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Evolution (also known as biological, genetic or organic evolution) is the change in the inherited traits of a population of organisms through successive generations. Over time variants with particular heritable traits become more, or less, common. A trait is a particular characteristicâ€â€anatomical, biochemical or behaviouralâ€â€that is the result of gene–environment interaction. One source of heritable variation is mutation, various types of which are passed on during the genetic recombination that happens at reproduction. Having occurred once, these changes can sometimes be passed on successfully to further generations, and may thus give rise to new variant traits in populations. Under certain circumstances, variation can also be increased by the transfer of genes between species, and by the extremely rare, but significant, wholesale incorporation of genomes through endosymbiosis. Two main processes cause variants to become more common or rarer in a population. One is natural selection, through which traits that aid reproduction become more common, while traits that hinder reproduction become rarer. Natural selection occurs because only a small proportion of individuals in each generation will reproduce, since resources are limited and organisms produce many more offspring than their environment can support. Over many generations, heritable variation in traits is filtered by natural selection and the beneficial changes are successively retained through differential survival and reproduction. This iterative process means that traits which are better suited to an organism’s environment become more common. These adjustments are called adaptations. However, not all change is adaptive. Another cause of evolution is genetic drift, which leads to random changes in how common traits are in a population. Genetic drift is most important when traits do not strongly influence survivalâ€â€particularly so in small populations, in which chance plays a disproportionate role in the frequency of traits passed on to the next generation. Genetic drift is important in the neutral theory of molecular evolution, and plays a role in the molecular clocks that are used in phylogenetic studies. A notable result of evolution is speciation, in which a single ancestral species splits and diversifies into multiple distinct new populations that are new species. There are several ways in which this occurs. Ultimately, all living (and extinct) species are descended from a common ancestor via a long series of speciation events. These events stretch back in a diverse "tree of life" which has grown over the 3.5 billion years during which life has existed on Earth. This is visible in anatomical, genetic and other similarities between groups of organisms, geographical distribution of related species, the fossil record and the recorded genetic changes in living organisms over many generations. Evolutionary biologists document the fact that evolution occurs, and also develop and test theories which explain its causes. The study of evolutionary biology began in the mid-nineteenth century, when research into the fossil record and the diversity of living organisms convinced most scientists that species changed over time. The mechanism driving these changes remained unclear until the theory of natural selection was independently proposed by Charles Darwin and Alfred Wallace. In 1859, Darwin’s seminal work On the Origin of Species brought the new theory of evolution by natural selection to a wide audience, leading to the overwhelming acceptance of evolution among scientists. In the 1930s, Darwinian natural selection became understood in combination with Mendelian inheritance, forming the modern evolutionary synthesis, which connected the substrate of evolution (inherited genetics) and the mechanism of evolution (natural selection). This powerful explanatory and predictive theory has become the central organizing principle of modern biology, directing research and providing a unifying explanation for the history and diversity of life on Earth. Evolution is applied and studied in fields as diverse as agriculture, anthropology, conservation biology, ecology, medicine, paleontology, philosophy, and psychology along with other specific topics in the previous listed fields.  White peppered moth  Black morph in peppered moth evolution Further information: Population genetics 
A chart showing three types of selection. 1.Disruptive selection 2.Stabilizing selection 3.Directional selection Natural selection within a population for a trait that can vary across a range of values, such as height, can be categorised into three different types. The first is directional selection, which is a shift in the average value of a trait over time  for example, organisms slowly getting taller. Secondly, disruptive selection is selection for extreme trait values and often results in two different values becoming most common, with selection against the average value. This would be when either short or tall organisms had an advantage, but not those of medium height. Finally, in stabilizing selection there is selection against extreme trait values on both ends, which causes a decrease in variance around the average value and less diversity. This would, for example, cause organisms to slowly become all the same height. A special case of natural selection is sexual selection, which is selection for any trait that increases mating success by increasing the attractiveness of an organism to potential mates. Traits that evolved through sexual selection are particularly prominent in males of some animal species, despite traits such as cumbersome antlers, mating calls or bright colours that attract predators, decreasing the survival of individual males. This survival disadvantage is balanced by higher reproductive success in males that show these hard to fake, sexually selected traits. Natural selection most generally makes nature the measure against which individuals, and individual traits, are more or less likely to survive. "Nature" in this sense refers to an ecosystem, that is, a system in which organisms interact with every other element, physical as well as biological, in their local environment. Eugene Odum, a founder of ecology, defined an ecosystem as: "Any unit that includes all of the organisms...in a given area interacting with the physical environment so that a flow of energy leads to clearly defined trophic structure, biotic diversity, and material cycles (ie: exchange of materials between living and nonliving parts) within the system." Each population within an ecosystem occupies a distinct niche, or position, with distinct relationships to other parts of the system. These relationships involve the life history of the organism, its position in the food chain, and its geographic range. This broad understanding of nature enables scientists to delineate specific forces which, together, comprise natural selection. An active area of research is the unit of selection, with natural selection being proposed to work at the level of genes, cells, individual organisms, groups of organisms and species. None of these are mutually exclusive and selection can act on multiple levels simultaneously. An example of selection occurring below the level of the individual organism are genes called transposons, which can replicate and spread throughout a genome. Selection at a level above the individual, such as group selection, may allow the evolution of co-operation, as discussed below. From Wikipedia, the free encyclopedia : Wholesale of live animals |
