Introduction to evolution
Evolution is the process of change in all forms of life over generations, evolutionary biology is the study of how evolution occurs. Biological populations evolve through genetic changes that correspond to changes in the organisms' observable traits. Genetic changes include mutations, which are caused by damage or replication errors in organisms' DNA; as the genetic variation of a population drifts randomly over generations, natural selection leads traits to become more or less common based on the relative reproductive success of organisms with those traits. The age of the Earth is about 4.54 billion years. The earliest undisputed evidence of life on Earth dates at least from 3.5 billion years ago. Evolution does not attempt to explain the origin of life, but it does explain how early lifeforms evolved into the complex ecosystem that we see today. Based on the similarities between all present-day organisms, all life on Earth is assumed to have originated through common descent from a last universal ancestor from which all known species have diverged through the process of evolution.
All individuals have hereditary material in the form of genes received from their parents, which they pass on to any offspring. Among offspring there are variations of genes due to the introduction of new genes via random changes called mutations or via reshuffling of existing genes during sexual reproduction; the offspring differs from the parent in minor random ways. If those differences are helpful, the offspring is more to survive and reproduce; this means that more offspring in the next generation will have that helpful difference and individuals will not have equal chances of reproductive success. In this way, traits that result in organisms being better adapted to their living conditions become more common in descendant populations; these differences accumulate resulting in changes within the population. This process is responsible for the many diverse life forms in the world; the modern understanding of evolution began with the 1859 publication of Charles Darwin's On the Origin of Species.
In addition, Gregor Mendel's work with plants helped to explain the hereditary patterns of genetics. Fossil discoveries in paleontology, advances in population genetics and a global network of scientific research have provided further details into the mechanisms of evolution. Scientists now have a good understanding of the origin of new species and have observed the speciation process in the laboratory and in the wild. Evolution is the principal scientific theory that biologists use to understand life and is used in many disciplines, including medicine, conservation biology, forensics and other social-cultural applications; the main ideas of evolution may be summarized as follows: Life forms reproduce and therefore have a tendency to become more numerous. Factors such as predation and competition work against the survival of individuals; each offspring differs from their parent in random ways. If these differences are beneficial, the offspring is more to survive and reproduce; this makes it that more offspring in the next generation will have beneficial differences and fewer will have detrimental differences.
These differences accumulate over generations. Over time, populations can branch off into new species; these processes, collectively known as evolution, are responsible for the many diverse life forms seen in the world. In the 19th century, natural history collections and museums were popular; the European expansion and naval expeditions employed naturalists, while curators of grand museums showcased preserved and live specimens of the varieties of life. Charles Darwin was an English graduate trained in the disciplines of natural history; such natural historians would collect, catalogue and study the vast collections of specimens stored and managed by curators at these museums. Darwin served as a ship's naturalist on board HMS Beagle, assigned to a five-year research expedition around the world. During his voyage, he observed and collected an abundance of organisms, being interested in the diverse forms of life along the coasts of South America and the neighboring Galápagos Islands. Darwin gained extensive experience as he collected and studied the natural history of life forms from distant places.
Through his studies, he formulated the idea that each species had developed from ancestors with similar features. In 1838, he described; the size of a population depends on how much. For the population to remain the same size year after year, there must be an equilibrium, or balance between the population size and available resources. Since organisms produce more offspring than their environment can support, not all individuals can survive out of each generation. There must be a competitive struggle for resources; as a result, Darwin realised. Instead, survival of an organism depends on the differences of each individual organism, or "traits," that aid or hinder survival and reproduction. Well-adapted individuals are to leave more offspring than their less well-adapted competitors. Traits that hinder survival and reproduction would disappear over generations. Traits that help an organism survive and reproduce would accumulate over generations. Darwin realised that the unequal ability of individuals to survive and reproduce could cause gradual changes in the population and used the term natural selection to describe this process.
