Mammals are vertebrate animals constituting the class Mammalia, characterized by the presence of mammary glands which in females produce milk for feeding their young, a neocortex, fur or hair, three middle ear bones. These characteristics distinguish them from reptiles and birds, from which they diverged in the late Triassic, 201–227 million years ago. There are around 5,450 species of mammals; the largest orders are the rodents and Soricomorpha. The next three are the Primates, the Cetartiodactyla, the Carnivora. In cladistics, which reflect evolution, mammals are classified as endothermic amniotes, they are the only living Synapsida. The early synapsid mammalian ancestors were sphenacodont pelycosaurs, a group that produced the non-mammalian Dimetrodon. At the end of the Carboniferous period around 300 million years ago, this group diverged from the sauropsid line that led to today's reptiles and birds; the line following the stem group Sphenacodontia split off several diverse groups of non-mammalian synapsids—sometimes referred to as mammal-like reptiles—before giving rise to the proto-mammals in the early Mesozoic era.
The modern mammalian orders arose in the Paleogene and Neogene periods of the Cenozoic era, after the extinction of non-avian dinosaurs, have been among the dominant terrestrial animal groups from 66 million years ago to the present. The basic body type is quadruped, most mammals use their four extremities for terrestrial locomotion. Mammals range in size from the 30–40 mm bumblebee bat to the 30-meter blue whale—the largest animal on the planet. Maximum lifespan varies from two years for the shrew to 211 years for the bowhead whale. All modern mammals give birth to live young, except the five species of monotremes, which are egg-laying mammals; the most species-rich group of mammals, the cohort called placentals, have a placenta, which enables the feeding of the fetus during gestation. Most mammals are intelligent, with some possessing large brains, self-awareness, tool use. Mammals can communicate and vocalize in several different ways, including the production of ultrasound, scent-marking, alarm signals and echolocation.
Mammals can organize themselves into fission-fusion societies and hierarchies—but can be solitary and territorial. Most mammals are polygynous. Domestication of many types of mammals by humans played a major role in the Neolithic revolution, resulted in farming replacing hunting and gathering as the primary source of food for humans; this led to a major restructuring of human societies from nomadic to sedentary, with more co-operation among larger and larger groups, the development of the first civilizations. Domesticated mammals provided, continue to provide, power for transport and agriculture, as well as food and leather. Mammals are hunted and raced for sport, are used as model organisms in science. Mammals have been depicted in art since Palaeolithic times, appear in literature, film and religion. Decline in numbers and extinction of many mammals is driven by human poaching and habitat destruction deforestation. Mammal classification has been through several iterations since Carl Linnaeus defined the class.
No classification system is universally accepted. George Gaylord Simpson's "Principles of Classification and a Classification of Mammals" provides systematics of mammal origins and relationships that were universally taught until the end of the 20th century. Since Simpson's classification, the paleontological record has been recalibrated, the intervening years have seen much debate and progress concerning the theoretical underpinnings of systematization itself through the new concept of cladistics. Though field work made Simpson's classification outdated, it remains the closest thing to an official classification of mammals. Most mammals, including the six most species-rich orders, belong to the placental group; the three largest orders in numbers of species are Rodentia: mice, porcupines, beavers and other gnawing mammals. The next three biggest orders, depending on the biological classification scheme used, are the Primates including the apes and lemurs. According to Mammal Species of the World, 5,416 species were identified in 2006.
