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Evolution

Mechanisms and processes

Adaptation Genetic drift Gene flow Mutation Natural selection Speciation Natural selection is the process by which individual organisms with favorable traits are more likely to survive and reproduce. ... For the song by Girls Aloud see Biology (song) Biology studies the variety of life (clockwise from top-left) E. coli, tree fern, gazelle, Goliath beetle Biology (from Greek: Βιολογία - βίος, bio, life; and λόγος, logos, speech lit. ... This article is about evolution in biology. ... Image File history File links Tree_of_life. ... For other uses, see Adaptation (disambiguation). ... In population genetics, genetic drift is the statistical effect that results from the influence that chance has on the success of alleles (variants of a gene). ... In population genetics, gene flow (also known as gene migration) is the transfer of alleles of genes from one population to another. ... For linguistic mutation, see Apophony. ... Charles Darwins first sketch of an evolutionary tree from his First Notebook on Transmutation of Species (1837) Speciation is the evolutionary process by which new biological species arise. ...

Research and history

Evidence Evolutionary history of life History Modern synthesis Social effect / Objections While on board HMS Beagle, Charles Darwin collected numerous specimens, many new to science, which supported his later theory of evolution by natural selection. ... The evolutionary history of life and the origin of life are fields of ongoing geological and biological research. ... Evolutionary thought has roots in antiquity as philosophical ideas conceived during the Ancient Greek and Roman eras, but until the 18th century, biological thought was dominated by essentialism, the idea that living forms are static and unchanging in time. ... The modern evolutionary synthesis refers to a set of ideas from several biological specialities that were brought together to form a unified theory of evolution accepted by the great majority of working biologists. ... The theory of transmutation had early origins in the speculations and hypotheses of Erasmus Darwin, and Jean-Baptiste Lamarck. ... There have been numerous objections to evolution since alternative evolutionary ideas came to be hotly debated around the start of the nineteenth century. ...

Evolutionary biology fields

Cladistics Ecological genetics Evolutionary development Human evolution Molecular evolution Phylogenetics Population genetics It has been suggested that Clade be merged into this article or section. ... Ecological genetics is the study of genetics (itself a field of biology) from an ecological perspective. ... Evolutionary developmental biology (evolution of development or informally, evo-devo) is a field of biology that compares the developmental processes of different animals in an attempt to determine the ancestral relationship between organisms and how developmental processes evolved. ... For the history of humans on Earth, see History of the world. ... Molecular evolution is the process of the genetic material in populations of organisms changing over time. ... Phylogenetic groups, or taxa, can be monophyletic, paraphyletic, or polyphyletic. ... Population genetics is the study of the distribution of and change in allele frequencies under the influence of the four evolutionary forces: natural selection, genetic drift, mutation, and migration. ...

Biology Portal · v • d • e

Natural selection is the process by which favorable heritable traits become more common in successive generations of a population of reproducing organisms, and unfavorable heritable traits 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. The phenotype's genetic basis, genotype associated with the favorable phenotype, will increase in frequency over the following generations. 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. In other words, natural selection is the mechanism by which evolution may take place in a population of a specific organism. Heredity (the adjective is hereditary) is the transfer of characteristics from parent to offspring, either through their genes or through the social institution called inheritance (for example, a title of nobility is passed from individual to individual according to relevant customs and/or laws). ... In biology, a trait or character is a feature of an organism. ... // Traditionally, a generation has been defined as “the average interval of time between the birth of parents and the birth of their offspring. ... Domains and Kingdoms Nanobes Acytota Cytota Bacteria Neomura Archaea Eukaryota Bikonta Apusozoa Rhizaria Excavata Archaeplastida Rhodophyta Glaucophyta Plantae Heterokontophyta Haptophyta Cryptophyta Alveolata Unikonta Amoebozoa Opisthokonta Choanozoa Fungi Animalia An ericoid mycorrhizal fungus Life on Earth redirects here. ... Individuals in the mollusk species Donax variabilis show diverse coloration and patterning in their phenotypes. ... Biological reproduction is the biological process by which new individual organisms are produced. ... This article is about the general scientific term. ... This article does not cite any references or sources. ... Allele frequency is a measure of the relative frequency of an allele on a genetic locus in a population. ... For other uses, see Adaptation (disambiguation). ... Two lichens on a rock, in two different ecological niches In ecology, a niche; (pronounced nich, neesh or nish)[1] is a term describing the relational position of a species or population in its ecosystem[1]. The ecological niche; describes how an organism or population responds to the distribution of... Charles Darwins first sketch of an evolutionary tree from his First Notebook on Transmutation of Species (1837) Speciation is the evolutionary process by which new biological species arise. ...

