Portal:Evolutionary biology/Selected articles

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Geologic clock.jpg
The evolutionary history of life and origin of life are fields on ongoing geological and biological research. Although not necessary conditions for the acceptance of evolution by natural selection, the origin of life and its evolutionary history can nonetheless help shed light on evolutionary processes. The current scientific consensus is that the complex biochemistry that makes up life came from simpler chemical reactions, but it is unclear how this occurred. Not much is certain about the earliest developments in life, the structure of the first living things, or the identity and nature of any last universal common ancestor or ancestral gene pool. Consequently, there is no scientific consensus on how life began, but proposals include self-replicating molecules such as RNA, and the assembly of simple cells. The first simple, sea dwelling organic structures appeared about 3,400 million years ago. It is considered that they may have formed when certain chemical (organic) molecules joined together. Prokaryotes, single-celled micro-organisms like blue green algae, were able to photosynthesize and produce oxygen. Around thousand million years later, sufficient oxygen had built up in the atmosphere and hence it allow multicellular organisms to proliferate in the Precambrian seas.


DNA chemical structure
Deoxyribonucleic acid (DNA) is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms. The main role of DNA molecules is the long-term storage of information. DNA is often compared to a set of blueprints, since it contains the instructions needed to construct other components of cells, such as proteins and RNA molecules. The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in regulating the use of this genetic information. Chemically, DNA is a long polymer of simple units called nucleotides, with a backbone made of sugars and phosphate groups joined by ester bonds. Attached to each sugar is one of four types of molecules called bases. It is the sequence of these four bases along the backbone that encodes information. This information is read using the genetic code, which specifies the sequence of the amino acids within proteins. The code is read by copying stretches of DNA into the related nucleic acid RNA, in a process called transcription. Most of these RNA molecules are used to synthesize proteins, but others are used directly in structures such as ribosomes and spliceosomes.


Artist's impression of a major impact event.
The Cretaceous–Paleogene extinction event was the large-scale mass extinction of animal and plant species in a geologically short period of time, approximately 66 million years ago (mya). It is widely known as the K–T extinction event and is associated with a geological signature, usually a thin band dated to that time and found in various parts of the world, known as the K–T boundary. K is the traditional abbreviation for the Cretaceous Period, and T is the abbreviation for the Tertiary Period. The event marks the end of the Mesozoic Era, and the beginning of the Cenozoic Era. Non-avian dinosaur fossils are only found below the K–T boundary and became extinct immediately before or during the event. A very small number of dinosaur fossils have been found above the K-T boundary, but they have been explained as reworked, that is, fossils that have been eroded from their original locations then preserved in later sedimentary layers. Mosasaurs, plesiosaurs, pterosaurs and many species of plants and invertebrates also became extinct. Mammalian and bird clades passed through the boundary with few extinctions, and radiation from those Maastrichtian clades occurred well past the boundary. Rates of extinction and radiation varied across different clades of organisms. Many scientists theorize that the K-T extinctions were caused by one or more catastrophic events such as massive asteroid impacts or increased volcanic activity. Several impact craters and massive volcanic activity in the Deccan traps have been dated to the approximate time of the extinction event. These geological events may have reduced sunlight and hindered photosynthesis, leading to a massive disruption in Earth's ecology. Other researchers believe the extinction was more gradual, resulting from slower changes in sea level or climate.


Darwins first tree
Evolutionary thought has roots in antiquity as philosophical ideas known to the Greeks, Romans, Indians, Chinese and Muslims. Until the 18th century, however, Western biological thought was dominated by essentialism, the idea that living forms are static and unchanging in time. During the Enlightenment, evolutionary cosmology and the mechanical philosophy spread from the physical sciences to natural history, and naturalists such as Maupertuis and Georges-Louis Leclerc, Comte de Buffon focused on the variability of species. The emergence of paleontology (and with it the notion of extinction), as well as the dramatic expansion of known species, helped undermine the traditional static view of nature. The first full theory of evolution was proposed by Jean-Baptiste Lamarck in the early 19th century; Lamarck's theory was based on the idea that species had an innate drive that pushed them up the great chain of being and that the mechanism of inheritance of acquired characteristics helped them adapt to local conditions. The evolutionary theory often referred to as Darwinism was first publicly put forth by Charles Darwin and Alfred Russel Wallace and discussed in detail in On the Origin of Species, published by Darwin in 1859. Darwinism, which unlike Lamarck's theory proposed common descent and a branching tree of life, was based on natural selection, and synthesized a wide range of evidence from animal husbandry, biogeography, geology, morphology, and embryology. The debate over Origin would play a key role in the displacement of natural theology by methodological naturalism in the life sciences, and raised profound questions about human nature and the place of humanity in the natural world.


