The Miocene is the first geological epoch of the Neogene Period and extends from about 23.03 to 5.333 million years ago. The Miocene was named by Charles Lyell; the Miocene is followed by the Pliocene. As the earth went from the Oligocene through the Miocene and into the Pliocene, the climate cooled towards a series of ice ages; the Miocene boundaries are not marked by a single distinct global event but consist rather of regionally defined boundaries between the warmer Oligocene and the cooler Pliocene Epoch. The Apes first evolved and diversified during the early Miocene, becoming widespread in the Old World. By the end of this epoch and the start of the following one, the ancestors of humans had split away from the ancestors of the chimpanzees to follow their own evolutionary path during the final Messinian stage of the Miocene; as in the Oligocene before it, grasslands continued to forests to dwindle in extent. In the seas of the Miocene, kelp forests made their first appearance and soon became one of Earth's most productive ecosystems.
The plants and animals of the Miocene were recognizably modern. Mammals and birds were well-established. Whales and kelp spread; the Miocene is of particular interest to geologists and palaeoclimatologists as major phases of the geology of the Himalaya occurred during the Miocene, affecting monsoonal patterns in Asia, which were interlinked with glacial periods in the northern hemisphere. The Miocene faunal stages from youngest to oldest are named according to the International Commission on Stratigraphy: Regionally, other systems are used, based on characteristic land mammals. Of the modern geologic features, only the land bridge between South America and North America was absent, although South America was approaching the western subduction zone in the Pacific Ocean, causing both the rise of the Andes and a southward extension of the Meso-American peninsula. Mountain building took place in western North America and East Asia. Both continental and marine Miocene deposits are common worldwide with marine outcrops common near modern shorelines.
Well studied continental exposures occur in Argentina. India continued creating dramatic new mountain ranges; the Tethys Seaway continued to shrink and disappeared as Africa collided with Eurasia in the Turkish–Arabian region between 19 and 12 Ma. The subsequent uplift of mountains in the western Mediterranean region and a global fall in sea levels combined to cause a temporary drying up of the Mediterranean Sea near the end of the Miocene; the global trend was towards increasing aridity caused by global cooling reducing the ability of the atmosphere to absorb moisture. Uplift of East Africa in the late Miocene was responsible for the shrinking of tropical rain forests in that region, Australia got drier as it entered a zone of low rainfall in the Late Miocene. During the Oligocene and Early Miocene the coast of northern Brazil, south-central Peru, central Chile and large swathes of inland Patagonia were subject to a marine transgression; the transgressions in the west coast of South America is thought to be caused by a regional phenomenon while the rising central segment of the Andes represents an exception.
While there are numerous registers of Oligo-Miocene transgressions around the world it is doubtful that these correlate. It is thought that the Oligo-Miocene transgression in Patagonia could have temporarily linked the Pacific and Atlantic Oceans, as inferred from the findings of marine invertebrate fossils of both Atlantic and Pacific affinity in La Cascada Formation. Connection would have occurred through narrow epicontinental seaways that formed channels in a dissected topography; the Antarctic Plate started to subduct beneath South America 14 million years ago in the Miocene, forming the Chile Triple Junction. At first the Antarctic Plate subducted only in the southernmost tip of Patagonia, meaning that the Chile Triple Junction lay near the Strait of Magellan; as the southern part of Nazca Plate and the Chile Rise became consumed by subduction the more northerly regions of the Antarctic Plate begun to subduct beneath Patagonia so that the Chile Triple Junction advanced to the north over time.
The asthenospheric window associated to the triple junction disturbed previous patterns of mantle convection beneath Patagonia inducing an uplift of ca. 1 km that reversed the Oligocene–Miocene transgression. Climates remained moderately warm, although the slow global cooling that led to the Pleistocene glaciations continued. Although a long-term cooling trend was well underway, there is evidence of a warm period during the Miocene when the global climate rivalled that of the Oligocene; the Miocene warming b
Northern white-cheeked gibbon
The northern white-cheeked gibbon is a species of gibbon native to South East Asia. It is related to the southern white-cheeked gibbon, with which it was considered conspecific; the females of the two species are indistinguishable in appearance. The genome of N. leucogenys was sequenced and published in 2011. A "substantial" population of 455 critically endangered northern white-cheeked crested gibbons has been found living in the Pù Mát National Park in Nghệ An Province, northern Vietnam, near the border with Laos. Conservation International report they are living at high altitudes, far from human settlements; this population, representing two-thirds of the total known in Vietnam are the "only confirmed viable population" of this variety in the world. Northern white-cheeked gibbons are sexually dimorphic, with males and females having different colourations and the former being larger. Males have black hair over their entire bodies, except for distinct white patches on their cheeks, as well as a prominent tuft of hair on the crown of head, a gular sac.
