The estrous cycle or oestrus cycle is the recurring physiological changes that are induced by reproductive hormones in most mammalian therian females. Estrous cycles start after sexual maturity in females and are interrupted by anestrous phases or by pregnancies. Estrous cycles continue until death; some animals may display bloody vaginal discharge mistaken for menstruation. Mammals share the same reproductive system, including the regulatory hypothalamic system that produces gonadotropin-releasing hormone in pulses, the pituitary gland that secretes follicle-stimulating hormone and luteinizing hormone, the ovary itself that releases sex hormones including estrogens and progesterone. However, species vary in the detailed functioning. One difference is that animals that have estrous cycles resorb the endometrium if conception does not occur during that cycle. Animals that have menstrual cycles shed the endometrium through menstruation instead. Another difference is sexual activity. In species with estrous cycles, females are only sexually active during the estrus phase of their cycle.
This is referred to as being "in heat". In contrast, females of species with menstrual cycles can be sexually active at any time in their cycle when they are not about to ovulate. Humans have menstrual cycles rather than estrous cycles. They, unlike most other species, have concealed ovulation, a lack of obvious external signs to signal estral receptivity at ovulation. There are, subtle signs to which human males may favorably respond, including changes in a woman's scent and facial appearance; some research suggests that women tend to have more sexual thoughts and are more prone to sexual activity right before ovulation. Animals with estrous cycles have unmistakable outward displays of receptivity, ranging from engorged and colorful genitals to behavioral changes like mating calls. See Menstrual cycle § Cycles and phases for more information. Estrus is derived from Greek οἶστρος oîstros; this refers to the gadfly in Ancient Greek mythology that Hera sent to torment Io, won in her heifer form by Zeus.
Euripides used oestrus to indicate'frenzy', to describe madness. Homer uses the word to describe panic. Plato uses it to refer to an irrational drive and to describe the soul "driven and drawn by the gadfly of desire". Somewhat more aligned to current meaning and usage of estrus, Herodotus uses oîstros to describe the desire of fish to spawn; the earliest use in English was with a meaning of'frenzied passion'. In 1900, it was first used to describe'rut in animals. In British and most Commonwealth English, the spelling is œstrus. In all English spellings, the noun ends in - the adjective in - ous, thus in North American English, a mammal may be described as "in estrus" when it is in that particular part of the estrous cycle. A four-phase terminology is used in reference to animals with estrous cycles. One or several follicles of the ovary start to grow, their number is species specific. This phase can last as little as one day or as long as three weeks, depending on the species. Under the influence of estrogen the lining in the uterus starts to develop.
Some animals may experience vaginal secretions. The female is not yet sexually receptive. Variant terms for proestrus include pro-oestrus and pro-oestrum. Estrus or oestrus refers to the phase. Under regulation by gonadotropic hormones, ovarian follicles mature and estrogen secretions exert their biggest influence; the female exhibits sexually receptive behavior, a situation that may be signaled by visible physiologic changes. Estrus is seen in the mammalian species, including primates, it is thought that this increased sexual receptivity serves the function of helping the female obtain mates with superior genetic quality. This phase is sometimes called oestrum. In some species, the labia are reddened. Ovulation may occur spontaneously in some species. Among quadrupeds, a signal trait of estrus is the lordosis reflex, in which the animal spontaneously elevates her hindquarters; this phase is characterized by the activity of the corpus luteum. The signs of estrogen stimulation subside and the corpus luteum starts to form.
