1.6+Evolution+and+Natural+Selection


 * Evolution **

** Father of Evolution (Charles Darwin) ** media type="youtube" key="Ci9jfMvoLb4" width="420" height="315"

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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. [|[1]] This change results from interactions between processes that introduce [|variation] into a population, and other processes that remove it. As a result, variants with particular traits become more, or less, common. A trait is a particular characteristic— [|anatomical], [|biochemical] or [|behavioural] —that is the result of [|gene–environment interaction]. The main source of variation is [|mutation], which introduces genetic changes. These changes are [|heritable] (can be passed on through [|reproduction] ), and may give rise to alternative traits in organisms. Another source of variationa is [|genetic recombination], which shuffles the genes into new combinations which can result in organisms exhibiting different traits. 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 survival and reproduction become more common, while traits that hinder survival and reproduction become rarer. Natural selection occurs because only a small proportion of individuals in each generation will survive and 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] adjusts traits so they become better suited to an organism's environment: these adjustments are called [|adaptations]. Natural selection    [|Natural selection] is the process by which genetic mutations that enhance reproduction become, and remain, more common in successive generations of a population. It has often been called a "self-evident" mechanism because it necessarily follows from three simple facts:
 * Heritable variation exists within populations of organisms.
 * Organisms produce more offspring than can survive.
 * These offspring vary in their ability to survive and reproduce.

These conditions produce competition between organisms for survival and reproduction. Consequently, organisms with traits that give them an advantage over their competitors pass these advantageous traits on, while traits that do not confer an advantage are not passed on to the next generation. The central concept of natural selection is the [|evolutionary fitness] of an organism. Fitness is measured by an organism's ability to survive and reproduce, which determines the size of its genetic contribution to the next generation. However, fitness is not the same as the total number of offspring: instead fitness is indicated by the proportion of subsequent generations that carry an organism's genes. For example, if an organism could survive well and reproduce rapidly, but its offspring were all too small and weak to survive, this organism would make little genetic contribution to future generations and would thus have low fitness.

If an allele increases fitness more than the other alleles of that gene, then with each generation this allele will become more common within the population. These traits are said to be "selected //for//". Examples of traits that can increase fitness are enhanced survival, and increased [|fecundity]. Conversely, the lower fitness caused by having a less beneficial or deleterious allele results in this allele becoming rarer — they are "selected //against//". Importantly, the fitness of an allele is not a fixed characteristic; if the environment changes, previously neutral or harmful traits may become beneficial and previously beneficial traits become harmful. However, even if the direction of selection does reverse in this way, traits that were lost in the past may not re-evolve in an identical form (see [|Dollo's law] ).

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 Adaptation    Adaptation is one of the basic phenomena of biology, and is the //process// whereby an organism becomes better suited to its [|habitat]. Also, the term adaptation may refer to a [|trait] that is important for an organism's survival. For example, the adaptation of horses' teeth to the grinding of grass, or the ability of horses to run fast and escape predators. By using the term //adaptation// for the evolutionary process, and //adaptive trait// for the product (the bodily part or function), the two senses of the word may be distinguished. Adaptations are produced by [|natural selection]. [|[149]] The following definitions are due to [|Theodosius Dobzhansky]. 1. //Adaptation// is the evolutionary process whereby an organism becomes better able to live in its [|habitat] or habitats. 2. //Adaptedness// is the state of being adapted: the degree to which an organism is able to live and reproduce in a given set of habitats. 3. An //adaptive trait// is an aspect of the developmental pattern of the organism which enables or enhances the probability of that organism surviving and reproducing.

Adaptation may cause either the gain of a new feature, or the loss of an ancestral feature. An example that shows both types of change is bacterial adaptation to [|antibiotic] selection, with genetic changes causing [|antibiotic resistance] by both modifying the target of the drug, or increasing the activity of transporters that pump the drug out of the cell. Other striking examples are the bacteria // [|Escherichia coli] // evolving the ability to use [|citric acid] as a nutrient in a [|long-term laboratory experiment], // [|Flavobacterium] // evolving a novel enzyme that allows these bacteria to grow on the by-products of [|nylon] manufacturing, [|[155]] and the soil bacterium // [|Sphingobium] // evolving an entirely new [|metabolic pathway] that degrades the synthetic [|pesticide] [|pentachlorophenol]. An interesting but still controversial idea is that some adaptations might increase the ability of organisms to generate genetic diversity and adapt by natural selection (increasing organisms' [|evolvability] ).

A [|baleen whale] skeleton, //a// and //b// label [|flipper] bones, which were [|adapted] from front [|leg] bones: while //c// indicates [|vestigial] leg bones, suggesting an adaptation from land to sea.

Adaptation occurs through the gradual modification of existing structures. Consequently, structures with similar internal organization may have different functions in related organisms. This is the result of a single [|ancestral structure] being adapted to function in different ways. The bones within [|bat] wings, for example, are very similar to those in [|mice] feet and [|primate] hands, due to the descent of all these structures from a common mammalian ancestor. However, since all living organisms are related to some extent, even organs that appear to have little or no structural similarity, such as [|arthropod], squid and vertebrate eyes, or the limbs and wings of arthropods and vertebrates, can depend on a common set of homologous genes that control their assembly and function; this is called [|deep homology]. During adaptation, some structures may lose their original function and become [|vestigial structures]. Such structures may have little or no function in a current species, yet have a clear function in ancestral species, or other closely related species. Examples include [|pseudogenes], the non-functional remains of eyes in blind cave-dwelling fish, wings in flightless birds, and the presence of hip bones in whales and snakes. Examples of [|vestigial structures in humans] include [|wisdom teeth], the [|coccyx] , the [|vermiform appendix] , and other behavioral vestiges such as [|goose bumps] , and [|primitive reflexes].

However, many traits that appear to be simple adaptations are in fact [|exaptations] : structures originally adapted for one function, but which coincidentally became somewhat useful for some other function in the process. One example is the African lizard //Holaspis guentheri//, which developed an extremely flat head for hiding in crevices, as can be seen by looking at its near relatives. However, in this species, the head has become so flattened that it assists in gliding from tree to tree—an [|exaptation]. Within cells, [|molecular machines] such as the bacterial [|flagella] and [|protein sorting machinery] evolved by the recruitment of several pre-existing proteins that previously had different functions. Another example is the recruitment of enzymes from [|glycolysis] and [|xenobiotic metabolism] to serve as structural proteins called [|crystallins] within the lenses of organisms' [|eyes].

A critical principle of [|ecology] is that of [|competitive exclusion] : no two species can occupy the same niche in the same environment for a long time. Consequently, natural selection will tend to force species to adapt to different [|ecological niches]. This may mean that, for example, two species of [|cichlid] fish adapt to live in different [|habitats], which will minimise the competition between them for food.

An area of current investigation in [|evolutionary developmental biology] is the [|developmental] basis of adaptations and exaptations. This research addresses the origin and evolution of [|embryonic development] and how modifications of development and developmental processes produce novel features. These studies have shown that evolution can alter development to create new structures, such as embryonic bone structures that develop into the jaw in other animals instead forming part of the middle ear in mammals. It is also possible for structures that have been lost in evolution to reappear due to changes in developmental genes, such as a mutation in [|chickens] causing embryos to grow teeth similar to those of [|crocodiles]. It is now becoming clear that most alterations in the form of organisms are due to changes in a small set of conserved genes