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Natural selection

2007 Schools Wikipedia Selection. Related subjects: Evolution and
reproduction

   The Galápagos Islands hold 13 species of finches that are closely
   related and differ most markedly in the shape of their beaks. The beak
   of each species is suited to its preferred food, suggesting that beak
   shapes evolved by natural selection. See also character displacement,
   adaptive radiation, divergent evolution.
   Enlarge
   The Galápagos Islands hold 13 species of finches that are closely
   related and differ most markedly in the shape of their beaks. The beak
   of each species is suited to its preferred food, suggesting that beak
   shapes evolved by natural selection. See also character displacement,
   adaptive radiation, divergent evolution.

   Natural selection is the process by which individual organisms with
   favorable traits are more likely to survive and reproduce than those
   with unfavorable traits. It works on the whole individual, but only the
   heritable component of a trait will be passed on to the offspring, with
   the result that favorable, heritable traits become more common in the
   next generation. Given enough time, this passive process results in
   adaptations and speciation (see evolution).

   Natural selection is one of the cornerstones of modern biology. The
   term was introduced by Charles Darwin in his 1859 book The Origin of
   Species, by analogy with artificial selection, by which a farmer
   selects his breeding stock.

An example: antibiotic resistance

   Figure 1: Schematic representation of how antibiotic resistance is
   enhanced by natural selection. The top section represents a population
   of bacteria before exposure to an antibiotic. The middle section shows
   the population directly after exposure, the phase in which selection
   took place. The last section shows the distribution of resistance in a
   new generation of bacteria. The legend indicates the resistance levels
   of individuals.
   Enlarge
   Figure 1: Schematic representation of how antibiotic resistance is
   enhanced by natural selection. The top section represents a population
   of bacteria before exposure to an antibiotic. The middle section shows
   the population directly after exposure, the phase in which selection
   took place. The last section shows the distribution of resistance in a
   new generation of bacteria. The legend indicates the resistance levels
   of individuals.

   A well-known example of natural selection in action is the development
   of antibiotic resistance in microorganisms. Antibiotics have been used
   to fight bacterial diseases since the discovery of penicillin in 1928
   by Alexander Fleming. However, the widespread use and especially misuse
   of antibiotics has led to increased microbial resistance against
   antibiotics, to the point that the methicillin-resistant Staphylococcus
   aureus (MRSA) has been described as a ' superbug' because of the threat
   it poses to health and its relative invulnerability to existing drugs.

   Natural populations of bacteria contain, among their vast numbers of
   individual members, considerable variation in their genetic material,
   primarily as the result of mutations. When exposed to antibiotics, most
   bacteria die quickly, but some may have mutations that make them a
   little less susceptible. If the exposure to antibiotics is short, these
   individuals will survive the treatment. This selective elimination of
   maladapted individuals from a population is natural selection.

   These surviving bacteria will then reproduce again, producing the next
   generation. Due to the elimination of the maladapted individuals in the
   past generation, this population contains more bacteria that have some
   resistance against the antibiotic. At the same time, new mutations
   occur, contributing new genetic variation to the existing genetic
   variation. Spontaneous mutations are very rare, very few have any
   effect at all, and usually any effect is deleterious. However,
   populations of bacteria are enormous, and so a few individuals will
   have beneficial mutations. If a new mutation reduces their
   susceptibility to an antibiotic, these individuals are more likely to
   survive when next confronted with that antibiotic. Given enough time,
   and repeated exposure to the antibiotic, a population of
   antibiotic-resistant bacteria will emerge.

   Recently, several new strains of MRSA have emerged that are resistant
   to vancomycin and teicoplanin. This is an example of what is sometimes
   called an ' arms race', in which bacteria continue to develop strains
   that are less susceptible to antibiotics, while medical researchers
   continue to develop new antibiotics that can kill them. A similar
   situation occurs with pesticide resistance in plants and insects.

General principles

   Natural selection acts on the phenotype. The phenotype is the overall
   result of an individual's genetic make-up ( genotype), the environment,
   and the interactions between genes and between genes and the
   environment. Often, natural selection acts on specific traits of an
   individual, and the terms phenotype and genotype are sometimes used
   narrowly to indicate these specific traits.