Females are reddish-tan in colour, lack a cranial tuft, have a crest of black or dark brown fur running from the crown to the nape of the neck. They are reported to have an average weight of 7.5 kg, although this is based on only a small number of wild individuals, those in captivity appear to be larger. Like other members of their genus, both males and females have unusually long arms for gibbons, with the arms being 1.2 to 1.4 times as long as the legs. They are more muscular, with heavier thighs and shoulders that suggest a greater bodily strength. Adults have been shown to demonstrate a hand preference while swinging through the trees, with individuals being likely to be right or left handed; the species resembles the southern white-cheeked gibbon, but has longer body hair and subtly different vocalisations. The males can be distinguished by the shape of the white patches on their cheeks. Both males and females have been reported to produce reddish-brown secretions from glands around their upper chest and ankles.
However, samples of sweat taken from the axillae and chest possess lower levels of steroids in white-cheeked gibbons than in many other species of ape, suggesting that olfactory signals may be less important in these animals than in their relatives. Today, the northern white-cheeked gibbon is found only in northern Laos, they were also known from southern China, in Yunnan province, where they were reported to be on the edge of extirpation in 2008. No subspecies are recognised, although the southern white-cheeked gibbon was considered to be a subspecies of N. leucogenys. The gibbon inhabits primary evergreen subtropical forest between 1,650 metres in elevation; the northern white-cheeked gibbon is arboreal in habits, herbivorous, feeding on fruits, with some leaves and flowers. However, up to 10 % of their diet may be composed of other small animals, they are sociable, living in groups of up to six individuals. Individual groups do not travel far, are believed to be territorial, they are diurnal, spend the night sleeping in high branches embracing one another tightly.
Behavioural studies have demonstrated. The calls of northern white-cheeked gibbons are among the most complex of those produced by gibbons, are different between males and females; the most distinctive calls are those made as part of male-female duets. These begin with the female making a series of 15 to 30 notes with an increasing pitch, followed by the male complex call with rapid changes of frequency modulation; the cycle, which lasts less than 20 seconds repeats with increasing intensity for five to 17 minutes. In the related southern species, such duets are most common at dawn, are only made on sunny days. In captive studies and females that sing duets together the most are the most to mate, indicating this may play a key role in pair-bonding. Similar calls are sometimes made solo by both sexes, juveniles sometimes join in, to create a full'chorus'. In addition to the duet and solo great calls, males can make booming sounds with their gular sacs, short single notes. Northern white-cheeked gibbons are monogamous, with long-lasting pair bonds.
The ovarian cycle has been reported to last an average of 22 days, gestation lasts 200 to 212 days. At birth, both sexes are covered in yellow-buff fur, weigh an average of 480 g. Around one year of age, the fur in both sexes changes to a black colour, with pale cheek patches, with the sexually dimorphic adult coats only growing when they reach four or five years. During this period, the juveniles sing the female form of call, engage in play behaviour. Northern white-cheeked gibbons reach sexual maturity at seven or eight years, have lived for at least 28 years in the wild. Northern White-cheeked Gibbon at Animal Diversity Web View the gibbon genome on Ensembl
The genus Pan consists of two extant species: the common chimpanzee and the bonobo. Taxonomically, these two ape species are collectively termed panins. Together with humans and orangutans they are part of the family Hominidae. Native to sub-Saharan Africa, common chimpanzees and bonobos are both found in the Congo jungle, while only the common chimpanzee is found further north in West Africa. Both species are listed as endangered on the IUCN Red List of Threatened Species, in 2017 the Convention on Migratory Species selected the common chimpanzee for special protection; the common chimpanzee who live north of the Congo River, the bonobo who live south of it, were once considered to be the same species, but since 1928 they have been recognized as distinct. In addition, P. troglodytes is divided into four subspecies. Based on genome sequencing, these two extant Pan species diverged around one million years ago; the most obvious differences are that chimpanzees are somewhat larger, more aggressive and male-dominated, while the bonobos are more gracile and female-dominated.