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^[1] 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. Although Gregor Mendel, the father of modern genetics, was a contemporary of Darwin's, his work would lie in obscurity until the early 20th century. The union of traditional Darwinian evolution with subsequent discoveries in classical and molecular genetics is termed the modern evolutionary synthesis. Although other mechanisms of molecular evolution, such as the neutral theory advanced by Motoo Kimura, have been identified as important causes of genetic diversity, natural selection remains the single primary explanation for adaptive evolution. For the song by Girls Aloud see Biology (song) Biology studies the variety of life (clockwise from top-left) E. coli, tree fern, gazelle, Goliath beetle Biology (from Greek: Βιολογία - βίος, bio, life; and λόγος, logos, speech lit. ... For other people of the same surname, and places and things named after Charles Darwin, see Darwin. ... Charles Darwins Origin of Species (publ. ... This Chihuahua mix and Great Dane show the wide range of dog breed sizes created using artificial selection. ... This article or section does not adequately cite its references or sources. ... This article is about the general scientific term. ... “Mendel” redirects here. ... Charles Darwin Darwinism is a term for the underlying theory in those ideas of Charles Darwin concerning evolution and natural selection. ... Classical genetics consists of the techniques and methodologies of genetics that predate the advent of molecular biology. ... Molecular genetics is the field of biology which studies the structure and function of genes at a molecular level. ... The modern evolutionary synthesis refers to a set of ideas from several biological specialities that were brought together to form a unified theory of evolution accepted by the great majority of working biologists. ... The neutral theory of molecular evolution (also, simply the neutral theory of evolution) is an influential theory that was introduced with provocative effect by Motoo Kimura in the late 1960s and early 1970s. ... Motoo Kimura (木村資生, born on November 13, 1924 in Okazaki, Aichi Prefecture - November 13, 1994) was a highly influential Japanese mathematical biologist, working in the field of theoretical population genetics. ... Genetic diversity is a characteristic of ecosystems and gene pools that describes an attribute which is commonly held to be advantageous for survival -- that there are many different versions of otherwise similar organisms. ...

General principles

Darwin's illustrations of beak variation in the finches of the Galápagos Islands, which hold 13 closely related species that 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.

See also: Genotype-phenotype distinction.

Natural selection acts on an organism's phenotype, or physical characteristics. Phenotype is determined by an organism's genetic make-up (genotype) and the environment in which the organism lives. Often, natural selection acts on specific traits of an individual, and the terms phenotype and genotype are used narrowly to indicate these specific traits. Image File history File links Darwin's_finches. ... Image File history File links Darwin's_finches. ... For other people of the same surname, and places and things named after Charles Darwin, see Darwin. ... The beak, bill or rostrum is an external anatomical structure of birds which, in addition to eating, is used for grooming, manipulating objects, killing prey, probing for food, courtship, and feeding their young. ... Genera Geospiza Camarhynchus Certhidea Pinaroloxias demon hickes and darwin were on a boat for seven years smoking crack and drinking rum!!!! The birds are all about the same size (10–20 cm). ... Galápagos redirects here. ... For other uses, see Species (disambiguation). ... Character displacement refers to the phenomenon where differences among similar species whose distributions overlap geographically are accentuated in regions where the species co-occur but are minimized or lost where the species’ distributions do not overlap. ... Four of the 13 finch species found on the Galápagos Archipelago, and thought to have evolved by an adaptive radiation that diversified their beak shapes to adapt them to different food sources. ... Divergent evolution occurs when two or more biological characteristics have a common evolutionary origin but have diverged over evolutionary time. ... The genotype-phenotype distinction refers to the fact that while genotype and phenotype of an organism are related, they do not necessarily coincide. ... Individuals in the mollusk species Donax variabilis show diverse coloration and patterning in their phenotypes. ... This article does not cite any references or sources. ... In biology, a trait or character is a feature of an organism. ...