These were grouped into 153 families and 29 orders. In 2008, the International Union for Conservation of Nature completed a five-year Global Mammal Assessment for its IUCN Red List, which counted 5,488 species. According to a research published in the Journal of Mammalogy in 2018, the number of recognized mammal species is 6,495 species included 96 extinct; the word "mammal" is modern, from the scientific name Mammalia coined by Carl Linnaeus in 1758, derived from the Latin mamma. In an influential 1988 paper, Timothy Rowe defined Mammalia phylogenetically as the crown group of mammals, the clade consisting of the most recent common ancestor of living monotremes and therian m
Ground sloths are a diverse group of extinct sloths, in the mammalian superorder Xenarthra. The term is used as a reference for all extinct sloths because of the large size of the earliest forms discovered, as opposed to existing tree sloths; the Caribbean ground sloths, the most recent survivors, lived in the Antilles until 1550 BC. However, radiocarbon dating suggests an age of between 2819 and 2660 BC for the last occurrence of Megalocnus in Cuba. Ground sloths had been extinct on the mainland of South America for 10,000 years or more, their survival in the Caribbean correlates with the colonization of this area by humans. Some island populations persisted 5,000–6,000 years longer than their continental relatives. Much ground sloth evolution took place during the late Paleogene and Neogene of South America while the continent was isolated. At their earliest appearance in the fossil record, the ground sloths were distinct at the family level; the presence of intervening islands between the American continents in the Miocene allowed a dispersal of forms into North America.
A number of mid- to small-sized forms are believed to have dispersed to the Antilles. They were hardy as evidenced by their diverse numbers and dispersals into remote areas given the finding of their remains in Patagonia and parts of Alaska. Sloths, xenarthrans as a whole, represent one of the more successful South American groups during the Great American Interchange. During the interchange, many more taxa moved from North America into South America than in the other direction. At least five genera of ground sloths have been identified in North American fossils. Paleontologists assign more than 80 genera of ground sloths to multiple families; the megalonychid ground sloths first appeared in the Late Eocene, about 35 million years ago, in Patagonia. Megalonychids first reached North America by island-hopping, prior to the formation of the Isthmus of Panama; some lineages of megalonychids increased in size. The first species of these were small and may have been tree-dwelling, whereas the Pliocene species were approximately half the size of the huge Late Pleistocene Megalonyx jeffersonii from the last ice age.
Some West Indian island species were as small as a large cat. This small size enabled them a degree of arboreality. Megalonyx, which means "giant claw", was a widespread North American genus that lived past the close of the last glaciation, when so many large mammals died out. Remains have been found as far north as the Yukon. Ongoing excavations at Tarkio Valley in southwestern Iowa may reveal something of the familial life of Megalonyx. An adult was found in direct association with two juveniles of different ages, suggesting that adults cared for young of different generations; the earliest known North American megalonychid, Pliometanastes protistus, lived in the southern U. S. is believed to have been the predecessor of Megalonyx. Several species of Megalonyx have been named. A broader perspective on the group, accounting for age, sex and geographic differences, indicates that only three species are valid in the late Pliocene and Pleistocene of North America, although work by McDonald lists five species.
Jefferson's ground sloth has a special place in modern paleontology, for Thomas Jefferson's letter on Megalonyx, read before the American Philosophical Society of Philadelphia in August 1796, marked the beginning of vertebrate paleontology in North America. When Lewis and Clark set out, Jefferson instructed Meriwether Lewis to keep an eye out for ground sloths, he was hoping. Megalonyx jeffersonii was appropriately named after Thomas Jefferson; the megatheriid ground sloths are relatives of the megalonychids. Megatheriids appeared in the Oligocene, some 30 million years ago in South America; the group includes the built Megatherium and Eremotherium. The skeletal structure of these ground sloths indicates, their thick bones and thicker joints gave their appendages tremendous power that, combined with their size and fearsome claws, provided a formidable defense against predators. The earliest megatheriid in North America was Eremotherium eomigrans which arrived 2.2 million years ago, after crossing the formed Panamanian land bridge.