Observations of variations in animals and plants formed the basis of the theory of natural sele
John Gould FRS was an English ornithologist and bird artist. He published a number of monographs on birds, illustrated by plates that he produced with the assistance of his wife, Elizabeth Gould, several other artists including Edward Lear, Henry Constantine Richter, Joseph Wolf and William Matthew Hart, he has been considered the father of bird study in Australia and the Gould League in Australia is named after him. His identification of the birds now nicknamed "Darwin's finches" played a role in the inception of Darwin's theory of evolution by natural selection. Gould's work is referenced On the Origin of Species. Gould was born in Lyme Regis the first son of a gardener, he and the boy had a scanty education. Shortly afterwards his father obtained a position on an estate near Guildford, in 1818 Gould became foreman in the Royal Gardens of Windsor, he was for some time under the care of the Royal Gardens of Windsor. The young Gould started training as a gardener, being employed under his father at Windsor from 1818 to 1824, he was subsequently a gardener at Ripley Castle in Yorkshire.
He became an expert in the art of taxidermy. In 1824 he set himself up in business in London as a taxidermist, his skill helped him to become the first Curator and Preserver at the museum of the Zoological Society of London in 1827. Gould's position brought him into contact with the country's leading naturalists; this meant that he was the first to see new collections of birds given to the Zoological Society of London. In 1830 a collection of birds arrived from the Himalayas, many not described. Gould published these birds in A Century of Birds from the Himalaya Mountains; the text was by Nicholas Aylward Vigors and the illustrations were drawn and lithographed by Gould's wife Elizabeth Coxen Gould. Most of Gould's work were rough sketches on paper from which other artists created the lithographic plates; this work was followed by four more in the next seven years, including Birds of Europe in five volumes. It was completed in 1837; the plates were lithographed by Elizabeth Coxen Gould. A few of the illustrations were made by Edward Lear as part of his Illustrations of the Family of Psittacidae in 1832.
Lear, was in financial difficulty, he sold the entire set of lithographs to Gould. The books were published in a large size, imperial folio, with magnificent coloured plates. 41 of these volumes were published, with about 3000 plates. They appeared in parts at £3 3s. A number, subscribed for in advance, in spite of the heavy expense of preparing the plates, Gould succeeded in making his ventures pay, realising a fortune; this was a busy period for Gould who published Icones Avium in two parts containing 18 leaves of bird studies on 54 cm plates as a supplement to his previous works. No further monographs were published as in 1838 he and his wife moved to Australia to work on the Birds of Australia. Shortly after their return to England, his wife died in 1841. Elizabeth Gould completed 84 plates for Birds of Australia before her death; when Charles Darwin presented his mammal and bird specimens collected during the second voyage of HMS Beagle to the Zoological Society of London on 4 January 1837, the bird specimens were given to Gould for identification.
He set aside his paying work and at the next meeting on 10 January reported that birds from the Galápagos Islands which Darwin had thought were blackbirds, "gross-bills" and finches were in fact "a series of ground Finches which are so peculiar" as to form "an new group, containing 12 species." This story made the newspapers. In March, Darwin met Gould again, learning that his Galápagos "wren" was another species of finch and the mockingbirds he had labelled by island were separate species rather than just varieties, with relatives on the South American mainland. Subsequently, Gould advised that the smaller southern Rhea specimen, rescued from a Christmas dinner was a separate species which he named Rhea darwinii, whose territory overlapped with the northern rheas. Darwin had not bothered to label his finches by island, but others on the expedition had taken more care, he now sought specimens collected by crewmen. From them he was able to establish that the species were unique to islands, an important step on the inception of his theory of evolution by natural selection.
Gould's work on the birds was published between 1838 and 1842 in five numbers as Part 3 of Zoology of the Voyage of H. M. S. Beagle, edited by Charles Darwin. Elizabeth Gould illustrated all the plates for Part 3. In 1838 the Goulds sailed to Australia, intending to study the birds of that country and be the first to produce a major work on the subject, they took with them the collector John Gilbert. They arrived in Tasmania in September, making the acquaintance of the governor Sir John Franklin and his wife. Gould and Gilbert collected on the island. In February 1839 Gould sailed to Sydney, he travelled to his brother-in-law's station at Yarrundi, spending his time searching for bowerbirds in the Liverpool Range. In April he returned to Tasmania for the birth of his son. In May he sailed to Adelaide to meet Charles Sturt, preparing to lead an expedition to the Murray River. Gould collected in the Mount Lofty range, the Murray Scrubs and Kangaroo Island, returning again to Hobart in July, he travelled with his wife to Yarrundi.
They returned home to England in May 1840. The result of the trip was The Birds of Australia, it included a total of 600 plates in seven volumes.