When different organisms in a population possess different versions of a gene for a certain trait, each of these versions is known as an allele. It is this genetic variation that underlies phenotypic traits. A typical example is that certain combinations of genes for eye color in humans which, for instance, give rise to the phenotype of blue eyes. (On the other hand, when all the organisms in a population share the same allele for a particular trait, and this state is stable over time, the allele is said to be fixed in that population.) For other uses, see Gene (disambiguation). ... An allele (pronounced , ) (from the Greek αλληλος, meaning each other) is one member of a pair or series of different forms of a gene. ... Hazel Eyes redirects here. ... In population genetics, fixation occurs when every individual within a population has the same allele at a particular locus. ...

Some traits are governed by only a single gene, but most traits are influenced by the interactions of many genes. A variation in one of the many genes that contributes to a trait may have only a small effect on the phenotype; together, these genes can produce a continuum of possible phenotypic values.^[2]

Nomenclature and usage

The term "natural selection" has slightly different definitions in different contexts. In simple terms, "natural selection" is most often defined to operate on heritable traits, but can sometimes refer to the differential reproductive success of phenotypes regardless of whether those phenotypes are heritable. Natural selection is "blind" in the sense that individuals' level of reproductive success is a function of the phenotype and not of whether or to what extent that phenotype is heritable. Following Darwin's primary usage^[1] the term is often used to refer to both the consequence of blind selection and to its mechanisms.^[3][4] It is sometimes helpful to explicitly distinguish between selection's mechanisms and its effects; 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.'^[5]

Traits that cause greater reproductive success of an organism are said to be selected for whereas those that reduce success are selected against. Selection for a trait may also result in the selection of other correlated traits that do not themselves directly influence fitness. This may occur as a result of pleiotropy or gene linkage.^[6] Pleiotropy occurs when a single gene influences multiple phenotypic traits. ... Genetic linkage occurs when particular alleles are inherited together. ...

Fitness

Main article: Fitness (biology)

The concept of fitness is central to natural selection. However, as with Natural selection above, there is serious divergence of opinion over the precise meaning of the term, and Richard Dawkins manages in his later books to avoid it entirely. (He devotes a chapter of his The Extended Phenotype to discussing the various senses in which the term is used.) Although fitness is sometimes colloquially understood as a quality that promotes survival of a particular individual - as illustrated in the well-known phrase survival of the fittest - modern evolutionary theory defines fitness in terms of individual reproduction. The basis of this approach is: if an organism lives half as long as others of its species, but has twice as many offspring surviving to productive adulthood, its genes will become more common in the adult population of the next generation. This is known as differential reproduction. Fitness (often denoted in population genetics models) is a central concept in evolutionary theory. ... Fitness (often denoted in population genetics models) is a central concept in evolutionary theory. ... For other uses, see Survival of the fittest (disambiguation). ...

Though natural selection acts on individuals, its average effect on all individuals with a particular genotype corresponds to the fitness of that genotype. Very low-fitness genotypes cause their bearers to have few or no offspring on average; examples include many human genetic disorders like cystic fibrosis. Conditions like sickle-cell anemia may have low fitness in the general human population, but because it confers immunity from malaria, it has high fitness value in populations which have high malaria infection rates. Broadly speaking, an organism's fitness is a function of its alleles' fitnesses. Since fitness is an averaged quantity, however, it is possible a favorable mutation may arise in an individual that does not survive to adulthood for unrelated reasons. A genetic disorder is a condition caused by abnormalities in genes or chromosomes. ... Sickle-shaped red blood cells Sickle-cell anemia or anaemia (also sickle-cell disease) is a genetic disorder in which red blood cells may change shape under certain circumstances. ...

Types of selection

Natural selection can act on any phenotypic trait, and selective pressure can be produced by any aspect of the environment, including mates and conspecifics, or members of the same species. 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 by eliminating less fit variants. Illustration from The Descent of Man and Selection in Relation to Sex by Charles Darwin showing the Tufted Coquette Lophornis ornatus, female on left, ornamented male on right. ... Competition is the act of striving against others for the purpose of achieving gain, such as income, pride, amusement, or dominance. ...

The unit of selection can be the individual or it can be another level within the hierarchy of biological organisation, such as genes, cells, and kin groups. There is still debate about whether natural selection acts at the level of groups or species to produce adaptations that benefit a larger, non-kin group. Selection at a different level such as 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, in a process called intragenomic conflict. 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. This article does not cite any references or sources. ... In evolutionary biology, kin selection refers to changes in gene frequency across generations that are driven at least in part by interactions between related individuals, and this forms much of the conceptual basis of the theory of social evolution. ... In evolutionary biology, group selection refers to the idea that alleles can become fixed or spread in a population because of the benefits they bestow on groups, regardless of the fitness of individuals within that group. ... The selfish gene theory postulates that natural selection will increase the frequency of those genes whose phenotypic effects ensure their successful replication. ...