Their hair is black or brown. Males and females differ in appearance. Both chimps and bonobos are some of the most social great apes, with social bonds occurring throughout large communities. Fruit is the most important component of a chimpanzee's diet, they can live over 30 years in both the captivity. Chimpanzees and bonobos are humanity's closest living relatives; as such, they are among the largest-brained and most intelligent primates: they use a variety of sophisticated tools and construct elaborate sleeping nests each night from branches and foliage. Their learning abilities have been extensively studied. There may be distinctive cultures within populations. Field studies of Pan troglodytes were pioneered by primatologist Jane Goodall. Both Pan species are considered to be endangered as human activities have caused severe declines in the populations and ranges of both species. Threats to wild panina populations include poaching, habitat destruction, the illegal pet trade. Several conservation and rehabilitation organisations are dedicated to the survival of Pan species in the wild.
The genus name Pan was first introduced by Lorenz Oken in 1816. An alternative Theranthropus was suggested by Brookes 1828 and Chimpansee by Voigt 1831. Troglodytes was not available, as it had been given as the name of a genus of wren in 1809; the International Commission on Zoological Nomenclature adopted Pan as the only official name of the genus in 1895. The name is a reference to the Greek god of nature and wilderness. In his book The Third Chimpanzee, Jared Diamond proposes that P. troglodytes and P. paniscus belong with H. sapiens in the genus Homo, rather than in Pan. He argues that other species have been reclassified by genus for less genetic similarity than that between humans and chimpanzees; the first use of the name "chimpanze" is recorded in The London Magazine in 1738, glossed as meaning "mockman" in a language of "the Angolans". The spelling chimpanzee is found in a 1758 supplement to Chamber's Cyclopædia; the colloquialism "chimp" was most coined some time in the late 1870s. The common chimpanzee was named Simia troglodytes by Johann Friedrich Blumenbach in 1776.
The species name troglodytes is a reference to the Troglodytae, an African people described by Greco-Roman geographers. Blumenbach first used it in his De generis humani varietate nativa liber in 1776, Linnaeus 1758 had used Homo troglodytes for a hypothetical mixture of human and orangutan; the bonobo, in the past referred to as the "pygmy chimpanzee", was given the species name of paniscus by Ernst Schwarz, a diminutive of the theonym Pan. There are two species of the genus Pan, both called Chimpanzees: Common chimpanzees or Pan troglodytes, are found exclusively in the forested regions of Central and West Africa. With at least four accepted subspecies, their population and distribution is much more extensive than the Bonobos, in the past called'Pygmy Chimpanzee'. Bonobos, Pan paniscus, are found only in Central Africa, south of the Congo River and north of the Kasai River, in the humid forests of the Democratic Republic of Congo of Central Africa; the genus Pan is part of the subfamily Homininae, to which humans belong.
The lineages of chimpanzees and humans separated in a process of speciation between five to twelve million years ago, making them humanity's closest living relative. Research by Mary-Claire King in 1973 found 99 % identical DNA between human chimpanzees. For some time, research modified that finding to about 94% commonality, with some of the difference occurring in noncoding DNA, but more recent knowledge states the difference in DNA between humans and bonobos at just about 1%–1.2% again. The chimpanzee fossil record has long been absent and thought to have been due to the preservation bias in relation to their environment. However, in 2005, chimpanzee fossils were discovered and described by Sally McBrearty and colleagues. Existing chimpanzee populations in West and Central Africa are separate from the major human fossil sites in East Africa.
The siamang is an arboreal black-furred gibbon native to the forests of Indonesia and Thailand. The largest of the gibbons, the siamang can be twice the size of other gibbons, reaching 1 m in height, weighing up to 14 kg; the siamang is the only species in the genus Symphalangus. The siamang is distinctive for two reasons; the first is that two digits on each foot are joined by a membrane—hence the name "syndactylus", from the Ancient Greek sun-, "united" + daktulos, "finger". The second is the large gular sac, a throat pouch that can be inflated to the size of the siamang's head, allowing it to make loud, resonating calls or songs. Two subspecies of the siamang may exist. If so, they are the Malaysian siamang. Otherwise, the Malaysian individuals are only a population; the siamang occurs sympatrically with other gibbons. The siamang can live to around 40 years in captivity. While the illegal pet trade takes a toll on wild populations, the principal threat to the siamang is habitat loss in both Indonesia and Malaysia.