Timeline of the evolutionary history of life
This timeline of the evolutionary history of life represents the current scientific theory outlining the major events during the development of life on planet Earth. In biology, evolution is any change across successive generations in the heritable characteristics of biological populations. Evolutionary processes give rise to diversity at every level of biological organization, from kingdoms to species, individual organisms and molecules, such as DNA and proteins; the similarities between all present day organisms indicate the presence of a common ancestor from which all known species and extinct, have diverged through the process of evolution. More than 99 percent of all species, amounting to over five billion species, that lived on Earth are estimated to be extinct. Estimates on the number of Earth's current species range from 10 million to 14 million, of which about 1.2 million have been documented and over 86 percent have not yet been described. However, a May 2016 scientific report estimates that 1 trillion species are on Earth, with only one-thousandth of one percent described.
While the dates given in this article are estimates based on scientific evidence, there has been controversy between more traditional views of increased biodiversity through a cone of diversity with the passing of time and the view that the basic pattern on Earth has been one of annihilation and diversification and that in certain past times, such as the Cambrian explosion, there was great diversity. Species go extinct as environments change, as organisms compete for environmental niches, as genetic mutation leads to the rise of new species from older ones. Biodiversity on Earth takes a hit in the form of a mass extinction in which the extinction rate is much higher than usual. A large extinction-event represents an accumulation of smaller extinction- events that take place in a brief period of time; the first known mass extinction in earth's history was the Great Oxygenation Event 2.4 billion years ago. That event led to the loss of most of the planet's obligate anaerobes. Researchers have identified five major extinction events in earth's history since: End of the Ordovician: 440 million years ago, 86% of all species lost, including graptolites Late Devonian: 375 million years ago, 75% of species lost, including most trilobites End of the Permian, "The Great Dying": 251 million years ago, 96% of species lost, including tabulate corals, most extant trees and synapsids End of the Triassic: 200 million years ago, 80% of species lost, including all of the conodonts End of the Cretaceous: 66 million years ago, 76% of species lost, including all of the ammonites, ichthyosaurs, plesiosaurs and nonavian dinosaurs Smaller extinction-events have occurred in the periods between these larger catastrophes, with some standing at the delineation points of the periods and epochs recognized by scientists in geologic time.
The Holocene extinction event is under way. Factors in mass extinctions include continental drift, changes in atmospheric and marine chemistry and other aspects of mountain formation, changes in glaciation, changes in sea level, impact events. In this timeline, Ma means "million years ago," ka means "thousand years ago," and ya means "years ago." 4000 Ma and earlier. 4000 Ma – 2500 Ma 2500 Ma – 542 Ma. Contains the Palaeoproterozoic and Neoproterozoic eras. 542 Ma – present The Phanerozoic Eon the "period of well-displayed life," marks the appearance in the fossil record of abundant, shell-forming and/or trace-making organisms. It is subdivided into three eras, the Paleozoic and Cenozoic, which are divided by major mass extinctions. 542 Ma – 251.0 Ma and contains the Cambrian, Silurian, Devonian and Permian periods. From 251.4 Ma to 66 Ma and containing the Triassic and Cretaceous periods. 66 Ma – present Dawkins, Richard. The Ancestor's Tale: A Pilgrimage to the Dawn of Life. Boston: Houghton Mifflin Company.
ISBN 978-0-618-00583-3. LCCN 2004059864. OCLC 56617123. "Understanding Evolution: your one-stop resource for information on evolution". University of California, Berkeley. Retrieved 2015-03-18. "Life on Earth". Tree of Life Web Project. University of Arizona. January 1, 1997. Retrieved 2015-03-18. Explore complete phylogenetic tree interactively Brandt, Niel. "Evolutionary and Geological Timelines". TalkOrigins Archive. Houston, TX: The TalkOrigins Foundation, Inc. Retrieved 2015-03-18. "Palaeos: Life Through Deep Time". Palaeos. Retrieved 2015-03-18. Kyrk, John. "Evolution". Cell Biology Animation. Retrieved 2015-03-18. Interactive timeline from Big Bang to present "Plant Evolution". Plant and Animal Evolution. University of Waikato. Retrieved 2015-03-18. Sequence of Plant Evolution "The History of Animal Evolution". Plant and Animal Evolution. University of Waikato. Retrieved 2015-03-18. Sequence of Animal Evolution Yeo, Dannel. "History of Life on Earth". Archived from the original on 2015-03-15. Retrieved 2015-03-19.