A drone fly exhibits Batesian mimicry by resembling a honey bee
In evolutionary ecology, mimicry describes a situation where one organism, the mimic, has evolved to share common outward characteristics with another organism, the model, through the selective action of a signal-receiver or "dupe". Collectively this is known as a mimicry complex. The model is usually another species except in cases of automimicry. The signal-receiver is typically another intermediate organism, e.g the common predator of two species, but may actually be the model itself (such as an orchid resembling a female wasp). As an interaction, mimicry is in most cases advantageous to the mimic and harmful to the receiver, but may increase, reduce or have no effect on the fitness of the model depending on the situation. Models themselves are difficult to define in some cases, for example eye spots may not bear resemblance to any specific organism's eyes, and camouflage often cannot be attributed to any particular model. Camouflage, in which a species appears similar to its surroundings, is essentially a form of visual mimicry, but usually is restricted to cases where the model is non-living or abiotic. In between camouflage and mimicry is mimesis, in which the mimic takes on the properties of a specific object or organism, but one to which the dupe is indifferent. The lack of a true distinction between the two phenomena can be seen in animals that resemble twigs, bark, leaves or flowers, in that they are often classified as camouflaged (a plant constitutes its "surroundings"), but are sometimes classified as mimics (a plant is also an organism). Crypsis is a broader concept that encompasses all forms of detection evasion, such as mimicry, camouflage, hiding etc.


Origin of Species title page.jpg
Charles Darwin's The Origin of Species (publ. 1859) is a seminal work in scientific literature and arguably the pivotal work in evolutionary biology. The book's full title is On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life. It introduced the theory that populations evolve over the course of generations through a process of natural selection. It was controversial because it contradicted religious beliefs which underlay the then current theories of biology. Darwin's book was the culmination of evidence he had accumulated on the voyage of the Beagle in the 1830s and expanded through continuing investigations and experiments since his return. The book is readable even for the non-specialist and attracted widespread interest on publication. The book was controversial, and generated much discussion on scientific, philosophical, and religious grounds. The scientific theory of evolution has itself evolved since Darwin first presented it, but natural selection remains the most widely accepted scientific model of how species evolve. The at-times bitter creation-evolution controversy continues to this day.


Mutation and selection diagram.svg
Natural selection is the process by which favorable traits that are heritable become more common in successive generations of a population of reproducing organisms, and unfavorable traits that are heritable become less common. Natural selection acts on the phenotype, or the observable characteristics of an organism, such that individuals with favorable phenotypes are more likely to survive and reproduce than those with less favorable phenotypes. If these phenotypes have a genetic basis, then the genotype associated with the favorable phenotype will increase in frequency in the next generation. Over time, this process can result in adaptations that specialize organisms for particular ecological niches and may eventually result in the emergence of new species. Natural selection is one of the cornerstones of modern biology. The term was introduced by Charles Darwin in his groundbreaking 1859 book The Origin of Species in which natural selection was described by analogy to artificial selection, a process by which animals with traits considered desirable by human breeders are systematically favored for reproduction. The concept of natural selection was originally developed in the absence of a valid theory of inheritance; at the time of Darwin's writing, nothing was known of modern genetics.


In biology and ecology, extinction is the cessation of existence of a species or group of taxa, reducing biodiversity. The moment of extinction is generally considered to be the death of the last individual of that species (although the capacity to breed and recover may have been lost before this point). Because a species' potential range may be very large, determining this moment is difficult, and is usually done retrospectively. This difficulty leads to phenomena such as Lazarus taxa, where a species presumed extinct abruptly "re-appears" (typically in the fossil record) after a period of apparent absence. Through evolution, new species arise through the process of speciation — where new varieties of organisms arise and thrive when they are able to find and exploit an ecological niche — and species become extinct when they are no longer able to survive in changing conditions or against superior competition. A typical species becomes extinct within 10 million years of its first appearance, although some species, called living fossils, survive virtually unchanged for hundreds of millions of years. Only one in a thousand species that have existed remain today.


Punctuated equilibrium is a theory in evolutionary biology which proposes that most sexually reproducing species will experience little evolutionary change for most of their geological history (in an extended state called stasis). When evolution occurs, it is localized in rare, rapid events of branching speciation (called cladogenesis). Cladogenesis is simply the process by which species split into two distinct species, rather than one species gradually transforming into another. Punctuated equilibrium is commonly contrasted against the theory of phyletic gradualism, which states that evolution generally occurs uniformly and by the steady and gradual transformation of whole lineages (anagenesis). In this view, evolution is seen as generally smooth and continuous. In 1972 paleontologists Niles Eldredge and Stephen Jay Gould published a landmark paper developing this theory and called it punctuated equilibria. Their paper was built upon Ernst Mayr's theory of geographic speciation, I. Michael Lerner's theories of developmental and genetic homeostasis, as well as their own empirical research. Eldredge and Gould proposed that the degree of gradualism commonly attributed to Charles Darwin was virtually nonexistent in the fossil record, and that stasis dominates the history of most fossil species.


Richard Dawkins
Clinton Richard Dawkins, FRS (born March 26, 1941) is a British ethologist, evolutionary biologist and popular science writer. He holds the Charles Simonyi Chair for the Public Understanding of Science at the University of Oxford. Born in Nairobi, Kenya, Dawkins moved to England with his parents at the age of eight, and completed his education at the University of Oxford. He first came to prominence with his 1976 book The Selfish Gene, which popularised the gene-centered view of evolution and introduced the term meme, playing a significant role in the foundation of memetics as a scientific field of study. In 1982, he made a widely cited contribution to evolutionary biology with the theory, presented in his book The Extended Phenotype, that phenotypic effects are not limited to an organism's body but can stretch far into the environment, which includes the bodies of other organisms. He has since written several best-selling popular books, and made regular appearances on television and radio programmes discussing evolution, creationism, intelligent design, and religion. In addition to his biological work, Dawkins is well-known for his views on religion. He is an outspoken antitheist and atheist; a secular humanist, sceptic, scientific rationalist, and a supporter of the Brights movement.