With more than five tons in weight, 6 meters in length, able to reach as high as 17 feet, it was larger than an African bush elephant bull. Unlike relatives, this species retained a plesiomorphic extra claw. While other species of Eremotherium had four fingers with only two or three claws, E. eomigrans had five fingers, four of them with claws up to nearly a foot long. Recognized, ground sloths of Nothrotheriidae are associated with those of the Megatheriidae, together the two form the superfamily Megatheroidea; the most prominent members of the group are the South American genus Thalassocnus, known for being aquatic, Nothrotheriops from North America. The last ground sloths in North America belonging to Nothrotheriops died so that their subfossil dung has remained undisturbed in some caves. One of the skeletons, found in a lava tube at
Sir Richard Owen was an English biologist, comparative anatomist and paleontologist. Despite being a controversial figure, Owen is considered to have been an outstanding naturalist with a remarkable gift for interpreting fossils. Owen produced a vast array of scientific work, but is best remembered today for coining the word Dinosauria. An outspoken critic of Charles Darwin's theory of evolution by natural selection, Owen agreed with Darwin that evolution occurred, but thought it was more complex than outlined in Darwin's On the Origin of Species. Owen's approach to evolution can be seen as having anticipated the issues that have gained greater attention with the recent emergence of evolutionary developmental biology. Owen was the first president of the Microscopical Society of London in 1839 and edited many issues of its journal – known as The Microscopic Journal. Owen campaigned for the natural specimens in the British Museum to be given a new home; this resulted in the establishment, in 1881, of the now world-famous Natural History Museum in South Kensington, London.
Bill Bryson argues that, "by making the Natural History Museum an institution for everyone, Owen transformed our expectations of what museums are for". His contributions to science and public learning notwithstanding, Owen's driving ambition vicious temperament, determination to succeed meant that he was not always popular with his fellow scientists. Owen was feared and hated by some contemporaries such as Thomas Henry Huxley, his career was tainted by controversies, many of which involved accusations that he took credit for other people's work. Owen was born in Lancaster in 1804, one of six children of a West Indian Merchant named Richard Owen, his mother, Catherine Longworth, was descended from Huguenots and he was educated at Lancaster Royal Grammar School. In 1820, he was apprenticed to a local surgeon and apothecary and, in 1824, he proceeded as a medical student to the University of Edinburgh, he left the university in the following year and completed his medical course in St Bartholomew's Hospital, where he came under the influence of the eminent surgeon John Abernethy.
In July 1835 Owen married Caroline Amelia Clift in St Pancras by whom he had William Owen. He outlived both son. After his death, in 1892, he was survived by his three grandchildren and daughter-in-law Emily Owen, to whom he left much of his £33,000 fortune. Upon completing his education, he contemplated the usual professional career, but his bent was evidently in the direction of anatomical research, he was induced by Abernethy to accept the position of assistant to William Clift, conservator of the museum of the Royal College of Surgeons. This congenial occupation soon led him to abandon his intention of medical practice and his life thenceforth was devoted to purely scientific labours, he prepared an important series of catalogues of the Hunterian Collection, in the Royal College of Surgeons and, in the course of this work, he acquired the unrivalled knowledge of comparative anatomy that enabled him to enrich all departments of the science and facilitated his researches on the remains of extinct animals.
In 1836, Owen was appointed Hunterian professor, in the Royal College of Surgeons and, in 1849, he succeeded Clift as conservator. He held the latter office until 1856, when he became superintendent of the natural history department of the British Museum, he devoted much of his energies to a great scheme for a National Museum of Natural History, which resulted in the removal of the natural history collections of the British Museum to a new building at South Kensington: the British Museum. He retained office until the completion of this work, in December, 1883, when he was made a knight of the Order of the Bath, he lived in retirement at Sheen Lodge, Richmond Park, until his death in 1892. His career was tainted by accusations that he failed to give credit to the work of others and tried to appropriate it in his own name; this came to a head in 1846, when he was awarded the Royal Medal for a paper he had written on belemnites. Owen had failed to acknowledge that the belemnite had been discovered by Chaning Pearce, an amateur biologist, four years earlier.
As a result of the ensuing scandal, he was voted off the councils of the Zoological Society and the Royal Society. Owen always tended to support orthodox men of the status quo; the royal family presented him with the cottage in Richmond Park and Robert Peel put him on the Civil List. In 1843, he was elected a foreign member of the Royal Swedish Academy of Sciences, he is buried in the churchyard at Ham near Richmond, Surrey. While occupied with the cataloguing of the Hunterian collection, Owen did not confine his attention to the preparations before him but seized every opportunity to dissect fresh subjects, he was allowed to examine all animals that died in London Zoo's gardens and, when the Zoo began to publish scientific proceedings, in 1831, he was the most prolific contributor of anatomical papers. His first notable publication, was his Memoir on the Pearly Nautilus, soon recognized as a classic. Henceforth, he continued to make important contributions to every department of comparative anatomy and zoology for a period of over fifty years.