The uterine lining begins to appear. In the absence of pregnancy the diestrus phase terminates with the regression of the corpus luteum; the lining in the uterus is reorganized for the next cycle. Other spellings include metoestrus, metoestrum, dioestrus and dioestrum. Anestrus refers to the phase; this is a seasonal event and controlled by light exposure through the pineal gland that releases melatonin. Melatonin may repress stimulation of reproduction in long-day breeders and stimulate reproduction in short-day breeders. Melatonin is thought to act by regulating the hypothalamic pulse activity of the gonadotropin-releasing hormone. Anestrus is induced by time of year, lactation, significant illness, chro
Siberia is an extensive geographical region spanning much of Eurasia and North Asia. Siberia has been a part of modern Russia since the 17th century; the territory of Siberia extends eastwards from the Ural Mountains to the watershed between the Pacific and Arctic drainage basins. The Yenisei River conditionally divides Siberia into two parts and Eastern. Siberia stretches southwards from the Arctic Ocean to the hills of north-central Kazakhstan and to the national borders of Mongolia and China. With an area of 13.1 million square kilometres, Siberia accounts for 77% of Russia's land area, but it is home to 36 million people—27% of the country's population. This is equivalent to an average population density of about 3 inhabitants per square kilometre, making Siberia one of the most sparsely populated regions on Earth. If it were a country by itself, it would still be the largest country in area, but in population it would be the world's 35th-largest and Asia's 14th-largest. Worldwide, Siberia is well known for its long, harsh winters, with a January average of −25 °C, as well as its extensive history of use by Russian and Soviet administrations as a place for prisons, labor camps, exile.
The origin of the name is unknown. Some sources say that "Siberia" originates from the Siberian Tatar word for "sleeping land". Another account sees the name as the ancient tribal ethnonym of the Sirtya, an ethnic group which spoke a Paleosiberian language; the Sirtya people were assimilated into the Siberian Tatars. The modern usage of the name was recorded in the Russian language after the Empire's conquest of the Siberian Khanate. A further variant claims; the Polish historian Chyliczkowski has proposed that the name derives from the proto-Slavic word for "north", but Anatole Baikaloff has dismissed this explanation. He said that the neighbouring Chinese and Mongolians, who have similar names for the region, would not have known Russian, he suggests that the name might be a combination of two words with Turkic origin, "su" and "bir". The region has paleontological significance, as it contains bodies of prehistoric animals from the Pleistocene Epoch, preserved in ice or in permafrost. Specimens of Goldfuss cave lion cubs and another woolly mammoth from Oymyakon, a woolly rhinoceros from the Kolyma River, bison and horses from Yukagir have been found.
The Siberian Traps were formed by one of the largest-known volcanic events of the last 500 million years of Earth's geological history. Their activity continued for a million years and some scientists consider it a possible cause of the "Great Dying" about 250 million years ago, – estimated to have killed 90% of species existing at the time. At least three species of human lived in Southern Siberia around 40,000 years ago: H. sapiens, H. neanderthalensis, the Denisovans. In 2010 DNA evidence identified the last as a separate species. Siberia was inhabited by different groups of nomads such as the Enets, the Nenets, the Huns, the Scythians and the Uyghurs; the Khan of Sibir in the vicinity of modern Tobolsk was known as a prominent figure who endorsed Kubrat as Khagan of Old Great Bulgaria in 630. The Mongols conquered a large part of this area early in the 13th century. With the breakup of the Golden Horde, the autonomous Khanate of Sibir was established in the late 15th century. Turkic-speaking Yakut migrated north from the Lake Baikal region under pressure from the Mongol tribes during the 13th to 15th century.
Siberia remained a sparsely populated area. Historian John F. Richards wrote: "... it is doubtful that the total early modern Siberian population exceeded 300,000 persons."The growing power of Russia in the West began to undermine the Siberian Khanate in the 16th century. First, groups of traders and Cossacks began to enter the area; the Russian Army was directed to establish forts farther and farther east to protect new settlers from European Russia. Towns such as Mangazeya, Tara and Tobolsk were developed, the last being declared the capital of Siberia. At this time, Sibir was the name of a fortress at Qashlik, near Tobolsk. Gerardus Mercator, in a map published in 1595, marks Sibier both as the name of a settlement and of the surrounding territory along a left tributary of the Ob. Other sources contend that the Xibe, an indigenous Tungusic people, offered fierce resistance to Russian expansion beyond the Urals; some suggest. By the mid-17th century, Russia had established areas of control; some 230,000 Russians had settled in Siberia by 1709.
