Natural selection

Natural selection is the phrase Charles Darwin used in 1859 in his book The Origin of Species to name the natural process he proposed to be responsible for the origin of new species and the adaptation of organisms to their environments. Darwin coined the term as an analogy to the way farmers select crops or livestock for breeding (see artificial selection). Since Darwin, evolutionary biologists have produced several lines of evidence confirming that Natural selection is the only evolutionary force that drives and has driven adaptive evolutionary change throughout the history of life on earth. In addition, classic population geneticists and modern molecular evolutionists have shown that the bulk of natural selection in natural populations is centered around the elimination of deleterious mutations.

Natural selection occurs when individuals differ in reproductive output for functional reasons, i.e., when differences in reproduction follow from the fact that individuals differ from each other in their ability to tackle the challenges posed by their internal biology and by the biological and physical environment. This ability is a function of the physical structures (traits) of life forms and of how these structures affect their ability to tackle the aforementioned challenges.

Natural selection results in adaptive evolution when traits that enhance organismic abilities and thus the reproductive output of individuals that display them, are heritable. Such traits should become more frequent over the generations.

Background and context
Before Darwin, many natural historians viewed differences between individuals within a species as uninteresting departures from each species' Platonic ideal (or typus) of created kinds. However, by the early 19th century the Platonic world view was under siege because of increasing evidence that colossal geological change (see Uniformitarianism) had occurred throughout the history of the earth; and some evolutionists were embracing the view that adaptive evolution can occur when organisms transmit to their progeny the modifications they acquire while dealing with the environment (which proved to be false; see Lamarckism). In contrast, Darwin argued that adaptive evolution results from the culling by nature of heritable variations that arise without directionality.

Evolutionary change can occur without natural selection, by, for example, genetic drift or founder effect. Adaptive evolution, however, requires natural selection to drive it, because the possibility that favorable traits become more frequent across generations because of random fluctuations in occurrence, is negligible. Favorable traits that owe their occurrence in a population to the fact that the genes encoding them became more frequent through evolution by natural selection are called adaptations.

Introduction
Organisms can differ in ways that affect their biological function, and thus their (or their groups', see Unit of selection), probability of surviving and reproducing. For instance, gazelles that run faster than others might survive for additional breeding seasons if this allows them to escape better from predators and/or, given their superior speed, they may be able to feed closer to predators so that they are later able to invest more energy in reproduction than slower gazelles. Faster gazelles therefore end up reproducing more than slower gazelles.

The processes that result in the enhanced reproduction of some and the reduced reproduction of others were described metaphorically by Darwin as an act of 'selection by nature'. Modern evolutionary biology, however, envisages natural selection as a complex causal chain that goes from the genetics and developmental biology of the generation of trait differences between individuals, to the biomechanics and ecology of how such differences result in differences in the performance of individuals or groups, and ultimately to the reproductive biology of how such differences translate into differences in reproduction.

In the case of the faster gazelle, this chain includes how the fertilized egg develops into a gazelle endowed with bodily structures that enable it to run faster, how the structurally superior gazelle uses its body to run faster, how the gazelle's faster running allows it to escape from predators, and how the gazelle's ability to avoid predation allows it to produce more offspring, e.g., during additional breeding seasons.

Therefore, natural selection cannot be summarized by the circular phrase "preferential reproduction of those who reproduce most", as often charged by those who neglect this chain of causation.

Overview
The basic concept of natural selection is that "nature" (the physical and biological environment) "selects" variations in characteristics or traits that improve individual survival and reproduction (adaptive traits), and selects against unfavourable traits (maladaptive traits). In Chapter 4 of The Origin of Species, Darwin wrote:


 * Can it, then, be thought improbable, seeing that variations useful to man have undoubtedly occurred, that other variations useful in some way to each being in the great and complex battle of life, should sometimes occur in the course of thousands of generations? If such do occur, can we doubt (remembering that many more individuals are born than can possibly survive) that individuals having any advantage, however slight, over others, would have the best chance of surviving and of procreating their kind? On the other hand, we may feel sure that any variation in the least degree injurious would be rigidly destroyed. This preservation of favourable individual differences and variations, and the destruction of those which are injurious, I have called Natural Selection. (pp 80-81)

Individuals with maladaptive traits might not survive until reproduction, or might reach reproduction in bad condition and only be able to produce fewer or lower-quality progeny, while individuals with favorable traits might be more likely to survive until reproduction, or be able to produce more or higher-quality progeny. While environmental conditions remain constant, the traits' adaptive values will not change, and if the traits are heritable, adaptive traits will become more common and maladaptive ones rarer over the generations. Sudden or gradual changes in the physical and biological environment, including changes brought about by the activities of the very population of interest, can change the adaptive value of a trait regardless of the trait's previous evolutionary history.