The palm oil production industry is clearing large swaths of forest, reducing the habitat of the siamang, along with that of other species, such as the Sumatran tiger. The siamang has long, shaggy hair, the darkest shade of all gibbons; the ape has gangling arms that are longer than its legs. The average length of a siamang is 90 cm, but the largest they have grown is 1 m 50 cm; the face of this large gibbon is hairless apart from a thin mustache. The siamang inhabits the forest remnants of Sumatra Island and the Malay Peninsula, is distributed from lowland forest to mountain forest—even rainforest—and can be found at altitudes up to 3800 m; the siamang lives in groups of up to six individuals with an average home range of 23 hectares. Their day ranges are smaller than those of sympatric Hylobates species less than 1 km; the siamang's melodious singing breaks the forest's silence in the early morning after the agile gibbons' or lar gibbons' calls. The siamang in Sumatra and the Malay Peninsula are similar in appearance, but some behaviors differ between the two populations.
The siamang eats various parts of plants. The Sumatran siamang is more frugivorous than its Malayan relative, with fruit making up to 60% of its diet; the siamang eats at least 160 species of plants, from vines to woody plants. Its major food is figs (Ficus spp; the siamang prefers to eat ripe rather than unripe fruit, young rather than old leaves. It eats flowers and a few animals insects; when the siamang eats large flowers, it eats only the corollae, but it eats all parts of smaller flowers, with the small fruit collected in its hand before being consumed. When it eats big and hard seeds or seeds with sharp edges, it peels out the fruit flesh and throws away the seed. Although its diet consists of substantial portions of fruit, it is the most folivorous of all members of Hylobatidae; as it is the largest gibbon, it fits well with the general primate dietary trend in which larger primates tend to be more folivorous. A group of siamangs consists of an adult dominant male, an adult dominant female, with offspring and sometimes a subadult.
The subadult leaves the group after attaining the age of 6–8 years. Siamang gestation period is between 6.2 and 7.9 months. Siamang males tend to offer more paternal care than do other members of the family Hylobatidae, taking up a major role in carrying an infant after it is about 8 months old; the infant returns to its mother to sleep and nurse. The infant begins to travel independently from its parents by its third year of life. Siamangs are known to have monogamous mating pairs, which have been documented to spend more time in close proximity to each other, in comparison to other gibbon species. However, both monogamous and polyandrous groups are found in south Sumatra. In studying these populations, infants belonging to monogamous groups were found to receive more overall male care than infants in the polyandrous groups; this reduced care is most due to reduced certainty of paternity in these groups. Habitat disturbance affects siamang group composition; the burnt, regrown forest population contained more adult and subadults than the intact forest population, which had more infants, small juveniles, large juveniles.
Infant survival rates in burnt, regrown forest groups are lower than in intact forest groups. The number of individuals in the latter is higher than in the former; the siamang in disturbed forests live in small groups and have a density lower than in intact forests because of lack of food resources and trees for living. In the 1980s, the Indonesian population of the siamang in the wild was estimated to be 360,000 individuals; this seems overestimated today, as an example, Bukit Barisan Selatan National Park is the third-largest protected area in Sumatra, of which 2,570 km² remain under forest cover inhabited by 22,390 siamangs. In Sumatra, the siamang prefers to inhabit 1000 m above sea level; the siamang tends to rest for more than 50% of its waking period, followed by feeding, moving and social activities. It takes more rest d
The Hominini, or hominins, form a taxonomic tribe of the subfamily Homininae. Hominini excludes the genus Gorilla; as of 2019, there is no consensus on whether it should include the genus Pan, the question being tied to the complex speciation process connecting humans and chimpanzees and the development of bipedalism in proto-humans. The tribe was introduced by John Edward Gray, long before any details on the speciation of Pan and Homo were known. Gray's tribe Hominini by definition includes both Homo; this definition is still adhered to in the proposal by Mann and Weiss, which divides Hominini into three subtribes, Panina and Australopithecina. Alternatively, Hominini is taken to exclude Pan. In this case, Panini may be used to refer to the tribe containing Pan as its only genus. Minority dissenting nomenclatures include Gorilla in Hominini and Pan in Homo, or both Pan and Gorilla in Homo. By convention, the adjectival term "hominin" refers to the tribe Hominini, while the members of the Hominina subtribe are referred to as "homininan".