Siberia was a destination for sending exiles. The first great modern change in Siberia was the Trans-Siberian Railway, constructed during 1891–1916, it linked Siberia more to the industrialising Russia of Nicholas II. Around seven million people moved to Siberia from European Russia between 1801 and 1914. From 1859 to 1917, more than half a million people migrated to the Russian Far East. Siberia has extensive natural resources. During the 20th century, large-scale exploitation of these was developed, industrial towns cropped up throughout the region. At 7:15 a.m. on 30 June 1908, millions of trees were felled near the Podkamennaya Tunguska River in central Siberia in the Tunguska Event. Most scientists believe this resulted from the air burst of a comet. Though no crater has been found, the landscape in the area still bears the scars of this event. In the early decades of the Soviet Union (
The even-toed ungulates are ungulates - hoofed animals - which bear weight on two of the five toes: their third and fourth toes. The other three toes are either present, vestigial, or pointing posteriorly. By contrast, odd-toed ungulates bear weight on one of the five toes: the third toe. Another difference between the two is that even-toed ungulates digest plant cellulose in one or more stomach chambers rather than in their intestine as the odd-toed ungulates do; the aquatic cetaceans evolved from even-toed ungulates, so modern taxonomic classification sometimes combines the Artiodactyla and Cetacea into the Cetartiodactyla. The 220 land-based even-toed ungulate species include pigs, hippopotamuses, llamas, mouse deer, giraffes, sheep and cattle. Many of these are of great dietary and cultural importance to humans; the oldest fossils of even-toed ungulates date back to the early Eocene. Since these findings simultaneously appeared in Europe and North America, it is difficult to determine the origin of artiodactyls.
The fossils are classified as belonging to the family Dichobunidae. These were small animals, some as small as a hare, with a slim build, lanky legs, a long tail, their hind legs were much longer than their front legs. The early to middle Eocene saw the emergence of the ancestors of most of today's mammals. Two widespread, but now extinct, families of even-toed ungulates were Enteledontidae and Anthracotheriidae. Entelodonts existed from the middle Eocene to the early Miocene in North America, they had a stocky body with short legs and a massive head, characterized by two humps on the lower jaw bone. Anthracotheres had a large, porcine build, with an elongated muzzle; this group appeared in the middle Eocene up until the Pliocene, spread throughout Eurasia and North America. Anthracothereres are thought to be the ancestors of hippos, probably led a similar aquatic lifestyle. Hippopotamuses appeared in the late Miocene and occupied Africa and Asia – they never got to the Americas; the camels were, during large parts of the Cenozoic, limited to North America.
Among the North American camels were groups like the short-legged Merycoidodontidae. They first developed a great diversity of species in North America. Only in the late Miocene or early Pliocene did they migrate from North America into Eurasia; the North American varieties became extinct around 10,000 years ago. Suina have been around since the Eocene. In the late Eocene or the Oligocene, two families stayed in Africa. South America was settled by even-toed ungulates only in the Pliocene, after the land bridge at the Isthmus of Panama formed some three million years ago. With only the peccaries and various species of capreoline deer, South America has comparatively fewer artiodactyl families than other continents, except Australia, which has no native species; the classification of artiodactyls was hotly debated because the ocean-dwelling cetaceans evolved from the land-dwelling even-toed ungulates. Some semiaquatic even-toed ungulates are more related to the ocean-dwelling cetaceans than to the other even-toed ungulates.