   Some traits are determined by just a single gene, but most are affected
   by many different genes. Variation in most of these genes has only a
   small effect on the phenotypic value of a trait, and the study of the
   genetics of these quantitative traits is called quantitative genetics.

   The key element in understanding natural selection is the concept of
   fitness. Natural selection acts on individuals, but its average effect
   on all individuals with a particular genotype is the fitness of that
   genotype. Fitness is measured as the proportion of progeny that
   survives, multiplied by the average fecundity, and it is equivalent to
   the reproductive success of a genotype. A fitness value of greater than
   one indicates that the frequency of that genotype in the population
   increases, while a value of less than one indicates that it decreases.
   The relative fitness of a genotype is estimated as the proportion of
   the fitness of a reference genotype. Related to relative fitness is the
   selection coefficient, which is the difference between the relative
   fitness of two genotypes. The larger the selection coefficient, the
   stronger natural selection will act against the genotype with the
   lowest fitness.

   Natural selection can act on any phenotypic trait, and any aspect of
   the environment, including mates and conspecifics, can produce
   selective pressure. However, this does not imply that natural selection
   is always directional and results in adaptive evolution; natural
   selection often results in the maintenance of the status quo through
   purifying selection. The unit of selection is not limited to the level
   of individuals, but includes other levels within the hierarchy of
   biological organisation, such as genes, cells and relatives. There is
   still debate, however, about whether natural selection acts at the
   level of groups or species, (i.e. selection for adaptations that
   benefit the group or species, rather than the individual). Selection at
   a different level than the individual, for example the gene, can result
   in an increase in fitness for that gene, while at the same time
   reducing the fitness of the individuals carrying that gene (see
   intragenomic conflict for more details). Overall, the combined effect
   of all selection pressures at various levels determines the overall
   fitness of an individual, and hence the outcome of natural selection.
   Figure 2: The life cycle of a sexually reproducing organism. Various
   components of natural selection are indicated for each life stage.
   Enlarge
   Figure 2: The life cycle of a sexually reproducing organism. Various
   components of natural selection are indicated for each life stage.

   Natural selection occurs at every life stage of an individual (see
   Figure 2), and selection at any of these stages can affect the
   likelihood that an individual will survive and reproduce. After an
   individual is born, it has to survive until adulthood before it can
   reproduce, and selection of those that reach this stage is called
   viability selection. In many species, adults must compete with each
   other for mates ( sexual selection), and success in this competition
   determines who will parent the next generation. When species reproduce
   more than once, a longer survival in the reproductive phase increases
   the number of offspring ( survival selection). The fecundity of both
   females (e.g. how many eggs a female bird produces) and males (e.g.
   giant sperm in certain species of Drosophila) can be limited (
   fecundity selection). The viability of produced gametes can differ,
   while intragenomic conflict (meiotic drive) between the haploid gametes
   can result in gametic or genic selection. Finally, the union of some
   combinations of eggs and sperm might be more compatible than others
   (compatibility selection).

Ecological selection and sexual selection

   It is also useful to make a mechanistic distinction between ecological
   selection and sexual selection. Ecological selection covers any
   mechanism of selection as a result of the environment (including
   relatives, e.g. kin selection, and conspecifics, e.g. competition,
   infanticide), while sexual selection refers specifically to competition
   between conspecifics for mates. Sexual selection includes mechanisms
   such as mate choice and male-male competition although the two forms
   can act in combination in some species, when females choose the winners
   of the male-male competition. Mate choice, or intersexual selection,
   typically involves female choice, as it is usually the females who are
   most choosy, but in some sex-role reversed species it is the males that
   choose. Some features that are confined to one sex only of a particular
   species can be explained by selection exercised by the other sex in the
   choice of a mate, e.g. the extravagant plumage of some male birds.
   Aggression between members of the same sex (intrasexual selection) is
   typically referred to as male-male competition, and is sometimes
   associated with very distinctive features, such as the antlers of
   stags, which are used in combat with other stags. More generally,
   intrasexual selection is often associated with sexual dimorphism,
   including differences in body size between males and females of a
   species.