This follows the proposal by Mann and Weiss, which presents tribe Hominini as including both Pan and Homo, placed in separate subtribes. The genus Pan is referred to subtribe Panina, genus Homo is included in the subtribe Hominina. However, there is an alternative convention which uses "hominin" to exclude members of Panina, i.e. either just for Homo or for both human and australopithecine species. This alternative convention is referenced in Dunbar. Potts in addition uses the name Hominini in a different sense, as excluding Pan, uses "hominins" for this, while a separate tribe for chimpanzees is introduced, under the name Panini. In this recent convention, contra Gray, the term "hominin" is applied to Homo, Australopithecus and others that arose after the split from the line that led to chimpanzees; this cladogram shows the clade of superfamily Hominoidea and its descendent clades, focussed on the division of Hominini. The family Hominidae comprises the tribes Ponginae and Hominini, the latter two forming the subfamily of Homininae.
Hominini is divided into Australopithecina. The Hominina are held to have emerged within the Australopithecina. Genetic analysis combined with fossil evidence indicates that hominoids diverged from the Old World monkeys about 25 million years ago, near the Oligocene-Miocene boundary; the most recent common ancestors of the subfamilies Homininae and Ponginae, lived about 15 million years ago. In the following cladogram, the approximate time the clades radiated newer clades indicated in millions of years ago. Both Sahelanthropus and Orrorin existed during the estimated duration of the ancestral chimpanzee-human speciation events, within the range of eight to four million years ago. Few fossil specimens have been found that can be considered directly ancestral to genus Pan. News of the first fossil chimpanzee, found in Kenya, was published in 2005. However, it is dated to recent times—between 545 and 284 thousand years ago; the divergence of a "proto-human" or "pre-human" lineage separate from Pan appears to have been a process of complex speciation-hybridization rather than a clean split, taking place over the period of anywhere between 13 million years ago and some 4 million years ago.
Different chromosomes appear to have split at different times, with broad-scale hybridization activity occurring between the two emerging lineages as late as the period 6.3 to 5.4 Mya, according to Patterson et al. This research group noted that one hypothetical late hybridization period was based in particular on the similarity of X chromosomes in the proto-humans and stem chimpanzees, suggesting the final divergence as recent as 4 Mya. Wakeley rejected these hypotheses. Most DNA studies find that humans and Pan are 99% identical, but one study found only 94% commonality, with some of the difference occurring in noncoding DNA, it is most that the australopithecines, dating from 3 to 4.4 Mya, evolved into the earliest members of genus Homo. In the year 2000, the discovery of Orrorin tugenensis, dated as early as 6.2 Mya challenged critical elements of that hypothesis, as it suggested that Homo did not in fact derive from australopithecine ancestors. All the listed fossil genera are evaluated for: 1) probability of being ancestral to Homo, 2) whether they are more related to Homo than to any other living primate—two traits that could identify them as hominins.
Some, including Paranthropus and Australopithecus, are broadly thought to be ancestral and related to Homo.
Gibbons are apes in the family Hylobatidae. The family contained one genus, but now is split into four genera and 18 species. Gibbons live in tropical and subtropical rainforests from eastern Bangladesh and northeast India to southern China and Indonesia. Called the smaller apes or lesser apes. Gibbons differ from great apes in being smaller, exhibiting low sexual dimorphism, not making nests. In certain anatomical details, they superficially more resemble monkeys than great apes do, but like all apes, gibbons are tailless. Unlike most of the great apes, gibbons form long-term pair bonds, their primary mode of locomotion, involves swinging from branch to branch for distances up to 15 m, at speeds as high as 55 km/h. They can make leaps up to 8 m, walk bipedally with their arms raised for balance, they are the most agile of all tree-dwelling, nonflying mammals. Depending on species and sex, gibbons' fur coloration varies from dark to light brown shades, any shade between black and white, though a "white" gibbon is rare.