The life cycle of a sexually reproducing organism. Various components of natural selection are indicated for each life stage.^[7]

Natural selection occurs at every life stage of an individual. An individual organism must 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 via sexual selection, and success in this competition determines who will parent the next generation. When individuals can reproduce more than once, a longer survival in the reproductive phase increases the number of offspring, called survival selection. The fecundity of both females and males (for example, giant sperm in certain species of Drosophila^[8]) can be limited via fecundity selection. The viability of produced gametes can differ, while intragenomic conflicts such as 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; this is termed compatibility selection. Image File history File links This is a lossless scalable vector image. ... Image File history File links This is a lossless scalable vector image. ... Illustration from The Descent of Man and Selection in Relation to Sex by Charles Darwin showing the Tufted Coquette Lophornis ornatus, female on left, ornamented male on right. ... Fecundity is the potential reproductive capacity of an organism or population, measured by the number of gametes (e. ... A spermatozoon or spermatozoan ( spermatozoa), from the ancient Greek σπέρμα (seed) and (living being) and more commonly known as a sperm cell, is the haploid cell that is the male gamete. ... Type species Drosophila funebris (Fabricius, 1787) Drosophila is a genus of small flies, belonging to the family Drosophilidae, whose members are often called fruit flies, or more appropriately vinegar flies, wine flies, pomace flies, grape flies, and picked fruit-flies, a reference to the characteristic of many species to linger... A gamete (from Ancient Greek γαμετης; translated gamete = wife, gametes = husband) is a cell that fuses with another gamete during fertilization (conception) in organisms that reproduce sexually. ... The selfish gene theory postulates that natural selection will increase the frequency of those genes whose phenotypic effects ensure their successful replication. ... Haploid (meaning simple in Greek) cells have only one copy of each chromosome. ...

Sexual selection

Main article: Sexual selection

It is also useful to make a mechanistic distinction between ecological selection and the narrower term 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.^[9] Sexual selection can be intrasexual, as in cases of competition among individuals of the same sex in a population, or intersexual, as in cases where one sex controls reproductive access by choosing among a population of available mates. Most commonly, intrasexual selection involves male-male competition and intersexual selection involves female choice of suitable males, due to the generally greater investment of resources for a female than a male in a single offspring organism. However, some species exhibit sex-role reversed behavior in which it is males that are most selective in mate choice; the best-known examples of this pattern occur in some fishes of the family Syngnathidae, though likely examples have also been found in amphibian and bird species.^[10] 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, for example, the extravagant plumage of some male birds. Similarly, aggression between members of the same sex 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.^[11] Illustration from The Descent of Man and Selection in Relation to Sex by Charles Darwin showing the Tufted Coquette Lophornis ornatus, female on left, ornamented male on right. ... Ecological selection (or environmental selection or survival selection or individual selection or asexual selection) refers to natural selection minus sexual selection, i. ... Illustration from The Descent of Man and Selection in Relation to Sex by Charles Darwin showing the Tufted Coquette Lophornis ornatus, female on left, ornamented male on right. ... In evolutionary biology, kin selection refers to changes in gene frequency across generations that are driven at least in part by interactions between related individuals, and this forms much of the conceptual basis of the theory of social evolution. ... Trees in this Bangladesh forest are in competition for light. ... In sociology and biology, infanticide is the practice of intentionally causing the death of an infant of a given species, by members of the same species - often by the mother. ... For other uses, see Fish (disambiguation). ... Syngnathidae is a family of fishes, including seahorses, pipehorse, pipefish and sea dragons, among others. ... ‹ The template below (Citations missing) is being considered for deletion. ... For other uses, see Bird (disambiguation). ... Genera About 15 in 4 subfamilies. ... Female (left) and male Common Pheasant, illustrating the dramatic difference in both color and size, between the sexes Sexual dimorphism is the systematic difference in form between individuals of different sex in the same species. ...

