiles. Vertebrate_sentence_74

An example based on Janvier (1981, 1997), Shu et al. Vertebrate_sentence_75

(2003), and Benton (2004) is given here: Vertebrate_sentence_76

†: Extinct Vertebrate_sentence_77

Vertebrate_unordered_list_2

  • Subphylum VertebrataVertebrate_item_2_12
    • PalaeospondylusVertebrate_item_2_13
    • Superclass Agnatha or Cephalaspidomorphi (lampreys and other jawless fishes)Vertebrate_item_2_14
    • Infraphylum Gnathostomata (vertebrates with jaws)Vertebrate_item_2_15
      • Class †Placodermi (extinct armoured fishes)Vertebrate_item_2_16
      • Class Chondrichthyes (cartilaginous fishes)Vertebrate_item_2_17
      • Class †Acanthodii (extinct spiny "sharks")Vertebrate_item_2_18
      • Superclass Osteichthyes (bony vertebrates)Vertebrate_item_2_19
        • Class Actinopterygii (ray-finned bony fishes)Vertebrate_item_2_20
        • Class Sarcopterygii (lobe-finned fishes, including the tetrapods)Vertebrate_item_2_21
      • Superclass Tetrapoda (four-limbed vertebrates)Vertebrate_item_2_22
        • Class Amphibia (amphibians, some ancestral to the amniotes)—now a paraphyletic groupVertebrate_item_2_23
        • Class Synapsida (mammals and the extinct mammal-like reptiles)Vertebrate_item_2_24
        • Class Sauropsida (reptiles and birds)Vertebrate_item_2_25

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. Vertebrate_sentence_78

For instance, descendants of the first reptiles include modern reptiles as well as mammals and birds; 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. Vertebrate_sentence_79

Most scientists working with vertebrates use a classification based purely on phylogeny, organized by their known evolutionary history and sometimes disregarding the conventional interpretations of their anatomy and physiology. Vertebrate_sentence_80

Phylogenetic relationships Vertebrate_section_12

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. Vertebrate_sentence_81

The one below is based on studies compiled by Philippe Janvier and others for the Tree of Life Web Project and Delsuc et al. Vertebrate_sentence_82

† denotes an entirely extinct clade. Vertebrate_sentence_83

Number of extant species Vertebrate_section_13

The number of described vertebrate species are split evenly between tetrapods and fish. Vertebrate_sentence_84

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. Vertebrate_sentence_85

Vertebrate_table_general_1

Vertebrate groupsVertebrate_header_cell_1_0_0 ImageVertebrate_header_cell_1_0_3 ClassVertebrate_header_cell_1_0_4 Estimated number of

described speciesVertebrate_header_cell_1_0_5

Group

totalsVertebrate_header_cell_1_0_6

Anamniote

lack amniotic membrane so need to reproduce in waterVertebrate_header_cell_1_1_0

JawlessVertebrate_header_cell_1_1_1 FishVertebrate_header_cell_1_1_2 Vertebrate_cell_1_1_3 Myxini

(hagfish)Vertebrate_cell_1_1_4

Vertebrate_cell_1_1_5 32,900Vertebrate_cell_1_1_6
Vertebrate_cell_1_2_0 Hyperoartia

(lamprey)Vertebrate_cell_1_2_1

Vertebrate_cell_1_2_2
JawedVertebrate_header_cell_1_3_0 Vertebrate_cell_1_3_1 cartilaginous fishVertebrate_cell_1_3_2 Vertebrate_cell_1_3_3
Vertebrate_cell_1_4_0 ray-finned fishVertebrate_cell_1_4_1 Vertebrate_cell_1_4_2
Vertebrate_cell_1_5_0 lobe-finned fishVertebrate_cell_1_5_1 Vertebrate_cell_1_5_2
TetrapodsVertebrate_header_cell_1_6_0 Vertebrate_cell_1_6_1 amphibiansVertebrate_cell_1_6_2 7,302Vertebrate_cell_1_6_3 33,278Vertebrate_cell_1_6_4
Amniote

have amniotic membrane adapted to reproducing on landVertebrate_header_cell_1_7_0