In the sponges, Owen was the first to describe the now well-known Venus' Flower Basket or Euplectella. Among Entozoa, his most noteworthy discovery was that of Trichina spiralis, the parasite infesting the muscles of man in the disease now termed trichinosis (se
The Neogene is a geologic period and system that spans 20.45 million years from the end of the Paleogene Period 23.03 million years ago to the beginning of the present Quaternary Period 2.58 Mya. The Neogene is sub-divided into two epochs, the earlier Miocene and the Pliocene; some geologists assert that the Neogene cannot be delineated from the modern geological period, the Quaternary. The term "Neogene" was coined in 1853 by the Austrian palaeontologist Moritz Hörnes. During this period and birds continued to evolve into modern forms, while other groups of life remained unchanged. Early hominids, the ancestors of humans, appeared in Africa near the end of the period; some continental movement took place, the most significant event being the connection of North and South America at the Isthmus of Panama, late in the Pliocene. This cut off the warm ocean currents from the Pacific to the Atlantic Ocean, leaving only the Gulf Stream to transfer heat to the Arctic Ocean; the global climate cooled over the course of the Neogene, culminating in a series of continental glaciations in the Quaternary Period that follows.
In ICS terminology, from upper to lower: The Pliocene Epoch is subdivided into 2 ages: Piacenzian Age, preceded by Zanclean AgeThe Miocene Epoch is subdivided into 6 ages: Messinian Age, preceded by Tortonian Age Serravallian Age Langhian Age Burdigalian Age Aquitanian AgeIn different geophysical regions of the world, other regional names are used for the same or overlapping ages and other timeline subdivisions. The terms Neogene System and upper Tertiary System describe the rocks deposited during the Neogene Period; the continents in the Neogene were close to their current positions. The Isthmus of Panama formed, connecting South America; the Indian subcontinent continued forming the Himalayas. Sea levels fell, creating land bridges between Africa and Eurasia and between Eurasia and North America; the global climate became seasonal and continued an overall drying and cooling trend which began at the start of the Paleogene. The ice caps on both poles began to grow and thicken, by the end of the period the first of a series of glaciations of the current Ice Age began.
Marine and continental flora and fauna have a modern appearance. The reptile group Choristodera became extinct in the early part of the period, while the amphibians known as Allocaudata disappeared at the end. Mammals and birds continued to be the dominant terrestrial vertebrates, took many forms as they adapted to various habitats; the first hominins, the ancestors of humans, may have appeared in southern Europe and migrated into Africa. In response to the cooler, seasonal climate, tropical plant species gave way to deciduous ones and grasslands replaced many forests. Grasses therefore diversified, herbivorous mammals evolved alongside it, creating the many grazing animals of today such as horses and bison. Eucalyptus fossil leaves occur in the Miocene of New Zealand, where the genus is not native today, but have been introduced from Australia; the Neogene traditionally ended at the end of the Pliocene Epoch, just before the older definition of the beginning of the Quaternary Period. However, there was a movement amongst geologists to include ongoing geological time in the Neogene, while others insist the Quaternary to be a separate period of distinctly different record.
The somewhat confusing terminology and disagreement amongst geologists on where to draw what hierarchical boundaries is due to the comparatively fine divisibility of time units as time approaches the present, due to geological preservation that causes the youngest sedimentary geological record to be preserved over a much larger area and to reflect many more environments than the older geological record. By dividing the Cenozoic Era into three periods instead of seven epochs, the periods are more comparable to the duration of periods in the Mesozoic and Paleozoic eras; the International Commission on Stratigraphy once proposed that the Quaternary be considered a sub-era of the Neogene, with a beginning date of 2.58 Ma, namely the start of the Gelasian Stage. In the 2004 proposal of the ICS, the Neogene would have consisted of the Miocene and Pliocene epochs; the International Union for Quaternary Research counterproposed that the Neogene and the Pliocene end at 2.58 Ma, that the Gelasian be transferred to the Pleistocene, the Quaternary be recognized as the third period in the Cenozoic, citing key changes in Earth's climate and biota that occurred 2.58 Ma and its correspondence to the Gauss-Matuyama magnetostratigraphic boundary.