An example: antibiotic resistance

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. Since the discovery of penicillin in 1928 by Alexander Fleming, antibiotics have been used to fight bacterial diseases. 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 slightly 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. Image File history File links This is a lossless scalable vector image. ... Image File history File links This is a lossless scalable vector image. ... Antibiotic resistance is the ability of a microorganism to withstand the effects of an antibiotic. ... A cluster of Escherichia coli bacteria magnified 10,000 times. ... Penicillin core structure Penicillin (abbreviated PCN) is a group of β-lactam antibiotics used in the treatment of bacterial infections caused by susceptible, usually Gram-positive, organisms. ... Sir Alexander Fleming (6 August 1881 – 11 March 1955) was a Scottish biologist and pharmacologist. ... Staphylococcus aureus - Antibiotics test plate. ... Phyla Actinobacteria Aquificae Chlamydiae Bacteroidetes/Chlorobi Chloroflexi Chrysiogenetes Cyanobacteria Deferribacteres Deinococcus-Thermus Dictyoglomi Fibrobacteres/Acidobacteria Firmicutes Fusobacteria Gemmatimonadetes Lentisphaerae Nitrospirae Planctomycetes Proteobacteria Spirochaetes Thermodesulfobacteria Thermomicrobia Thermotogae Verrucomicrobia Bacteria (singular: bacterium) are unicellular microorganisms. ... For linguistic mutation, see Apophony. ...

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, and advantageous mutations are even rarer. However, populations of bacteria are large enough that 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.

The widespread use and misuse of antibiotics has resulted in increased microbial resistance to antibiotics in clinical use, 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.^[12] Response strategies typically include the use of different, stronger antibiotics; however, new strains of MRSA have recently emerged that are resistant even to these drugs.^[13] This is an example of what is known as an evolutionary 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. Arms races are not necessarily induced by man; a well-documented example involves the elaboration of the RNA interference pathway in plants as means of innate immunity against viruses.^[14] MRSA redirects here. ... Antibiotic resistance is the ability of a microorganism to withstand the effects of an antibiotic. ... In biology, Strain can be used two ways. ... An evolutionary arms race is an evolutionary struggle between a predator species and its prey (including parasitism) that is said to resemble an arms race. ... the plane is spreading pesticide. ... Cells use dicer to trim double stranded RNA to form small interfering RNA or microRNA. An exogenous dsRNA or endogenous pre-miRNA can be processed by dicer and incorporated into the RNA-induced silencing complex (RISC), which targets single-stranded messenger RNA molecules and triggers translational repression;[1] incorporation into... Innate immunity is immunity that the body possesses naturally, as opposed to adaptive immunity. ... This article is about biological infectious particles. ...

Genetical theory of natural selection

Natural selection by itself is a simple concept, in which fitness differences between phenotypes play a crucial role. It is the union of natural selection as a mechanism with genetic material as a substrate that offers most of the theory's explanatory power

Directionality of selection

When some component of a trait is heritable, selection will alter the frequencies of the different alleles, or variants of the gene that produces the variants of the trait. Selection can be divided into three classes, on the basis of its effect on allele frequencies.^[15] An allele (pronounced , ) (from the Greek αλληλος, meaning each other) is one member of a pair or series of different forms of a gene. ...

Directional selection occurs when a certain allele has a greater fitness than others, resulting in an increase in frequency of that allele. This process can continue until the allele is fixed and the entire population shares the fitter phenotype. It is directional selection that is illustrated in the antibiotic resistance example above. In population genetics, directional selection (sometimes referred to as positive selection) occurs when natural selection favors a single allele and therefore allele frequency continuously shifts in one direction. ... In population genetics, fixation occurs when every individual within a population has the same allele at a particular locus. ...

Far more common is stabilizing selection (also known as purifying selection), which lowers the frequency of alleles that have a deleterious effect on the phenotype - that is, produce organisms of lower fitness. This process can continue until the allele is eliminated from the population. Purifying selection results in functional genetic features, such as protein-coding genes or regulatory sequences, being conserved over time due to selective pressure against deleterious variants. A chart showing three types of selection Stabilizing selection, also referred to as purifying selection, is a type of natural selection in which genetic diversity decreases as the population stabilizes on a particular trait value. ... Protein biosynthesis (synthesis) is the process in which cells build proteins. ... A regulatory sequence (also called regulatory region or ~ element) is a promoter, enhancer or other segment of DNA where regulatory proteins such as transcription factors bind preferentially. ... Conservation is a high degree of similarity in the primary or higher structure of homologous proteins amongst various phyla. ...