Gibbon species include the siamang, the white-handed or lar gibbon, the hoolock gibbons. Whole genome molecular dating analyses indicate that the gibbon lineage diverged from that of great apes around 16.8 million years ago. Adaptive divergence associated with chromosomal rearrangements led to rapid radiation of the four genera 5-7 Mya; each genus comprises a distinct, well-delineated lineage, but the sequence and timing of divergences among these genera has been hard to resolve with whole genome data, due to radiative speciations and extensive incomplete lineage sorting. An analysis based on morphology suggests that the four genera are ordered as A coalescent-based species tree analysis of genome-scale datasets suggests a phylogeny for the four genera ordered as. At the species level, estimates from mitochondrial DNA genome analyses suggest that Hylobates pileatus diverged from H. lar and H. agilis around 3.9 Mya, H. lar and H. agilis separated around 3.3 Mya. Whole genome analysis suggests divergence of Hylobates pileatus from Hylobates moloch 1.5-3.0 Mya.
The extinct Bunopithecus sericus is a gibbon or gibbon-like ape which, until was thought to be related to the hoolock gibbons. Family Hylobatidae: gibbons The family is divided into four genera based on their diploid chromosome number: Hylobates, Hoolock and Symphalangus. There is an extinct fifth genus named Bunopithecus, either a gibbon or gibbon-like ape. An extinct sixth genus, was identified in 2018 based on a partial skull found in China. Genus Hoolock Western hoolock gibbon, H. hoolock Eastern hoolock gibbon, H. leuconedys Skywalker hoolock gibbon, H. tianxing Genus Hylobates: dwarf gibbons Lar gibbon or white-handed gibbon, H. lar Malaysian lar gibbon, H. l. lar Carpenter's lar gibbon, H. l. carpenteri Central lar gibbon, H. l. entelloides Sumatran lar gibbon, H. l. vestitus Yunnan lar gibbon, H. l. yunnanensis Bornean white-bearded gibbon, H. albibarbis Agile gibbon or black-handed gibbon, H. agilis Müeller's gibbon, H. muelleri Müeller's grey gibbon, H. m. muelleri Abbott's grey gibbon, H. m. abbotti Northern grey gibbon, H. m. funereus Silvery gibbon, H. moloch Western silvery gibbon or western Javan gibbon, H. m. moloch Eastern silvery gibbon or central Javan gibbon, H. m. pongoalsoni Pileated gibbon or capped gibbon, H. pileatus Kloss's gibbon, Mentawai gibbon or bilou, H. klossii Genus Symphalangus Siamang, S. syndactylus Genus Nomascus: crested gibbons Northern buffed-cheeked gibbon, N. annamensis Concolor or black crested gibbon, N. concolor Tonkin black crested gibbon, N. c. concolor Laotian black crested gibbon, N. c. lu Central Yunnan black crested gibbon, N. c. jingdongensis West Yunnan black crested gibbon, N. c. furvogaster Eastern black crested gibbon or Cao Vit black crested gibbon, N. nasutus Hainan black crested gibbon, N. hainanus Northern white-cheeked gibbon, N. leucogenys Southern white-cheeked gibbon, N. siki Yellow-cheeked gibbon, N. gabriellae Genus Bunopithecus Bunopithecus sericus Genus Junzi Junzi imperialis Many gibbons are hard to identify based on fur coloration, so are identified either by song or genetics.
These morphological ambiguities have led to hybrids in zoos. Zoos receive gibbons of unknown origin, so they rely on morphological variation or labels that are impossible to verify to assign species and subspecies names, so separate species of gibbons are misidentified and housed together. Interspecific hybrids, hybrids within a genus, are suspected to occur in wild gibbons where their ranges overlap. However, no records exist of fertile hybrids between different gibbon genera, either in the wild or in captivity. One unique aspect of a gibbon's anatomy is the wrist, which functions something like a ball and socket joint, allowing for biaxial movement; this reduces the amount of energy needed in the upper arm and torso, while reducing stress on the shoulder joint. Gibbons have long hands and feet, with a deep cleft between the first and second digits of their hands, their fur is black, gray, or brownish with white markings on hands and face. Some species have an enlarged throat sac, which inflates and serves as a resonating chamber when the animals call.