Nomenclature and usage

   Scientists use several, slightly different definitions of the term
   "natural selection" in different contexts. In each generation, only
   some individuals will produce offspring themselves, and of those that
   reproduce, some will leave more offspring than others. This is seen as
   the "natural" process of reproductive selection. Individuals with
   beneficial traits are more likely to be 'selected' - that is, to have
   more offspring - than individuals with other, less beneficial traits.
   When those traits have a heritable component, they tend to become more
   common in the next generation. The mechanism of selection of
   individuals in a population does not "know" which traits are heritable;
   in this sense, the mechanisms of selection are "blind". However, the
   term natural selection is often used to encompass the consequence of
   blind selection as well as the mechanisms that describe the process
   resulting in the enrichment of the beneficial characteristics in the
   next generation.

   It is helpful to distinguish clearly between the mechanisms of
   selection and the effects of selection. When this distinction is
   important, scientists define "natural selection" specifically as "those
   mechanisms that contribute to the selection of individuals that
   reproduce," without regard to whether the basis of the selection is
   heritable. This is sometimes referred to as 'phenotypic natural
   selection.'

   Of particular importance is selection according to traits by which
   individuals differ from each other, and the effects of this selective
   process on the genetic characteristics of a population when some
   aspects of beneficial traits are heritable.

   Selection targets specific traits of an individual, and if such a trait
   has a heritable component, the trait will tend to become more common in
   the next generation. This trait is said to be "selected for". Selection
   for a specific trait therefore results in the selection of specific
   individuals. Selection for a trait can also result in the indirect
   selection of other traits ("free riders") when two or more traits are
   genetically linked through mechanisms such as pleiotropy (single gene
   that affects multiple traits) and linkage disequilibrium (non-random
   association of two genes).

Genetical theory of natural selection

   Natural selection by itself is a simple concept, in which fitness
   differences between phenotypes play a crucial role.

   However, the interplay of the actual selection mechanism with the
   underlying genetics is where the explanatory power of natural selection
   comes from.

Directionality of selection

   When some component of a trait is heritable, selection will alter the
   frequencies of the different alleles (variants of a gene) involved.
   Selection can be divided into three classes, on the basis of their
   effect on the allele frequencies.

   Positive or directional selection occurs when a certain allele has a
   greater fitness than others, resulting in an increase in frequency of
   that allele until it is fixed and the entire population expresses the
   fitter phenotype.

   Far more common is purifying or stabilizing selection, which lowers the
   frequency of alleles which have a deleterious effect on the phenotype
   (that is, lower fitness), until they are eliminated from the
   population. Purifying selection results in functional genetic features
   (e.g. protein-coding sequences or regulatory sequences) being conserved
   over time because of selective pressure against deleterious variants.

   Finally, a number of forms of balancing selection exist, which do not
   result in fixation, but maintain an allele at intermediate frequencies
   in a population. This can occur in diploid species (with two pair of
   chromosomes) when individuals with a combination of two different
   alleles at a single position at the chomosome ( heterozygote) have a
   higher fitness than individuals that have two of the same alleles (
   homozygote). This is called heterozygote advantage or overdominance.
   Maintenance of allelic variation can also occur through disruptive or
   diversifying selection, which favors genotypes that depart from the
   average in either direction (that is, the opposite of overdominance),
   and can result in a bimodal distribution of trait values. Finally, it
   can occur through frequency-dependent selection, where the fitness of
   one particular phenotype depends on the distribution of other
   phenotypes in the population (see also Game theory).

Selection and genetic variation

   A portion of all genetic variation is functionally neutral; i.e., it
   produces no phenotypic effect or significant differences in fitness.
   Previously, this was thought to encompass most of the genetic variation
   in non-coding DNA, but recent studies have shown that large parts of
   those sequences are highly conserved and under strong purifying
   selection; i.e. they do not vary as much from individual to individual,
   indicating that mutations in these regions have deleterious
   consequences). When genetic variation does not result in differences in
   fitness, selection cannot directly affect the frequency of such
   variation. As a result, the genetic variation at those sites will be
   higher than at sites where selection does have a result.