Vertebrate_cell_1_7_1 mammalsVertebrate_cell_1_7_2 5,513Vertebrate_cell_1_7_3
Vertebrate_cell_1_8_0 reptilesVertebrate_cell_1_8_1 10,711Vertebrate_cell_1_8_2
Vertebrate_cell_1_9_0 birdsVertebrate_cell_1_9_1 10,425Vertebrate_cell_1_9_2
Total described speciesVertebrate_cell_1_10_0 66,178Vertebrate_cell_1_10_6

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_sentence_86

Vertebrate species databases Vertebrate_section_14

The following databases maintain (more or less) up-to-date lists of vertebrate species: Vertebrate_sentence_87

Vertebrate_unordered_list_3

  • Fish: FishbaseVertebrate_item_3_26
  • Amphibians:Vertebrate_item_3_27
  • Reptiles: Reptile DatabaseVertebrate_item_3_28
  • Birds:Vertebrate_item_3_29
  • Mammals:Vertebrate_item_3_30

Reproductive systems Vertebrate_section_15

Nearly all vertebrates undergo sexual reproduction. Vertebrate_sentence_88

They produce haploid gametes by meiosis. Vertebrate_sentence_89

The smaller, motile gametes are spermatozoa and the larger, non-motile gametes are ova. Vertebrate_sentence_90

These fuse by the process of fertilisation to form diploid zygotes, which develop into new individuals. Vertebrate_sentence_91

Inbreeding Vertebrate_section_16

During sexual reproduction, mating with a close relative (inbreeding) often leads to inbreeding depression. Vertebrate_sentence_92

Inbreeding depression is considered to be largely due to expression of deleterious recessive mutations. Vertebrate_sentence_93

The effects of inbreeding have been studied in many vertebrate species. Vertebrate_sentence_94

In several species of fish, inbreeding was found to decrease reproductive success. Vertebrate_sentence_95

Inbreeding was observed to increase juvenile mortality in 11 small animal species. Vertebrate_sentence_96

A common breeding practice for pet dogs is mating between close relatives (e.g. between half- and full siblings). Vertebrate_sentence_97

This practice generally has a negative effect on measures of reproductive success, including decreased litter size and puppy survival. Vertebrate_sentence_98

Incestuous matings in birds result in severe fitness costs due to inbreeding depression (e.g. reduction in hatchability of eggs and reduced progeny survival). Vertebrate_sentence_99

Inbreeding avoidance Vertebrate_section_17

As a result of the negative fitness consequences of inbreeding, vertebrate species have evolved mechanisms to avoid inbreeding. Vertebrate_sentence_100

Numerous inbreeding avoidance mechanisms operating prior to mating have been described. Vertebrate_sentence_101

Toads and many other amphibians display breeding site fidelity. Vertebrate_sentence_102

Individuals that return to natal ponds to breed will likely encounter siblings as potential mates. Vertebrate_sentence_103

Although incest is possible, Bufo americanus siblings rarely mate. Vertebrate_sentence_104

These toads likely recognize and actively avoid close kin as mates. Vertebrate_sentence_105

Advertisement vocalizations by males appear to serve as cues by which females recognize their kin. Vertebrate_sentence_106

Inbreeding avoidance mechanisms can also operate subsequent to copulation. Vertebrate_sentence_107

In guppies, a post-copulatory mechanism of inbreeding avoidance occurs based on competition between sperm of rival males for achieving fertilization. Vertebrate_sentence_108

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. Vertebrate_sentence_109

When female sand lizards mate with two or more males, sperm competition within the female's reproductive tract may occur. Vertebrate_sentence_110

Active selection of sperm by females appears to occur in a manner that enhances female fitness. Vertebrate_sentence_111