Exploring Time. The Science Channel. 2007. Retrieved 2015-03-19. Roberts, Ben. "Plant evolution timeline". University of Cambridge. Archived from the original on 2015-03-13. Retrieved 2015-03-19. Art of the Nature Timelines on Wikipedia
In biology, adaptation has three related meanings. Firstly, it is the dynamic evolutionary process that fits organisms to their environment, enhancing their evolutionary fitness. Secondly, it is a state reached by the population during that process. Thirdly, it is a phenotypic or adaptive trait, with a functional role in each individual organism, maintained and has evolved through natural selection. Organisms face a succession of environmental challenges as they grow, show adaptive plasticity as traits develop in response to the imposed conditions; this gives them resilience to varying environments. Adaptation is an observable fact of life accepted by philosophers and natural historians from ancient times, independently of their views on evolution, but their explanations differed. Empedocles did not believe that adaptation required a final cause, but thought that it "came about since such things survived." Aristotle assumed that species were fixed. In natural theology, adaptation was interpreted as the work of a deity and as evidence for the existence of God.
William Paley believed that organisms were adapted to the lives they led, an argument that shadowed Gottfried Wilhelm Leibniz, who had argued that God had brought about "the best of all possible worlds." Voltaire's Dr. Pangloss is a parody of this optimistic idea, David Hume argued against design; the Bridgewater Treatises are a product of natural theology, though some of the authors managed to present their work in a neutral manner. The series was lampooned by Robert Knox, who held quasi-evolutionary views, as the Bilgewater Treatises. Charles Darwin broke with the tradition by emphasising the flaws and limitations which occurred in the animal and plant worlds. Jean-Baptiste Lamarck proposed a tendency for organisms to become more complex, moving up a ladder of progress, plus "the influence of circumstances," expressed as use and disuse; this second, subsidiary element of his theory is what is now called Lamarckism, a proto-evolutionary hypothesis of the inheritance of acquired characteristics, intended to explain adaptations by natural means.
Other natural historians, such as Buffon, accepted adaptation, some accepted evolution, without voicing their opinions as to the mechanism. This illustrates the real merit of Darwin and Alfred Russel Wallace, secondary figures such as Henry Walter Bates, for putting forward a mechanism whose significance had only been glimpsed previously. A century experimental field studies and breeding experiments by people such as E. B. Ford and Theodosius Dobzhansky produced evidence that natural selection was not only the'engine' behind adaptation, but was a much stronger force than had been thought; the significance of an adaptation can only be understood in relation to the total biology of the species. Adaptation is a process rather than a physical form or part of a body. An internal parasite can illustrate the distinction: such a parasite may have a simple bodily structure, but the organism is adapted to its specific environment. From this we see that adaptation is not just a matter of visible traits: in such parasites critical adaptations take place in the life cycle, quite complex.
However, as a practical term, "adaptation" refers to a product: those features of a species which result from the process. Many aspects of an animal or plant can be called adaptations, though there are always some features whose function remains in doubt. By using the term adaptation for the evolutionary process, adaptive trait for the bodily part or function, one may distinguish the two different senses of the word. Adaptation is one of the two main processes that explain the observed diversity of species, such as the different species of Darwin's finches; the other process is speciation, in which new species arise through reproductive isolation. A favourite example used today to study the interplay of adaptation and speciation is the evolution of cichlid fish in African lakes, where the question of reproductive isolation is complex. Adaptation is not always a simple matter where the ideal phenotype evolves for a given external environment. An organism must be viable at all stages of its development and at all stages of its evolution.
This places constraints on the evolution of development and structure of organisms. The main constraint, over which there has been much debate, is the requirement that each genetic and phenotypic change during evolution should be small, because developmental systems are so complex and interlinked. However, it is not clear what "relatively small" should mean, for example polyploidy in plants is a reasonably common large genetic change; the origin of eukaryotic endosymbiosis is a more dramatic example. All adaptations help; the adaptive traits may be behavioural or physiological. Structural adaptations are physical features of an organism, such as shape, body covering and internal organization. Behavioural adaptations are inherited systems of behaviour, whether inherited in detail as instincts, or as a neuropsychological capacity for learning. Examples include searching for food and vocalizations. Physiological adaptations permit the organism to perform special functions such as making venom, secreting slime, phototropism), but involve more general functions such as growth and development, temperature regulation, ionic balance and other aspects of homeostasis.