Darwin's theory of natural selection starts from the premise that traits vary in a non-preordained way among individuals. Darwin called the process by which variation between individuals is generated "individuation", but he did not make any claims as to how these differences arise. In general, phenotypic (trait) differences between individuals can result from environmental effects (e.g. bad nutrition) or from genetic differences. Differences caused by environmental factors are mostly not heritable, so the fitness differences they may cause do not alter gene frequencies and cannot result in adaptive evolution. Phenotypic differences triggered by heritable factors are necessary for natural selection to result in adaptive evolutionary change. Modern genetics has characterized several mechanisms that generate heritable genetic differences: Permanent alterations of the genetic material (DNA) can result from errors during DNA replication, or from damage during the transcription of genes and genetic recombination, or be caused by chemicals and physical agents (e.g. X rays, see mutagen); the natural mutation rate of humans is about 10-8 per nucleotide per generation. In sexual populations, genetic recombination and segregation/syngamy mix the DNA of two parents into that of offspring so that the offspring are certain to differ genetically from each other and from their parents.

Although Darwinian fitness is often thought to be partitionable into an ecological ability (viability) component and a fecundity component (which is often the component most affected by sexual selection), many traits are involved in determining more than one fitness component. For example, motor skills influence not only foraging success and survival but often make one attractive to mates. Sexual selection, therefore, might, but need not, lead to ecologically maladaptative traits. Recent modelling work suggests that even sexual selection for maladaptative traits can have beneficial overall fitness consequences, e.g., when it leads to positive assortative mating according to overall genetic quality, which can reduce strongly the genetic load on a population.

Both viability and fecundity can have an ecological component and a sexual-selection component. The ecological component is determined by the ability to negotiate environmental challenges not related directly to sexual competition (such as the ability to gather food, or to fend off or avoid predators). The sexual-selection component is determined by the ability to perform in the sometimes elaborate rituals that determine an individual's success at attracting mates and prevailing at such against other individuals of the same sex. Because of sexual selection's potential to affect total fitness, it can led to traits that are ecologically maladaptive; a famous example is the tails of male peacocks, which are very important in wooing females during courtship but are obviously detrimental to locomotion.

Natural selection is different from artificial selection which refers to the evolution of domesticated species as a result of human culling rather than culling by the "natural environment". However, the mechanisms of natural and artificial selection are similar, and outstanding cases of evolution by artificial selection, like the diversity of dog and pigeon breeds, were used by Darwin to illustrate how selection can produce evolution.

Mechanisms of natural selection
Differential reproduction can result from differences in functional performance at many levels of biological organization, not only at the level of individuals (see unit of selection). Historically, because of the focus on evolution by natural selection, the emphasis has been on the selection of individual organisms that differ in some trait(s) that affect individual performance and result in a higher or lower reproductive output (so called positive and negative selection). Elliott Sober in his book "The Nature of Selection" stressed that natural selection can entail the differential reproduction of many things ("selection of") but that what is most essential to natural selection as a natural process is what causes the differences in reproductive output ("selection for"), i.e., that the workings of natural selection do not depend on the factors that allow gene frequencies to react or not to eventual fitness differences.

In Chapter 4 of The Origin of Species, Darwin wrote:


 * It may be said that natural selection is daily and hourly scrutinising, throughout the world, every variation, even the slightest; rejecting that which is bad, preserving and adding up all that is good; silently and insensibly working, whenever and wherever opportunity offers, at the improvement of each organic being in relation to its organic and inorganic conditions of life. We see nothing of these slow changes in progress, until the hand of time has marked the long lapses of ages, and then so imperfect is our view into long past geological ages, that we only see that the forms of life are now different from what they formerly were. (pg. 84)

Whether a trait results in higher or lower fitness depends on the environment, including predators, food sources, challenges from the physical environment, etc. When populations of a species become separated, e.g. by a geographic barrier, they may have to negotiate different environments and thus may be selected in different ways, and might start evolving in different directions. If enough time goes by for the traits of separated populations to become very different, the populations might become different species. Thus Darwin suggested that all species today have evolved from a common ancestor, but he also stressed that a species can evolve into a new form without splitting. Modern evolutionary biology stresses the lack of interbreeding as critical for speciation but, as R.C. Lewontin has recently stressed, what allows sexual and asexual species to be around is enough ecological divergence for competitive exclusion not to take place.