This makes the Artiodactyla as traditionally defined a paraphyletic taxon, since it includes animals descended from a common ancestor, but does not include all of its descendants. Phylogenetic classification only recognizes monophyletic taxa. To address this problem, the traditional order Artiodactyla and infraorder Cetacea are sometimes subsumed into the more inclusive Cetartiodactyla taxon. An alternative approach is to include both land-dwelling even-toed ungulates and ocean-dwelling cetaceans in a revised Artiodactyla taxon. Order Artiodactyla/Clade CetartiodactylaSuborder Tylopoda Family †Anoplotheriidae? Family †Cainotheriidae Family †Merycoidodontidae Family †Agriochoeridae Family Camelidae: camels and lamoids or llamas Family †Oromerycidae Family †Xiphodontidae Clade Artiofabula Suborder Suina Family Suidae: pigs Family Tayassuidae: peccaries Family †Sanitheriidae Clade Cetruminantia Clade CetancodontamorphaGenus †Andrewsarchus? Family †Entelodontidae Suborder Whippomorpha Family †Raoellidae Superfamily Dichobunoidea – paraphyletic to Cetacea and Raoellidae Family †Dichobunidae Family †Helohyidae Family †Choeropotamidae Family †Cebochoeridae Family †Mixtotheriidae Infraorder Ancodonta Family †Anthracotheriidae – paraphyletic to Hippopotamidae Family Hippopotamidae: hippos Infraorder Cetacea: whales Parvorder †Archaeoceti Family †Pakicetidae Family †Ambulocetidae Family †Remingtonocetidae Family †Basilosauridae Parvorder Mysticeti: baleen whales Superfamily Balaenoidea: right whales Family Balaenidae: greater right whales Family Cetotheriidae: pygmy right whale Superfamily Balaenopteroidea: large baleen whales Family Balaenopteridae: slender-back rorquals and humpback whale Family Eschrichtiidae: gray whale Parvorder Odontoceti: toothed whales Superfamily Delphinoidea: oceanic dolphins and others Family Delphinidae: oceanic t
Binomial nomenclature called binominal nomenclature or binary nomenclature, is a formal system of naming species of living things by giving each a name composed of two parts, both of which use Latin grammatical forms, although they can be based on words from other languages. Such a name is called a binomen, binominal name or a scientific name; the first part of the name – the generic name – identifies the genus to which the species belongs, while the second part – the specific name or specific epithet – identifies the species within the genus. For example, humans belong within this genus to the species Homo sapiens. Tyrannosaurus rex is the most known binomial; the formal introduction of this system of naming species is credited to Carl Linnaeus beginning with his work Species Plantarum in 1753. But Gaspard Bauhin, in as early as 1623, had introduced in his book Pinax theatri botanici many names of genera that were adopted by Linnaeus; the application of binomial nomenclature is now governed by various internationally agreed codes of rules, of which the two most important are the International Code of Zoological Nomenclature for animals and the International Code of Nomenclature for algae and plants.
Although the general principles underlying binomial nomenclature are common to these two codes, there are some differences, both in the terminology they use and in their precise rules. In modern usage, the first letter of the first part of the name, the genus, is always capitalized in writing, while that of the second part is not when derived from a proper noun such as the name of a person or place. Both parts are italicized when a binomial name occurs in normal text, thus the binomial name of the annual phlox is now written as Phlox drummondii. In scientific works, the authority for a binomial name is given, at least when it is first mentioned, the date of publication may be specified. In zoology "Patella vulgata Linnaeus, 1758"; the name "Linnaeus" tells the reader who it was that first published a description and name for this species of limpet. "Passer domesticus". The original name given by Linnaeus was Fringilla domestica; the ICZN does not require that the name of the person who changed the genus be given, nor the date on which the change was made, although nomenclatorial catalogs include such information.
In botany "Amaranthus retroflexus L." – "L." is the standard abbreviation used in botany for "Linnaeus". "Hyacinthoides italica Rothm. – Linnaeus first named this bluebell species Scilla italica. The name is composed of two word-forming elements: "bi", a Latin prefix for two, "-nomial", relating to a term or terms; the word "binomium" was used in Medieval Latin to mean a two-term expression in mathematics. Prior to the adoption of the modern binomial system of naming species, a scientific name consisted of a generic name combined with a specific name, from one to several words long. Together they formed a system of polynomial nomenclature; these names had two separate functions. First, to designate or label the species, second, to be a diagnosis or description. In a simple genus, containing only two species, it was easy to tell them apart with a one-word genus and a one-word specific name; such "polynomial names" may sometimes look like binomials, but are different. For example, Gerard's herbal describes various kinds of spiderwort: "The first is called Phalangium ramosum, Branched Spiderwort.