In 2006 ICS and INQUA reached a compromise that made Quaternary a subera, subdividing Cenozoic into the old classical Tertiary and Quaternary, a compromise, rejected by International Union of Geological Sciences because it split both Neogene and Pliocene in two. Following formal discussions at the 2008 International Geological Congress in Oslo, the ICS decided in May 2009 to make the Quaternary the youngest period of the Cenozoic Era with its base at 2.58 Mya and including the Gelasian age, considered part of the Neogene Period and Pliocene Epoch. Thus the Neogene Period ends bounding the succeeding Quaternary Period at 2.58 Mya. "Digital Atlas of Neogene Life for the Southeastern United States". San Jose State University. Archived from the original on 2013-04-23. Retrieved 21 September 2018
The Ordovician is a geologic period and system, the second of six periods of the Paleozoic Era. The Ordovician spans 41.2 million years from the end of the Cambrian Period 485.4 million years ago to the start of the Silurian Period 443.8 Mya. The Ordovician, named after the Celtic tribe of the Ordovices, was defined by Charles Lapworth in 1879 to resolve a dispute between followers of Adam Sedgwick and Roderick Murchison, who were placing the same rock beds in northern Wales into the Cambrian and Silurian systems, respectively. Lapworth recognized that the fossil fauna in the disputed strata were different from those of either the Cambrian or the Silurian systems, placed them in a system of their own; the Ordovician received international approval in 1960, when it was adopted as an official period of the Paleozoic Era by the International Geological Congress. Life continued to flourish during the Ordovician as it did in the earlier Cambrian period, although the end of the period was marked by the Ordovician–Silurian extinction events.
Invertebrates, namely molluscs and arthropods, dominated the oceans. The Great Ordovician Biodiversification Event increased the diversity of life. Fish, the world's first true vertebrates, continued to evolve, those with jaws may have first appeared late in the period. Life had yet to diversify on land. About 100 times as many meteorites struck the Earth per year during the Ordovician compared with today; the Ordovician Period began with a major extinction called the Cambrian–Ordovician extinction event, about 485.4 Mya. It lasted for about 42 million years and ended with the Ordovician–Silurian extinction events, about 443.8 Mya which wiped out 60% of marine genera. The dates given are recent radiometric dates and vary from those found in other sources; this second period of the Paleozoic era created abundant fossils that became major petroleum and gas reservoirs. The boundary chosen for the beginning of both the Ordovician Period and the Tremadocian stage is significant, it correlates well with the occurrence of widespread graptolite and trilobite species.
The base of the Tremadocian allows scientists to relate these species not only to each other, but to species that occur with them in other areas. This makes it easier to place many more species in time relative to the beginning of the Ordovician Period. A number of regional terms have been used to subdivide the Ordovician Period. In 2008, the ICS erected a formal international system of subdivisions. There exist Baltoscandic, Siberian, North American, Chinese Mediterranean and North-Gondwanan regional stratigraphic schemes; the Ordovician Period in Britain was traditionally broken into Early and Late epochs. The corresponding rocks of the Ordovician System are referred to as coming from the Lower, Middle, or Upper part of the column; the faunal stages from youngest to oldest are: Late Ordovician Hirnantian/Gamach Rawtheyan/Richmond Cautleyan/Richmond Pusgillian/Maysville/Richmond Middle Ordovician Trenton Onnian/Maysville/Eden Actonian/Eden Marshbrookian/Sherman Longvillian/Sherman Soudleyan/Kirkfield Harnagian/Rockland Costonian/Black River Chazy Llandeilo Whiterock Llanvirn Early Ordovician Cassinian Arenig/Jefferson/Castleman Tremadoc/Deming/Gaconadian The Tremadoc corresponds to the Tremadocian.