Additionally, some scientists have theorized that an adaptation which serves to make the organism more adaptable will also tend to supplant its competitors even if it provides no specific advantage in the near term. Descendants of that organism will be more varied and therefore more resistant to extinction due to environmental catastrophes and extinction events. This has been proposed as one reason for the rise of mammals. While this form of selection is possible, it is more likely to be important in cases where selection for adaptation is continuous. For example, the Red Queen hypothesis suggests that sex might have evolved to help organisms adapt to deal with parasites.

Natural selection can be expressed as the following general law (from the conclusion of The Origin of Species):

The result is the evolutionary change of populations and eventually of species.
 * 1) If there are organisms that reproduce, and
 * 2) If offspring inherit traits from their parents(s), and
 * 3) If there is variability of traits, and
 * 4) If the environment limits the size of natural populations,
 * 5) Then those members of the population with maladaptive traits (as determined by the environment) will die out or reproduce less, and
 * 6) Then those members with adaptive traits (as determined by the environment) will survive to reproduction or reproduce more

This is a continuing process that accounts for how species change and can account for both the extinction of species and the origin of new ones. As the formulation is not explicit about how the environment determines whether traits are more or less adaptive, the formulation does not rule out selection occurring at biological levels other than the individual level (e.g., gene, group). Finally the formulation does not invoke specific mechanisms that generate new traits but their continuous action is postulated.

Darwin did not maintain that natural selection is the only mechanism of evolution. In the introduction to The Origin of Species, he wrote:


 * I am convinced that [it] has been the main, but not exclusive means of modification. (pg. 6)

Although natural selection is often called the mechanism of evolution, the generation of heritable phenotypic diversity is also crucial, as, without it, selection cannot result in adaptive evolution. Such variation is generated by the shuffling of genetic material (crossing over)during meiosis and syngamy, by random alterations of the genetic material like point mutations, insertions, and deletions, and by the insertion and deletion of self-replicating genetic elements like transposons as well as of viruses that integrate their genomes in that of their hosts.

History of the principle
Charles Darwin first formulated his theory of evolution by natural selection as an explanation for speciation in 1837 or 1838, and he outlined his theory in two unpublished manuscripts in 1842 and 1844. Darwin defined natural selection as the "principle, by which each slight variation [of a trait], if useful, is preserved". Inspired by economics of Thomas Malthus, which postulated that limitations on resources would eventually have massive demographic implications for humans, and his observations on the H.M.S. Beagle, Darwin elaborated upon his theory over the years and quietly acquired evidence for its action. As Darwin saw it, the concept was simple but powerful: Individuals best adapted to their environments are more likely to survive and reproduce. As long as there was some variation amongst animals, this would lead, in Darwin's view, to an inevitable and natural process of selecting the most advantageous variations out of a population and rewarding them with further reproduction, eventually differentiating populations into different species.

Darwin coined the term as an analogy to the way farmers selected crops or livestock for breeding, designated as artificial selection, and in early manuscripts especially referred explicitly to a "Nature" which would do the selection. Over the next twenty years, Darwin shared his theorizing with just a few acquaintances, while he gathered evidence and attempted to work through all of the possible objections in advance. In 1858, a young naturalist who often gave scientific data to Darwin, Alfred Russel Wallace independently discovered the principle and expressed it to Darwin in a letter. Not wanting to be scooped, Darwin contacted his scientific friends to determine what would be the most gentlemanly way to handle the situation, and it was decided that two short papers by the two would be read at the Linnean Society announcing the co-discovery of the principle. Darwin published his detailed theory the following year, as The Origin of Species, along with evidence and detailed discussion. Darwin's manuscript became a topic of great dispute, and though it resulted in evolutionary theories of some sort to become the primary scientific method of talking about speciation, natural selection itself did not predominate as the mechanism by which it happened. At the heart of what made natural selection a controversial idea was both whether or not it was powerful enough to effect speciation, and the fact that it was explicitly "unguided" (rather than "progressive"), something that even Darwin's avowed followers balked at (including Thomas Henry Huxley).

Similar ideas go back to ancient times. For example, 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 (most prominently, Pierre Louis Moreau de Maupertuis in 1745, and Darwin's grandfather Erasmus Darwin in 1794–1796), although none formulated it in the same terms as Darwin, or developed it along with compelling evidence, and they were not taken seriously as possible progenitors until well after Darwin's publication. As Darwin acknowledged in his introduction to the 6th edition of The Origin of Species, some others had proposed similar theories &mdash; notably William Charles Wells in 1813, and Patrick Matthew in 1831 &mdash; but had not presented them fully or in general scientific publications. Wells' hypothesis, applied to explain the origin of human races, was presented in person at the Royal Society. Matthew's hypothesis appeared in an appendix to his book on arboriculture. Edward Blyth has also been suggested as a having proposed a method of natural selection as a mechanism of keeping species constant.