The other... is aptly termed Phalangium Ephemerum Virginianum, Soon-Fading Spiderwort of Virginia". The Latin phrases are short descriptions, rather than identifying labels; the Bauhins, in particular Caspar Bauhin, took some important steps towards the binomial system, by pruning the Latin descriptions, in many cases to two words. The adoption by biologists of a system of binomial nomenclature is due to Swedish botanist and physician Carl von Linné, more known by his Latinized name Carl Linnaeus, it was in his 1753 Species Plantarum that he first began using a one-word "trivial name" together with a generic name in a system of binomial nomenclature. This trivial name is what is now known as specific name; the Bauhins' genus names were retained in many of these, but the descriptive part was reduced to a single word. Linnaeus's trivial names introduced an important new idea, namely that the function of a name could be to give a species a unique label; this meant. Thus Gerard's Phalangium ephemerum virginianum became Tradescantia virgi
The wild yak is a large wild bovid native to the Himalayas. It is the ancestor of the domestic yak; the ancestor of the wild and domestic yak is thought to have diverged from Bos primigenius at a point between one and five million years ago. The wild yak is now treated as a separate species from the domestic yak. Wild yaks are second only to the gaur in shoulder height, they are the largest native animal in their range. Wild yak adults weigh 305 -- 1,000 kg; the head and body length is 2.5 to 3.3 m. The females are about one-third the weight and are about 30% smaller in their linear dimensions when compared to bull wild yaks. Domesticated yaks are somewhat smaller, they are built animals with a bulky frame, sturdy legs, rounded cloven hooves. To protect against the cold, the udder in females and the scrotum in males are small and covered in a layer of hair. Females have four teats. Both sexes have long shaggy hair with a dense woolly undercoat over the chest and thighs for insulation against the cold.
In males this undercoat may form a long "skirt" that can reach the ground. The tail is horse-like rather than tufted like the tails of cattle or bison; the coat is black or dark brown covering most of the body, with a grey muzzle, although some wild golden-brown individuals have been reported. Wild yaks with gold coloured hair are known as the Wild Golden Yak, they are considered an endangered subspecies by China, with an estimated population of 170 left in the wild. Wild yaks are found in northern Tibet and western Qinghai, with some populations extending into the southernmost parts of Xinjiang, into Ladakh in India. Small, isolated populations of wild yak are found farther afield in western Tibet and eastern Qinghai. In historic times, wild yaks were found in Nepal and Bhutan, but they are now considered extinct in both countries; the primary habitat of wild yaks consists of treeless uplands between 3,000 and 5,500 m, dominated by mountains and plateaus. They are most found in alpine tundra with a thick carpet of grasses and sedges rather than the more barren steppe country.
The diet of wild yaks consists of grasses and sedges, such as Carex and Kobresia. They eat a smaller amount of herbs, winterfat shrubs, mosses, have been reported to eat lichen; the main natural predator of the wild yak has been the Tibetan wolf, but brown bears and snow leopards have been reported as predators in some areas of young or infirm wild yaks. Thubten Jigme Norbu, the elder brother of the 14th Dalai Lama, reported on his journey from Kumbum in Amdo to Lhasa in 1950: Before long I was to see the vast herds of drongs with my own eyes; the sight of those beautiful and powerful beasts who from time immemorial have made their home on Tibet's high and barren plateaux never ceased to fascinate me. Somehow these shy creatures manage to sustain themselves on the stunted grass roots, all that nature provides in those parts, and what a wonderful sight it is to see a great herd of them plunging head down in a wild gallop across the steppes. The earth shakes under their heels and a vast cloud of dust marks their passage.
At nights they will protect themselves from the cold by huddling up together, with the calves in the centre. They will stand like this in a snow-storm, pressed so close together that the condensation from their breath rises into the air like a column of steam; the nomads have tried to bring up young drongs as domestic animals, but they have never succeeded. Somehow once they live together with human beings they seem to lose their astonishing strength and powers of endurance, their immemorial relationship with humans has therefore remained that of game and hunter, for their flesh is tasty. Wild yaks are herd animals. Herds can contain several hundred individuals. Herds consist of females and their young, with a smaller number of adult males. On average female yaks graze 100m higher than males. Females with young tend to choose grazing ground on steep slopes; the remaining males are either solitary, or found in much smaller groups, averaging around six individuals. Groups move into lower altitude ranges during the winter.