Genetic linkage

   Genetic linkage occurs when two alleles are in close proximity to each
   other. During the formation of the gametes, recombination of the
   genetic material results in reshuffling of the alleles. However, the
   chance that such a reshuffle occurs between two alleles depends on the
   distance between those alleles; the closer the alleles are to each
   other, the less likely it is that such a reshuffle will occur.
   Consequently, when selection targets one allele, this automatically
   results in selection of the other allele as well; through this
   mechanism, selection can have a strong influence on patterns of
   variation in the genome.

Mutation-selection balance

   Natural selection results in the reduction of genetic variation through
   the elimination of maladapted individuals and, through that, of the
   mutations that causes the maladaptation. At the same time, new
   mutations occur, resulting in a mutation-selection balance. The exact
   outcome of the two processes depends both on the rate at which new
   mutations occurs and on the strength of the natural selection.
   Consequently, changes in the mutation rate or the selection pressure
   will result in a different mutation-selection balance.

Selective sweep

   Selective sweeps occur when an allele becomes more common in a
   population as a result of positive selection. As the prevalence of one
   allele increases, linked alleles (those nearby on the chromosome) can
   also become more common, whether they are neutral or even slightly
   deleterious. This is called genetic hitchhiking. A strong selective
   sweep results in a region of the genome where the positively selected
   haplotype (the allele and its neighbours) are essentially the only ones
   that exist in the population.

   Whether a selective sweep has occurred or not can be investigated by
   measuring linkage disequilibrium, i.e., whether a given haplotype is
   overrepresented in the population. Normally, genetic recombination
   results in a reshuffling of the different alleles within a haplotype,
   and none of the haplotypes will dominate the population. However,
   during a selective sweep, selection for a specific allele will also
   result in selection of neighbouring alleles. Therefore, the presence of
   strong linkage disequilibrium might indicate that there has been a
   'recent' selective sweep, and this can be used to identify sites
   recently under selection.

Background selection

   Background selection is the opposite of a selective sweep. If a
   specific site experiences strong and persistent purifying selection
   (perhaps as a result of mutation-selection balance), linked variation
   will tend to be weeded out along with it. Background selection,
   however, acts as a result of new mutations, which can occur randomly in
   any haplotype. It therefore produces no linkage disequilibrium, though
   it reduces the amount of variation in the region.

Evolution by means of natural selection

   A prerequisite for natural selection to result in adaptive evolution,
   novel traits and speciation, is the presence of heritable genetic
   variation that results in fitness differences. Genetic variation is the
   result of mutations, recombinations and alterations in the karyotype
   (the number, shape, size and internal arrangement of the chromosomes).
   Any of these changes might have an effect that is highly advantageous
   or highly disadvantageous, but large effects are very rare. In the
   past, most changes in the genetic material were considered neutral or
   close to neutral because they occurred in noncoding DNA or resulted in
   a synonymous substitution. However, recent research suggests that many
   mutations in non-coding DNA do have slight deleterious effects.
   Overall, of those mutations that do affect the fitness of the
   individual, most are slightly deleterious, some reduce the fitness
   dramatically and some increase the fitness.
   The exuberant tail of the peacock is thought to be the result of sexual
   selection by females. This peacock is an albino - it carries a mutation
   that makes it unable to produce melanin. Selection against albinos in
   nature is intense because they are easily spotted by predators or are
   unsuccessful in competition for mates, and so these mutations are
   usually rapidly eliminated by natural selection
   Enlarge
   The exuberant tail of the peacock is thought to be the result of sexual
   selection by females. This peacock is an albino - it carries a mutation
   that makes it unable to produce melanin. Selection against albinos in
   nature is intense because they are easily spotted by predators or are
   unsuccessful in competition for mates, and so these mutations are
   usually rapidly eliminated by natural selection