Few of these "precursors" had any effect on the history of evolutionary thought, except perhaps Erasmus Darwin who was certainly an influence on Charles, although the former's theory of evolution was not formulated in a scientific fashion. Most modern historians of science do not consider any of them to have any historical priority over Darwin, as they did not develop the theory or combine it rigorously with an argument for evolution. The historian of biology Peter J. Bowler has said that efforts to find "precursors" of this sort "misunderstand the whole point of the history of science."

Darwin's Origin of Species elevated the idea of evolution to the level of scientific discourse, and within a decade most scientists and educated lay-people had begun to believe that evolution had occurred in some form or another. Natural selection, however, was not a popular notion. Of the many ideas of evolution which surfaced in the years after Darwin, only in August Weismann's neo-Darwinism was natural selection the main evolutionary force, and this was considered as unjustified by his contemporaries. Most evolutionists &mdash; even Darwin's supporter Thomas Henry Huxley &mdash; believed that there was more "purpose" in evolution than natural selection afforded, and neo-Lamarckism was also popular.

After reading Darwin's works, Herbert Spencer introduced the term "survival of the fittest" to refer to natural selection. This phrase became popular among at the time (Alfred Russel Wallace marked up an entire edition of Origin of Species, replacing each instance of "natural selection" with Spencer's phrase), and is still often used today by non-biologists. Modern biologists reject it because it loses the analogy, and is tautological unless "fittest" is not read to mean "functionally superior".

During the "eclipse of Darwinism" from the late-nineteenth-century through the first decades of the twentieth century, evolution was largely accepted by scientists, but natural selection was not. Only after the integration of a theory of evolution with a complex statistical appreciation of Gregor Mendel's "re-discovered" laws of inheritance did natural selection become highly favored by the scientific community. The work by Ronald Fisher, J.B.S. Haldane, Sewall Wright, Theodosius Dobzhansky, and others, to form the modern evolutionary synthesis propelled Darwinism into the forefront of evolutionary theories, where it remains today.

Scope and role of natural selection
Natural selection need not apply solely to biological organisms; in theory, it applies to all systems in which entities reproduce in a way that includes both inheritance and variation. Thus, a form of natural selection can occur in the nonbiological realm. Computer-based systems (e.g., artificial life) have shown that natural selection can be highly effective in adapting entities to their environments; whether such systems have demonstrated that 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, as well as in his novel The Hacker and the Ants.

Impact of the idea
Perhaps the most radical claim of Darwin's 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 fundamental claim inspired some of Darwin's most ardent supporters&mdash;and has provoked the most profound opposition.

In addition, many theories of artificial selection have been proposed to suggest that economic or social fitness factors assessed by other humans or their built environments are somehow biological or inevitable&mdash;Social Darwinism. Others held that there was an evolution of societies analogous to that of species. Many theories of eugenics were created in an attempt to address these issues. Darwin's ideas, along with those of Adam Smith and Karl Marx, are considered by most historians to have had a profound influence on 19th-century thought.

Darwin's theory inspired Alfred Lotka in 1922 to propose that natural selection might be understood as a physical principle which can be energetically quantified. Through the work of Howard T. Odum this principle of natural selection has become known as the maximum power principle such that those evolutionary systems that have selective advantage maximise the rate of useful energy tranformation.

By effect on phenotypic composition of the population

 * Balancing selection - acts to maintain allelic diversity
 * Directional selection - selects for phenotypes that depart from the mean in one direction
 * Disruptive selection - selects for phenotypes that depart from the mean in either direction
 * Stabilizing selection or purifying selection - selects against extreme phenotypes, keeps phenotype constant
 * Frequency dependent selection - fitness of phenotypes depends upon distribution of other phenotypes in the population (see also Game theory)

By aspect of fitness affected

 * Ecological selection - viability and female fecundity
 * Sexual selection - Selection due to social interactions within a species. There are two distinct forms of sexual selection:
 * Mate choice (intersexual selection, typically referred to as "female choice", as it is usually the females who are most choosy, but in sex-role reversed species it is the males)
 * Aggression between members of the same sex (intrasexual selection, typically referred to as "male-male competition").

Trivia

 * In a letter to Charles Lyell in September 1860, Darwin regrets the use of the term natural selection, preferring the term Natural Preservation.