This structure can become quite large in some species, sometimes equaling the size of the animal's head. Their voices are much more powerful than that of any human singer, although they
Hybrid speciation is a form of speciation where hybridization between two different species leads to a new species, reproductively isolated from the parent species. From the 1940s, reproductive isolation between hybrids and their parents was thought to be difficult to achieve and thus hybrid species were thought to be rare. With DNA analysis becoming more accessible in the 1990s, hybrid speciation has been shown to be a common phenomenon in plants. In botanical nomenclature, a hybrid species is called a nothospecies. Hybrid species are by their nature polyphyletic. A hybrid may be better fitted to the local environment than the parental lineage and as such natural selection may favor these individuals. If reproductive isolation is subsequently achieved, a separate species may arise. Reproductive isolation may be genetic, behavioural, spatial, or a combination of these. If reproductive isolation fails to establish, the hybrid population may merge with either or both parent species; this will lead to an influx of foreign genes in the parent population, a situation called an introgression.
Introgression is a source of genetic variation, can in itself facilitate speciation. There is evidence that introgression is a ubiquitous phenomenon in plants and humans, where genetic material from Neanderthals and Denisovans is responsible for much of the immune genes in non-African populations. For a hybrid form to persist, it must be able to exploit the available resources better than either parent species, which, in most cases, it will have to compete with. While grizzly bears and polar bears may have offspring, a grizzly–polar bear hybrid will be less suited in either of the ecological roles than the parents themselves. Although the hybrid is fertile, this poor adaptation would prevent the establishment of a permanent population. Lions and tigers have overlapped in a portion of their range and can theoretically produce wild hybrids: ligers, which are a cross between a male lion and female tiger, tigons, which are a cross between a male tiger and a female lion. In both ligers and tigons, the females are fertile and the males are sterile.
One of these hybrids carries growth-inhibitor genes from both parents and thus is smaller than either parent species and might in the wild come into competition with smaller carnivores, e.g. the leopard. The other hybrid, the liger, ends up larger than either of its parents: about a thousand pounds fully-grown. No tiger-lion hybrids are known from the wild because each species is confined to geographically separated ranges; some situations may favour hybrid population. One example is rapid turnover of available environment types, like the historical fluctuation of water level in Lake Malawi, a situation that favors speciation. A similar situation can be found where related species occupy a chain of islands; this will allow any present hybrid population to move into new, unoccupied habitats, avoiding direct competition with parent species and giving a hybrid population time and space to establish. Genetics too can favour hybrids. In the Amboseli National Park in Kenya, yellow baboons and anubis baboons interbreed.
The hybrid males reach maturity earlier than their pure bred cousins, setting up a situation where the hybrid population may over time replace one or both of the parent species in the area. Genetics are more variable and malleable in plants than in animals reflecting the higher activity level in animals. Hybrids genetics will be less stable than those of species evolving through isolation, which explains why hybrid species appear more common in plants than in animals. Many agricultural crops are hybrids with double or triple chromosome sets. Having multiple sets of chromosomes is called polyploidy. Polyploidy is fatal in animals where extra chromosome sets upset fetal development, but is found in plants. A form of hybrid speciation, common in plants, occurs when an infertile hybrid becomes fertile after doubling of the chromosome number. Hybridization without change in chromosome number is called homoploid hybrid speciation; this is the situation found in most animal hybrids. For a hybrid to be viable, the chromosomes of the two organisms will have to be similar, i.e. the parent species must be related, or the difference in chromosome arrangement will make mitosis problematic.
With polyploid hybridization, this constraint is less acute. Super-numerary chromosome numbers can be unstable, which can lead to instability in the genetics of the hybrid; the European edible frog appears to be a species, but is triploid semi-permanent hybrids between pool frogs and marsh frogs. In most populations, the edible frog population is dependent on the presence of at least one of the parents species to be maintained as each individual need two gene sets from one parent species and one from the other; the male sex determination gene in the hybrids is only found in the genome of the pool frog, further undermining stability. Such instability can lead to rapid reduction of chromosome numbers, creating reproductive barriers and thus allowing speciation. Hybrid speciation in animals is homoploid. While not common, a few animal species are the result of hybridization insects such as the Lonicera fly, some fish, with a mammal, the clymene dolphin, a few birds. One is an unnamed species of Darwin's finches bred with Española cactus finch with a p