   By the definition of fitness, individuals with greater fitness are more
   likely to contribute offspring to the next generation, while
   individuals with lesser fitness are more likely to die early or they
   fail to reproduce. As a result, alleles which on average result in
   greater fitness become more abundant in the next generation, while
   alleles which generally reduce fitness become rarer. If the selection
   forces remain the same for many generations, beneficial alleles become
   more and more abundant, until they dominate the population, while
   alleles with a lesser fitness disappear. In every generation, new
   mutations and recombinations arise spontaneously, producing a new
   spectrum of phenotypes. Therefore, each new generation will be enriched
   by the increasing abundance of alleles that contribute to those traits
   that were favored by selection, enhancing these traits over successive
   generations.
   X-ray of the left hand of a ten year old boy with polydactyly
   Enlarge
   X-ray of the left hand of a ten year old boy with polydactyly

   Some mutations occur in so-called regulatory genes. Changes in these
   can have large effects on the phenotype of the individual because they
   regulate the function of many other genes. Most, but not all, mutations
   in regulatory genes result in non-viable zygotes. For example,
   mutations in some HOX genes in humans result in an increase in the
   number of fingers or toes or a cervical rib. When such mutations result
   in a higher fitness, natural selection will favour these phenotypes and
   the novel trait will spread in the population.

   Established traits are not immutable: an established trait may lose its
   fitness if environmental conditions change. In these circumstances, in
   the absence of natural selection to preserve the trait, the trait will
   become more variable and will deteriorate over time. The power of
   natural selection will also inevitably depend upon prevailing
   environmental factors; in general, the number of offspring is (far)
   greater than the number of individuals that can survive to the next
   generation, and there will be intense selection of the best adapted
   individuals for the next generation.

Speciation

   Speciation requires selective mating, which result in a reduced gene
   flow. Selective mating can be the result of, for example, a change in
   the physical environment (physical isolation by an extrinsic barrier),
   or by sexual selection resulting in assortative mating. Over time,
   these subgroups might diverge radically to become different species,
   either because of differences in selection pressures on the different
   subgroups, or because different mutations arise spontaneously in the
   different populations, or because of founder effects - some potentially
   beneficial alleles may, by chance, be present in only one or other of
   two subgroups when they first become separated. When the genetic
   changes result in increasing incompatibility between the genotypes of
   the two subgroups, gene flow between the groups will be reduced even
   more, and will stop altogether as soon as the mutations become fixed in
   the respective subgroups. As few as two mutations can result in
   speciation: if each mutation has a neutral or positive effect on
   fitness when they occur separately, but a negative effect when they
   occur together, then fixation of these genes in the respective
   subgroups will lead to two reproductively isolated populations.
   According to the biological species concept, these will be two
   different species.

Historical development

   The modern theory of natural selection derives from the work of Charles
   Darwin in the nineteenth century.
   Enlarge
   The modern theory of natural selection derives from the work of Charles
   Darwin in the nineteenth century.

Pre-Darwinian theories

   General concepts of biological evolution and species change date to
   ancient times; the Ionian physician Empedocles said that many races
   "must have been unable to beget and continue their kind. For in the
   case of every species that exists, either craft or courage or speed has
   from the beginning of its existence protected and preserved it".
   Several eighteenth-century thinkers wrote about similar theories,
   including Pierre Louis Moreau de Maupertuis in 1745, Lord Monboddo in
   his theories of species alteration, and Darwin's grandfather Erasmus
   Darwin in 1794–1796. However, these 'precursors' had little influence
   on the trajectory of evolutionary thought after Darwin.

   Until the early 19th century, the established view in Western societies
   was that differences between individuals of a species were
   uninteresting departures from their Platonic ideal (or typus) of
   created kinds. However, growing awareness of the fossil record led to
   the recognition that species that lived in the distant past were often
   very different from those that exist today. Naturalists of the time
   tried to reconcile this with the emerging ideas of uniformitarianism in
   geology - the notion that simple, weak forces, acting continuously over
   very long periods of time could have radical consequences, shaping the
   landscape as we know it today. Most importantly perhaps, these notions
   led to the awareness of the immensity of geological time, which makes
   it possible for slight causes to produce dramatic consequences. This
   opened the door to the notion that species might have arisen by descent
   with modification from ancestor species.