Adaptation affects all aspects of the life of an organism. The following definitions are given by the evolutionary biologist Theodosius Dobzhansky: 1. Adaptation is the evolutionary pr
The eclipse of Darwinism
Julian Huxley used the phrase “the eclipse of Darwinism” to describe the state of affairs prior to what he called the modern synthesis, when evolution was accepted in scientific circles but few biologists believed that natural selection was its primary mechanism. Historians of science such as Peter J. Bowler have used the same phrase as a label for the period within the history of evolutionary thought from the 1880s to around 1920, when alternatives to natural selection were developed and explored—as many biologists considered natural selection to have been a wrong guess on Charles Darwin's part, or at least as of minor importance. An alternative term, the interphase of Darwinism, has been proposed to avoid the incorrect implication that the putative eclipse was preceded by a period of vigorous Darwinian research. While there had been multiple explanations of evolution including vitalism and structuralism through the 19th century, four major alternatives to natural selection were in play at the turn of the 20th century: Theistic evolution was the belief that God directly guided evolution.
Neo-Lamarckism was the idea that evolution was driven by the inheritance of characteristics acquired during the life of the organism. Orthogenesis was the belief that organisms were affected by internal forces or laws of development that drove evolution in particular directions Mutationism was the idea that evolution was the product of mutations that created new forms or species in a single step. Theistic evolution disappeared from the scientific literature by the end of the 19th century as direct appeals to supernatural causes came to be seen as unscientific; the other alternatives had significant followings well into the 20th century. Ernst Mayr wrote. Evolution was accepted in scientific circles within a few years after the publication of On the Origin of Species, but acceptance of natural selection as its driving mechanism was much less. Six objections were raised to the theory in the 19th century: The fossil record was discontinuous, suggesting gaps in evolution; the physicist Lord Kelvin calculated in 1862 that the earth would have cooled in 100 million years or less from its formation, too little time for evolution.
It was argued that many structures were nonadaptive, so they could not have evolved under natural selection. Some structures seemed to have evolved on a regular pattern, like the eyes of unrelated animals such as the squid and mammals. Natural selection was argued not to be creative, while variation was admitted to be not of value; the engineer Fleeming Jenkin noted in 1868, reviewing The Origin of Species, that the blending inheritance favoured by Darwin would oppose the action of natural selection. Both Darwin and his close supporter Thomas Henry Huxley admitted, that selection might not be the whole explanation. By the end of the 19th century, criticism of natural selection had reached the point that in 1903 the German botanist, Eberhard Dennert, wrote that "We are now standing at the death bed of Darwinism", in 1907 the Stanford University entomologist Vernon Lyman Kellogg, who supported natural selection, asserted that "... the fair truth is that the Darwinian selection theory, considered with regard to its claimed capacity to be an independently sufficient mechanical explanation of descent, stands today discredited in the biological world."
He added, that there were problems preventing the widespread acceptance of any of the alternatives, as large mutations seemed too uncommon, there was no experimental evidence of mechanisms that could support either Lamarckism or orthogenesis. Ernst Mayr wrote that a survey of evolutionary literature and biology textbooks showed that as late as 1930 the belief that natural selection was the most important factor in evolution was a minority viewpoint, with only a few population geneticists being strict selectionists. A variety of different factors motivated people to propose other evolutionary mechanisms as alternatives to natural selection, some of them dating back before Darwin's Origin of Species. Natural selection, with its emphasis on death and competition, did not appeal to some naturalists because they felt it was immoral, left little room for teleology or the concept of progress in the development of life; some of these scientists and philosophers, like St. George Jackson Mivart and Charles Lyell, who came to accept evolution but disliked natural selection, raised religious objections.