The Floian corresponds to the lower Arenig. The Llanvirn occupies the rest of the Darriwilian, terminates with it at the base of the Late Ordovician; the Sandbian represents the first half of the Caradoc. During the Ordovician, the southern continents were collected into Gondwana. Gondwana started the period in equatorial latitudes and, as the period progressed, drifted toward the South Pole. Early in the Ordovician, the continents of Laurentia and Baltica were still independent continents, but Baltica began to move towards Laurentia in the period, causing the Iapetus Ocean between them to shrink; the small continent Avalonia separated from Gondwana and began to move north towards Baltica and Laurentia, opening the Rheic Ocean between Gondwana and Avalonia. The Taconic orogeny, a major mountain-building episode, was well under way in Cambrian times. In the early and middle Ordovician, temperatures were mild, but at the beginning of the Late Ordovician, from 460 to 450 Ma, volcanoes along the margin of the Iapetus Ocean spewed massive amounts of carbon dioxide, a greenhouse gas, into the atmosphere, turning the planet into a hothouse.
Sea levels were high, but as Gondwana moved south, ice accumulated into glaciers and sea levels dropped. At first, low-lying sea beds increased diversity, but glaciation led to mass extinctions as the seas drained and continental shelves became dry land. During the Ordovician, in fact during the Tremadocian, marine transgressions worldwide were the greatest for which evidence is preserved; these volcanic island arcs collided with proto North America to form the Appalachian mountains. By the end of the Late Ordovician the volcanic emissions had stopped. Gondwana had by that time neared the South Pole and was glaciated
In archaeological terms, a projectile point is an object, hafted to weapon, capable of being thrown or projected, such as a spear, dart, or arrow, or used as a knife. They are thus different from weapons presumed to have been kept in the hand, such as axes and maces, the stone mace or axe-heads attached to them. Stone tools, including projectile points, can survive for long periods, were lost or discarded, are plentiful at archaeological sites, providing useful clues to the human past, including prehistoric trade. A distinctive form of point, identified though lithic analysis of the way it was made, is a key diagnostic factor in identifying an archaeological industry or culture. Scientific techniques exist to track the specific kinds of rock or minerals that used to make stone tools in various regions back to their original sources; as well as stone, projectile points were made of worked bone, antler or ivory. In regions where metallurgy emerged, projectile points were made from copper, bronze, or iron, though the change was by no means immediate.
In North America, some late prehistoric points were fashioned from copper, mined in the Lake Superior region and elsewhere. A large variety of prehistoric arrowheads, dart points, spear points have been discovered. Flint, obsidian and many other rocks and minerals were used to make points in North America; the oldest projectile points found in North America were long thought to date from about 13,000 years ago, during the Paleo-Indian period, however recent evidence suggests that North American projectile points may date to as old as 15,500 years. Some of the more famous Paleo-Indian types include Clovis and Dalton points. Projectile points fall into two general types: dart/spear points, arrow points. Larger points were used to atlatl darts. Arrow points are smaller and lighter than dart points, were used to tip arrows; the question of how to distinguish an arrow point from a point used on a larger projectile is non-trivial. According to some investigators, the best indication is the width of the hafting area, thought to correlate to the width of the shaft.