   In the early years of the 19th century, radical evolutionists such as
   Jean Baptiste Lamarck had proposed that characteristics (adaptations)
   acquired by individuals might be inherited by their progeny, causing,
   in enough time, transmutation of species (see Lamarckism).

Darwin's hypothesis

   Between 1842 and 1844, Charles Darwin outlined his theory of evolution
   by natural selection as an explanation for adaptation and speciation.
   He defined natural selection as the "principle by which each slight
   variation [of a trait], if useful, is preserved". The concept was
   simple but powerful: individuals best adapted to their environments are
   more likely to survive and reproduce. As long as there is some
   variation between them, there will be an inevitable selection of
   individuals with the most advantageous variations. If the variations
   are inherited, then differential reproductive success will lead to a
   progressive evolution of particular populations of a species, and
   populations that evolve to be sufficiently different might eventually
   become different species.

   For Darwin, natural selection was synonymous with evolution by natural
   selection; other mechanisms of evolution such as evolution by genetic
   drift were not explicitly formulated at that time, and Darwin realised
   that: "I am convinced that [it] has been the main, but not exclusive
   means of modification."  Today, scientists use natural selection mainly
   to describe the mechanism. In this sense, natural selection includes
   any selection by a natural agent, including sexual selection and kin
   selection. Sometimes, sexual selection is distinguished from natural
   selection, but a more useful distinction is between sexual selection
   and ecological selection.

   Darwin thought of natural selection by analogy to how farmers select
   crops or livestock for breeding ( artificial selection); in his early
   manuscripts he referred to a 'Nature' which would do the selection. In
   the next twenty years, he shared these theories with just a few
   friends, while gathering evidence and trying to address all possible
   objections. In 1858, Alfred Russel Wallace, a young naturalist,
   independently conceived the principle and described it in a letter to
   Darwin. Not wanting to be scooped, Darwin contacted scientific friends
   to find an honorable way to handle this potentially embarrassing
   situation, and two short papers by the two were read at the Linnean
   Society announcing co-discovery of the principle. The following year,
   Darwin published The Origin of Species, along with his evidence and
   detailed discussion. This became a topic of great dispute; evolutionary
   theories became the primary way of talking about speciation, but
   natural selection did not predominate as the mechanism by which it
   happened. What made natural selection controversial was doubt about
   whether it was powerful enough to result in speciation, and that it was
   'unguided' rather than 'progressive', something that even Darwin's
   supporters balked at.

   Darwin's ideas were inspired by the observations that he had made on
   the Voyage of the Beagle, and by the economic theories of Thomas
   Malthus, who noted that population (if unchecked) increases
   exponentially whereas the food supply grows only arithmetically; thus
   inevitable limitations of resources would have demographic
   implications, leading to a "struggle for existence", in which only the
   fittest would survive. In the 6th edition of The Origin of Species
   Darwin acknowledged that others — notably William Charles Wells in
   1813, and Patrick Matthew in 1831 — had proposed similar theories, but
   had not presented them fully or in notable scientific publications.
   Wells presented his hypothesis to explain the origin of human races in
   person at the Royal Society, and Matthew published his as an appendix
   to his book on arboriculture. Edward Blyth had also proposed a method
   of natural selection as a mechanism of keeping species constant.

   Within a decade of The Origin of Species, most educated people had
   begun to accept that evolution had occurred in some form or another.
   However, of the many ideas of evolution that emerged, only August
   Weismann's saw natural selection as the main evolutionary force. Even
   T.H. Huxley believed that there was more "purpose" in evolution than
   natural selection afforded, and neo- Lamarckism was also popular. After
   reading Darwin, Herbert Spencer introduced the term survival of the
   fittest, which became a popular summary of the theory. Although the
   phrase is still often used by non-biologists, modern biologists avoid
   it because it is tautological if fittest is read to mean functionally
   superior. Survival of the fittest can actually be rephrased as "That
   which is mostly observed, is that which replicates the most", showing
   it as a tautology to the fullest extent. Such a statement, arguably,
   makes natual selection even more persuasive. In a letter to Charles
   Lyell in September 1860, Darwin regrets the use of the term 'Natural
   Selection', preferring the term 'Natural Preservation'.