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. Vertebrate_sentence_112

This preference may enhance the fitness of progeny by reducing inbreeding depression. Vertebrate_sentence_113

Outcrossing Vertebrate_section_18

Mating with unrelated or distantly related members of the same species is generally thought to provide the advantage of masking deleterious recessive mutations in progeny (see heterosis). Vertebrate_sentence_114

Vertebrates have evolved numerous diverse mechanisms for avoiding close inbreeding and promoting outcrossing (see inbreeding avoidance). Vertebrate_sentence_115

Outcrossing as a way of avoiding inbreeding depression has been especially well studied in birds. Vertebrate_sentence_116

For instance, inbreeding depression occurs in the great tit (Parus major) when the offspring are produced as a result of a mating between close relatives. Vertebrate_sentence_117

In natural populations of the great tit, inbreeding is avoided by dispersal of individuals from their birthplace, which reduces the chance of mating with a close relative. Vertebrate_sentence_118

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. Vertebrate_sentence_119

Southern pied babblers (Turdoides bicolor) appear to avoid inbreeding in two ways: through dispersal and by avoiding familiar group members as mates. Vertebrate_sentence_120

Although both males and females disperse locally, they move outside the range where genetically related individuals are likely to be encountered. Vertebrate_sentence_121

Within their group, individuals only acquire breeding positions when the opposite-sex breeder is unrelated. Vertebrate_sentence_122

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. Vertebrate_sentence_123

Female offspring rarely stay at home, dispersing over distances that allow them to breed independently or to join unrelated groups. Vertebrate_sentence_124

Parthenogenesis Vertebrate_section_19

Parthenogenesis is a natural form of reproduction in which growth and development of embryos occur without fertilization. Vertebrate_sentence_125

Reproduction in squamate reptiles is ordinarily sexual, with males having a ZZ pair of sex determining chromosomes, and females a ZW pair. Vertebrate_sentence_126

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. Vertebrate_sentence_127

The WW females are likely produced by terminal automixis. Vertebrate_sentence_128

Mole salamanders are an ancient (2.4–3.8 million year-old) unisexual vertebrate lineage. Vertebrate_sentence_129

In the polyploid unisexual mole salamander females, a premeiotic endomitotic event doubles the number of chromosomes. Vertebrate_sentence_130

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. Vertebrate_sentence_131

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. Vertebrate_sentence_132

Thus little, if any, genetic variation is produced. Vertebrate_sentence_133

Recombination between homeologous chromosomes occurs only rarely, if at all. Vertebrate_sentence_134

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. Vertebrate_sentence_135

Self-fertilization Vertebrate_section_20

The mangrove killifish (Kryptolebias marmoratus) produces both eggs and sperm by meiosis and routinely reproduces by self-fertilisation. Vertebrate_sentence_136

This capacity has apparently persisted for at least several hundred thousand years. Vertebrate_sentence_137

Each individual hermaphrodite normally fertilizes itself through uniting inside the fish's body of an egg and a sperm that it has produced by an internal organ. Vertebrate_sentence_138

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. Vertebrate_sentence_139

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. Vertebrate_sentence_140

Population trends Vertebrate_section_21

The Living Planet Index, following 16,704 populations of 4,005 species of vertebrates, shows a decline of 60% between 1970 and 2014. Vertebrate_sentence_141

Since 1970, freshwater species declined 83%, and tropical populations in South and Central America declined 89%. Vertebrate_sentence_142

The authors note that, "An average trend in population change is not an average of total numbers of animals lost." Vertebrate_sentence_143

According to WWF, this could lead to a sixth major extinction event. Vertebrate_sentence_144

The five main causes of biodiversity loss are land-use change, overexploitation of natural resources, climate change, pollution and invasive species. Vertebrate_sentence_145

See also Vertebrate_section_22

Vertebrate_unordered_list_4


Credits to the contents of this page go to the authors of the corresponding Wikipedia page: en.wikipedia.org/wiki/Vertebrate.