Others, such as Herbert Spencer, the botanist George Henslow, Samuel Butler, felt that evolution was an inherently progressive process that natural selection alone was insufficient to explain. Still others, including the American paleontologists Edward Drinker Cope and Alpheus Hyatt, had an idealist perspective and felt that nature, including the development of life, followed orderly patterns that natural selection could not explain. Another factor was the rise of a new faction of biologists at the end of the 19th century, typified by the geneticists Hugo DeVries and Thomas Hunt Morgan, who wanted to recast biology as an experimental laboratory science, they distrusted the work of naturalists like Darwin and Alfred Russel Wallace, dependent on field observations of
Speciation is the evolutionary process by which populations evolve to become distinct species. The biologist Orator F. Cook coined the term in 1906 for cladogenesis, the splitting of lineages, as opposed to anagenesis, phyletic evolution within lineages. Charles Darwin was the first to describe the role of natural selection in speciation in his 1859 book The Origin of Species, he identified sexual selection as a mechanism, but found it problematic. There are four geographic modes of speciation in nature, based on the extent to which speciating populations are isolated from one another: allopatric, peripatric and sympatric. Speciation may be induced artificially, through animal husbandry, agriculture, or laboratory experiments. Whether genetic drift is a minor or major contributor to speciation is the subject matter of much ongoing discussion. Rapid sympatric speciation can take place through polyploidy, such as by doubling of chromosome number. New species can be created through hybridisation followed, if the hybrid is favoured by natural selection, by reproductive isolation.
In addressing the question of the origin of species, there are two key issues: what are the evolutionary mechanisms of speciation, what accounts for the separateness and individuality of species in the biota? Since Charles Darwin's time, efforts to understand the nature of species have focused on the first aspect, it is now agreed that the critical factor behind the origin of new species is reproductive isolation. Next we focus on the second aspect of the origin of species. In On the Origin of Species, Darwin interpreted biological evolution in terms of natural selection, but was perplexed by the clustering of organisms into species. Chapter 6 of Darwin's book is entitled "Difficulties of the Theory." In discussing these "difficulties" he noted "Firstly, why, if species have descended from other species by insensibly fine gradations, do we not everywhere see innumerable transitional forms? Why is not all nature in confusion instead of the species being, as we see them, well defined?" This dilemma can be referred to as the rarity of transitional varieties in habitat space.
Another dilemma, related to the first one, is the absence or rarity of transitional varieties in time. Darwin pointed out that by the theory of natural selection "innumerable transitional forms must have existed," and wondered "why do we not find them embedded in countless numbers in the crust of the earth." That defined species do exist in nature in both space and time implies that some fundamental feature of natural selection operates to generate and maintain species. It has been argued that the resolution of Darwin's first dilemma lies in the fact that out-crossing sexual reproduction has an intrinsic cost of rarity; the cost of rarity arises. If, on a resource gradient, a large number of separate species evolve, each exquisitely adapted to a narrow band on that gradient, each species will, of necessity, consist of few members. Finding a mate under these circumstances may present difficulties when many of the individuals in the neighborhood belong to other species. Under these circumstances, if any species’ population size happens, by chance, to increase, this will make it easier for its members to find sexual partners.
The members of the neighboring species, whose population sizes have decreased, experience greater difficulty in finding mates, therefore form pairs less than the larger species. This has a snowball effect, with large species growing at the expense of the smaller, rarer species driving them to extinction. Only a few species remain, each distinctly different from the other; the cost of rarity not only involves the costs of failure to find a mate, but indirect costs such as the cost of communication in seeking out a partner at low population densities. Rarity brings with it other costs. Rare and unusual features are seldom advantageous. In most instances, they indicate a mutation, certain to be deleterious, it therefore behooves sexual creatures to avoid mates sporting unusual features. Sexual populations therefore shed rare or peripheral phenotypic features, thus canalizing the entire external appearance, as illustrated in the accompanying illustration of the African pygmy kingfisher, Ispidina picta.
This uniformity of all the adult members of a sexual species has stimulated the proliferation of field guides on birds, reptiles and many other taxa, in which a species can be described with a single illustration. Once a population has become as homogeneous in appearance as is typical of most species, its members will avoid mating with members of other populations that look different from themselves. Thus, the avoidance of mates displaying rare and unusual phenotypic features leads to reproductive isolation, one of the hallmarks of speciation. In the contrasting case of organisms that reproduce asexually, there is no cost of rarity. Thus, asexual organisms frequently show the continuous variation in form that Darwin expected evolution to produce, making their classification into "species" difficult. All forms of natural speciation have taken place over the course of evolution.