Modern evolutionary synthesis

   Only after the integration of a theory of evolution with a complex
   statistical appreciation of Mendel's 're-discovered' laws of
   inheritance did natural selection become generally accepted by
   scientists. The work of Ronald Fisher (who first attempted to explain
   natural selection in terms of the underlying genetic processes), J.B.S.
   Haldane (who introduced the concept of the 'cost' of natural
   selection), Sewall Wright (one of the founders of population genetics),
   Theodosius Dobzhansky (who established the idea that mutation, by
   creating genetic diversity, supplied the raw material for natural
   selection), William Hamilton (who conceived of kin selection), Ernst
   Mayr (who recognised the key importance of reproductive isolation for
   speciation) and many others formed the modern evolutionary synthesis.
   This propelled natural selection to the forefront of evolutionary
   theories, where it remains today.

Impact of the idea

   Darwin's ideas, along with those of Adam Smith and Karl Marx, had a
   profound influence on 19th-century thought. Perhaps the most radical
   claim of the theory of evolution through natural selection is that
   "elaborately constructed forms, so different from each other, and
   dependent on each other in so complex a manner" evolved from the
   simplest forms of life by a few simple principles. This claim inspired
   some of Darwin's most ardent supporters—and provoked the most profound
   opposition. The radicalism of natural selection, according to Stephen
   Jay Gould, lay in its power to "dethrone some of the deepest and most
   traditional comforts of Western thought". In particular, it challenged
   beliefs in nature's benevolence, order, and good design, the belief
   that humans occupy a summit of power and excellence, belief in an
   omnipotent, benevolent creator, and belief that nature has any
   meaningful direction, or that humans fit into any sensible pattern.

Social theory

   The social implications of the theory of evolution by natural selection
   also became the source of continuing controversy. Engels in 1872 wrote
   that "Darwin did not know what a bitter satire he wrote on mankind when
   he showed that free competition, the struggle for existence, which the
   economists celebrate as the highest historical achievement, is the
   normal state of the animal kingdom". That natural selection had
   apparently led to 'advancement' in intelligence and civilisation also
   became used as a justification for colonialism and policies of eugenics
   —see Social Darwinism. Konrad Lorenz won the Nobel Prize in 1973 for
   his analysis of animal behaviour in terms of the role of natural
   selection (particularly group selection). However, in Germany in 1940,
   in writings that he subsequently disowned, he used the theory as a
   justification for policies of the Nazi state. He wrote "... selection
   for toughness, heroism, and social utility...must be accomplished by
   some human institution, if mankind, in default of selective factors, is
   not to be ruined by domestication-induced degeneracy. The racial idea
   as the basis of our state has already accomplished much in this
   respect." Others have developed ideas that human societies and culture
   evolve by mechanisms that are analogous to those that apply to
   evolution of species. (see article on Sociocultural evolution).

Energetic theory

   In 1922, Alfred Lotka proposed that natural selection might be
   understood as a physical principle which can be energetically
   quantified. Through the work of Howard T. Odum this became known as the
   maximum power principle whereby evolutionary systems with selective
   advantage maximise the rate of useful energy transformation.

Information theory

   Natural selection need not apply only to biological organisms. In
   computer-based systems (e.g., artificial life), simulating natural
   selection can be very effective in 'adapting' entities to their
   environments. By combining this with simulated reproduction and random
   variation it is possible for instance to 'evolve' problem-solving
   abilities of computer-based systems. However, whether such systems show
   that evolution by means of natural selection per se can generate
   complexity is contested. The mathematician and science fiction writer
   Rudy Rucker explored the use of natural selection to create artificial
   intelligence in his best-known work, The Ware Tetralogy, and in his
   novel The Hacker and the Ants.

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