Although wild yaks can become aggressive when defending young, or during the rut, they avoid humans, may flee for great distances if approached. Wild yaks give birth to a single calf the following spring. Females only give birth every other year; the wild yak is considered to be Vulnerable by the IUCN. It was classified as Endangered, but was downlisted in 1996 based on the estimated rate of population decline and current population sizes; the latest assessment in 2008 suggested a total population of no more than 10,000 mature individuals. The wild yak is experiencing threats applied by several sources. Poaching, including commercial poaching, has remained the most serious threat. Disturbance by and interbreeding with livestock herds is common; this may include the transmission of cattle-borne diseases, although no direct evidence of this has yet been found. Conflicts with herders themselves, as in preventative and retaliatory killings for abduction of domestic yaks by wild herds occur but appear to be rare.
Weaning is the process of introducing an infant human or mammal to what will be its adult diet while withdrawing the supply of its mother's milk. The process takes place only in mammals; the infant is considered to be weaned once it is no longer fed any breast milk. How and when to wean a human infant is controversial; the American Academy of Pediatrics recommends feeding a baby only breast milk for the first six months of its life. Many mothers find breastfeeding challenging in modern times when many mothers have to return to work soon after the birth of their child; the American Academy of Pediatrics, the World Health Organization, the National Health Service Choices UK, the National Health & Medical Research Council in Australia recommend waiting until 6 months to introduce baby food. However, many baby food companies market their "stage 1" foods to children between 4 and 6 months old with the precaution that the food is meant to be consumed in addition to breast milk or formula and is just for "practice".
These practice foods are soft and runny. Examples include mashed fruit and vegetables. Certain foods are recommended to be avoided; the United Kingdom's NHS recommends withholding foods including those "that contain wheat, nuts, peanut products, liver, fish, cows’ milk and soft or unpasteurised cheese" until a baby is six months old, as they may cause food allergies or make the baby ill. However, recommendations such as these have been called into question by research that suggests early exposure to potential allergens does not increase the likelihood of allergies, in some cases reduces it. In many cultures around the world, weaning progresses with the introduction of feeding the child food, prechewed by the parent along with continued breastfeed, a practice known as premastication; the practice was important throughout human history in that it gave a child a improved protein source in addition to preventing iron deficiency. The prechewing of food gives the baby long-term immunological benefits through factors in the mother's saliva.
However, premasticated food from caregivers of lower socioeconomic status in areas of endemic diseases can result in the passing of the disease to the child. No matter what age baby food is introduced, it is a messy affair, as young children do not have the coordination to eat "neatly". Coordination for using utensils properly and eating with dexterity takes years to develop. Many babies begin using utensils between 10 and 14 months, but most will not be able to feed themselves sufficiently well until about 2 or 3 years of age. At this point, the mother tries to force the infant to cease nursing, while the infant attempts to force the mother to continue. From an evolutionary perspective, weaning conflict may be considered the result of the cost of continued nursing to the mother in terms of reduced ability to raise future offspring, exceeding the benefits to the mother in terms of increased survival of the current infant; this can come about because future offspring will be related to the mother as the current infant, but will share less than 100% of the current infant's genes.
So, from the perspective of the mother's evolutionary fitness, it makes sense for her to cease nursing the current infant as soon as the cost to future offspring exceeds the benefit to the current infant. But, assuming the current infant shares 50% of the future offspring's genes, from the perspective of the infant's own evolutionary fitness, it makes sense for the infant to continue nursing until the cost to future offspring exceeds twice the benefit to itself. Weaning conflict has been studied for a variety including primates and canines. There are significant cultural variations in regards to weaning. Scientifically, one can ask various questions. At what age do various societies normatively choose to wean? In comparison with other animals similar primates, by various measures; as there are significant ranges and skew in these numbers, looking at the median is more useful than looking at the average. Considering biological measures of maturity, notably investigated by Katherine Ann Dettwyler, yields a range of ages from 2 1/2 years to 7 years as the weaning age analogous to other primates – the "natural age of weaning".