Cladistics is an approach to biological classification in which organisms are categorized in groups based on the most recent common ancestor. Hypothesized relationships are based on shared derived characteristics that can be traced to the most recent common ancestor and are not present in more distant groups and ancestors. A key feature of a clade is that all its descendants are part of the clade. All descendants stay in their overarching ancestral clade. For example, if within a strict cladistic framework the terms animals, bilateria/worms, fishes/vertebrata, or monkeys/anthropoidea were used, these terms would include humans. Many of these terms are used paraphyletically, outside of cladistics, e.g. as a'grade'. Radiation results in the generation of new subclades by bifurcation; the techniques and nomenclature of cladistics have been applied to other disciplines. Cladistics is now the most used method to classify organisms; the original methods used in cladistic analysis and the school of taxonomy derived from the work of the German entomologist Willi Hennig, who referred to it as phylogenetic systematics.
Cladistics in the original sense refers to a particular set of methods used in phylogenetic analysis, although it is now sometimes used to refer to the whole field. What is now called the cladistic method appeared as early as 1901 with a work by Peter Chalmers Mitchell for birds and subsequently by Robert John Tillyard in 1921, W. Zimmermann in 1943; the term "clade" was introduced in 1958 by Julian Huxley after having been coined by Lucien Cuénot in 1940, "cladogenesis" in 1958, "cladistic" by Cain and Harrison in 1960, "cladist" by Mayr in 1965, "cladistics" in 1966. Hennig referred to his own approach as "phylogenetic systematics". From the time of his original formulation until the end of the 1970s, cladistics competed as an analytical and philosophical approach to systematics with phenetics and so-called evolutionary taxonomy. Phenetics was championed at this time by the numerical taxonomists Peter Sneath and Robert Sokal, evolutionary taxonomy by Ernst Mayr. Conceived, if only in essence, by Willi Hennig in a book published in 1950, cladistics did not flourish until its translation into English in 1966.
Today, cladistics is the most popular method for constructing phylogenies from morphological data. In the 1990s, the development of effective polymerase chain reaction techniques allowed the application of cladistic methods to biochemical and molecular genetic traits of organisms, vastly expanding the amount of data available for phylogenetics. At the same time, cladistics became popular in evolutionary biology, because computers made it possible to process large quantities of data about organisms and their characteristics; the cladistic method interprets each character state transformation implied by the distribution of shared character states among taxa as a potential piece of evidence for grouping. The outcome of a cladistic analysis is a cladogram – a tree-shaped diagram, interpreted to represent the best hypothesis of phylogenetic relationships. Although traditionally such cladograms were generated on the basis of morphological characters and calculated by hand, genetic sequencing data and computational phylogenetics are now used in phylogenetic analyses, the parsimony criterion has been abandoned by many phylogeneticists in favor of more "sophisticated" but less parsimonious evolutionary models of character state transformation.
Cladists contend. Every cladogram is based on a particular dataset analyzed with a particular method. Datasets are tables consisting of molecular, ethological and/or other characters and a list of operational taxonomic units, which may be genes, populations, species, or larger taxa that are presumed to be monophyletic and therefore to form, all together, one large clade. Different datasets and different methods, not to mention violations of the mentioned assumptions result in different cladograms. Only scientific investigation can show, more to be correct; until for example, cladograms like the following have been accepted as accurate representations of the ancestral relations among turtles, lizards and birds: If this phylogenetic hypothesis is correct the last common ancestor of turtles and birds, at the branch near the ▼ lived earlier than the last common ancestor of lizards and birds, near the ♦. Most molecular evidence, produces cladograms more like this: If this is accurate the last common ancestor of turtles and birds lived than the last common ancestor of lizards and birds.
Since the cladograms provide competing accounts of real events, at most one of them is correct. The cladogram to the right represents the current universally accepted hypothesis that all primates, including strepsirrhines like the lemurs and lorises, had a common ancestor all of whose descendants were primates, so form a clade. Within the primates, all anthropoids are hypothesized to have had a common ancestor all of whose descendants were anthropoids, so they form the clade called Anthropoidea; the "prosimians", on the other hand, form a paraphyletic taxon. The name Prosimii is not used in phylogenetic nomenclature, whic