An alternative approach is to distinguish arrow points by their smaller size. Projectile points come in an amazing variety of shapes and styles, which vary according to chronological periods, cultural identities, intended functions. Typological studies of projectile points have become more elaborate through the years. For instance, Gregory Perino began his categorical study of projectile point typology in the late 1950s. Collaborating with Robert Bell, he published a set of four volumes defining the known point types of that time. Perino followed this several years with a three-volume study of "Selected Preforms and Knives of the North American Indians". Another recent set of typological studies of North American projectile points has been produced by Noel Justice. Bare Island projectile point Cascade point Clovis point Cumberland point Eden point Folsom point Greene projectile point Jack's Reef pentagonal projectile point Lamoka projectile point Levanna projectile point Susquehanna broad projectile point Plano point Elf-arrows Levallois technique Lithic reduction
In biology, a species is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is defined as the largest group of organisms in which any two individuals of the appropriate sexes or mating types can produce fertile offspring by sexual reproduction. Other ways of defining species include their karyotype, DNA sequence, behaviour or ecological niche. In addition, paleontologists use the concept of the chronospecies since fossil reproduction cannot be examined. While these definitions may seem adequate, when looked at more they represent problematic species concepts. For example, the boundaries between related species become unclear with hybridisation, in a species complex of hundreds of similar microspecies, in a ring species. Among organisms that reproduce only asexually, the concept of a reproductive species breaks down, each clone is a microspecies. All species are given a two-part name, a "binomial"; the first part of a binomial is the genus.
The second part is called the specific epithet. For example, Boa constrictor is one of four species of the genus Boa. None of these is satisfactory definitions, but scientists and conservationists need a species definition which allows them to work, regardless of the theoretical difficulties. If species were fixed and distinct from one another, there would be no problem, but evolutionary processes cause species to change continually, to grade into one another. Species were seen from the time of Aristotle until the 18th century as fixed kinds that could be arranged in a hierarchy, the great chain of being. In the 19th century, biologists grasped. Charles Darwin's 1859 book The Origin of Species explained how species could arise by natural selection; that understanding was extended in the 20th century through genetics and population ecology. Genetic variability arises from mutations and recombination, while organisms themselves are mobile, leading to geographical isolation and genetic drift with varying selection pressures.
Genes can sometimes be exchanged between species by horizontal gene transfer. Viruses are a special case, driven by a balance of mutation and selection, can be treated as quasispecies. Biologists and taxonomists have made many attempts to define species, beginning from morphology and moving towards genetics. Early taxonomists such as Linnaeus had no option but to describe what they saw: this was formalised as the typological or morphological species concept. Ernst Mayr emphasised reproductive isolation, but this, like other species concepts, is hard or impossible to test. Biologists have tried to refine Mayr's definition with the recognition and cohesion concepts, among others. Many of the concepts are quite similar or overlap, so they are not easy to count: the biologist R. L. Mayden recorded about 24 concepts, the philosopher of science John Wilkins counted 26. Wilkins further grouped the species concepts into seven basic kinds of concepts: agamospecies for asexual organisms biospecies for reproductively isolated sexual organisms ecospecies based on ecological niches evolutionary species based on lineage genetic species based on gene pool morphospecies based on form or phenotype and taxonomic species, a species as determined by a taxonomist.
A typological species is a group of organisms in which individuals conform to certain fixed properties, so that pre-literate people recognise the same taxon as do modern taxonomists. The clusters of variations or phenotypes within specimens would differentiate the species; this method was used as a "classical" method of determining species, such as with Linnaeus early in evolutionary theory. However, different phenotypes are not different species. Species named in this manner are called morphospecies. In the 1970s, Robert R. Sokal, Theodore J. Crovello and Peter Sneath proposed a variation on this, a phenetic species, defined as a set of organisms with a similar phenotype to each other, but a different phenotype from other sets of organisms, it differs from the morphological species concept in including a numerical measure of distance or similarity to cluster entities based on multivariate comparisons of a reasonably large number of phenotypic traits. A mate-recognition species is a group of sexually reproducing organisms that recognize one another as potential mates.
Expanding on this to allow for post-mating isolation, a cohesion species is the most inclusive population of individuals having the potential for phenotypic cohesion through intrinsic cohesion mechanisms. A further development of the recognition concept is provided by the biosemiotic concept of species. In microbiology, genes can move even between distantly related bacteria extending to the whole bacterial domain; as a rule of thumb, microbiologists have assumed that kinds of Bacteria or Archaea with 16S ribosomal RNA gene sequences more similar than 97% to each other need to be checked by DNA-DNA hybridisation to decide if they belong to the same species or not. This concept was narrowed in 2006 to a similarity of 98.7%. DNA-DNA hybri