This depends on the measure, for example: weaning in non-human primates is associated with eruption of permanent molars. Other studies are possible, as in psychological factors. For example, Barbara Rogoff has noted, citing a 1953 study by Whiting & Child, that the most distressing time to wean a child is at 13–18 months. After this peak, weaning becomes progressively easier and less distressing for the child, with "older children wean themselves." In science, mice are used in laboratory experiments. When breeding laboratory mice in a controlled environment, the weaning is defined as the moment when the pups are transferred out of the mothers' cage. Weaning is recommended at 3 to 4 weeks after parturition. For pet carnivores such as do
Mitochondrial DNA is the DNA located in mitochondria, cellular organelles within eukaryotic cells that convert chemical energy from food into a form that cells can use, adenosine triphosphate. Mitochondrial DNA is only a small portion of the DNA in a eukaryotic cell. In humans, the 16,569 base pairs of mitochondrial DNA encode for only 37 genes. Human mitochondrial DNA was the first significant part of the human genome to be sequenced. In most species, including humans, mtDNA is inherited from the mother. However, in exceptional cases, human babies sometimes inherit mtDNA from both their fathers and their mothers resulting in mtDNA heteroplasmy. Since animal mtDNA evolves faster than nuclear genetic markers, it represents a mainstay of phylogenetics and evolutionary biology, it permits an examination of the relatedness of populations, so has become important in anthropology and biogeography. Nuclear and mitochondrial DNA are thought to be of separate evolutionary origin, with the mtDNA being derived from the circular genomes of the bacteria that were engulfed by the early ancestors of today's eukaryotic cells.
This theory is called the endosymbiotic theory. Each mitochondrion is estimated to contain 2–10 mtDNA copies. In the cells of extant organisms, the vast majority of the proteins present in the mitochondria are coded for by nuclear DNA, but the genes for some, if not most, of them are thought to have been of bacterial origin, having since been transferred to the eukaryotic nucleus during evolution; the reasons why mitochondria have retained some genes are debated. The existence in some species of mitochondrion-derived organelles lacking a genome suggests that complete gene loss is possible, transferring mitochondrial genes to the nucleus has several advantages; the difficulty of targeting remotely-produced hydrophobic protein products to the mitochondrion is one hypothesis for why some genes are retained in mtDNA. Recent analysis of a wide range of mtDNA genomes suggests that both these features may dictate mitochondrial gene retention. In most multicellular organisms, mtDNA is inherited from the mother.
Mechanisms for this include simple dilution, degradation of sperm mtDNA in the male genital tract and in the fertilized egg. Whatever the mechanism, this single parent pattern of mtDNA inheritance is found in most animals, most plants and in fungi. In sexual reproduction, mitochondria are inherited from the mother. Most mitochondria are present at the base of the sperm's tail, used for propelling the sperm cells. In 1999 it was reported that paternal sperm mitochondria are marked with ubiquitin to select them for destruction inside the embryo; some in vitro fertilization techniques injecting a sperm into an oocyte, may interfere with this. The fact that mitochondrial DNA is maternally inherited enables genealogical researchers to trace maternal lineage far back in time; this is accomplished on human mitochondrial DNA by sequencing the hypervariable control regions, sometimes the complete molecule of the mitochondrial DNA, as a genealogical DNA test. HVR1, for example, consists of about 440 base pairs.
These 440 base pairs are compared to the same regions of other individuals to determine maternal lineage. Most the comparison is made with the revised Cambridge Reference Sequence. Vilà et al. have published studies tracing the matrilineal descent of domestic dogs from wolves. The concept of the Mitochondrial Eve is based on the same type of analysis, attempting to discover the origin of humanity by tracking the lineage back in time. MtDNA is conserved, its slow mutation rates make it useful for studying the evolutionary relationships—phylogeny—of organisms. Biologists can determine and compare mtDNA sequences among different species and use the comparisons to build an evolutionary tree for the species examined. However, due to the slow mutation rates, it is hard to distinguish between related species to any large degree, so other methods of analysis must be used. Entities subject to uniparental inheritance and with little to no recombination may be expected to be subject to Muller's ratchet, the accumulation of deleterious mutations until functionality is lost.
Animal populations of mitochondria avoid this through a developmental process known as the mtDNA bottleneck. The bottleneck exploits random processes in the cell to increase the cell-to-cell variability in mutant load as an organism develops: a single egg cell with some proportion of mutant mtDNA thus produces an embryo in which different cells have different mutant loads. Cell-level selection may act to remove those cells with more mutant mtDNA, leading to a stabilisation or reduction in mutant load between generations; the mechanism underlying the bottleneck is debated, with a recent mathematical and experimental