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 (that is, those that have two pairs of chromosomes) when heterozygote individuals, who have different alleles on each chromosome at a single genetic locus, have a higher fitness than homozygote individuals that have two of the same alleles. This is called heterozygote advantage or overdominance, of which the best-known example is the malarial resistance observed in heterozygous humans who carry only one copy of the gene for sickle cell anemia. 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, balancing selection can occur through frequency-dependent selection, where the fitness of one particular phenotype depends on the distribution of other phenotypes in the population. The principles of game theory have been applied to understand the fitness distributions in these situations, particularly in the study of kin selection and the evolution of reciprocal altruism.^[16][17] Sickle-shaped red blood cells Balancing selection refers to forms of natural selection which work to maintain genetic polymorphisms (or multiple alleles) within a population. ... Diploid (meaning double in Greek) cells have two copies (homologs) of each chromosome (both sex- and non-sex determining chromosomes), usually one from the mother and one from the father. ... A scheme of a condensed (metaphase) chromosome. ... An organism is a heterozygote or heterozygous for a gene or trait if it has different alleles at the genes locus for each homologous chromosome. ... Short and long arms Chromosome. ... This article or section does not cite its references or sources. ... A heterozygote advantage (heterozygous advantage or overdominance) describes the case in which the heterozygote genotype has a higher relative fitness than either the homozygote dominant or homozygote recessive genotype. ... Malaria is a vector-borne infectious disease caused by protozoan parasites. ... Sickle-shaped red blood cells Sickle cell anemia (American English), sickle cell anaemia (British English) or sickle cell disease is a genetic disease in which red blood cells may change shape under certain circumstances. ... Disruptive selection is a type of evolution that simultaneously favors individuals at both extremes of the distribution. ... In statistics, a bimodal distribution is a distribution with two different peaks — that is, there are two distinct values that measurements tend to center around. ... Frequency dependent selection is the term given to an evolutionary process where the fitness of a phenotype is dependent on the relative frequency of other phenotypes in a given population. ... Game theory is a branch of applied mathematics that is often used in the context of economics. ... In evolutionary biology, kin selection refers to changes in gene frequency across generations that are driven at least in part by interactions between related individuals, and this forms much of the conceptual basis of the theory of social evolution. ... In evolutionary biology, reciprocal altruism is a form of altruism in which one organism provides a benefit to another in the expectation of future reciprocation. ...

Selection and genetic variation

A portion of all genetic variation is functionally neutral in that it produces no phenotypic effect or significant difference in fitness; the hypothesis that this variation accounts for a large fraction of observed genetic diversity is known as the neutral theory of molecular evolution and was originated by Motoo Kimura. Neutral variation was once thought to encompass most of the genetic variation in non-coding DNA, which was hypothesized to be composed of "junk DNA". However, more recently, the functional roles of non-coding DNA, such as the regulatory and developmental functions of RNA gene products, has been studied in depth;^[18] large parts of non-protein-coding DNA sequences are highly conserved under strong purifying selection and thus do not vary much from individual to individual, indicating that mutations in these regions have deleterious consequences.^[19][20] 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 variation does influence fitness.^[15] Genetic variation is the variation in the genetic material of a population, and includes the nuclear, mitochodrial, ribosomal genomes as well as the genomes of other organelles. ... Genetic diversity is a characteristic of ecosystems and gene pools that describes an attribute which is commonly held to be advantageous for survival -- that there are many different versions of otherwise similar organisms. ... The neutral theory of molecular evolution (also, simply the neutral theory of evolution) is an influential theory that was introduced with provocative effect by Motoo Kimura in the late 1960s and early 1970s. ... Motoo Kimura (木村資生, born on November 13, 1924 in Okazaki, Aichi Prefecture - November 13, 1994) was a highly influential Japanese mathematical biologist, working in the field of theoretical population genetics. ... In genetics, noncoding DNA describes DNA which does not contain instructions for making proteins (or other cell products such as RNAs). ... In molecular biology, junk DNA is a collective label for the portions of the DNA sequence of a chromosome or a genome for which no function has yet been identified. ... For other uses, see RNA (disambiguation). ... A gene product is the biochemical material, either RNA or protein, resulting from expression of a gene. ...

Mutation selection balance

Natural selection results in the reduction of genetic variation through the elimination of maladapted individuals and consequently of the mutations that caused 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 occur and on the strength of the natural selection, which is a function of how unfavorable the mutation proves to be. Consequently, changes in the mutation rate or the selection pressure will result in a different mutation-selection balance. mutation-selection balance is a balance between mutation and selection. ...

Genetic linkage

Genetic linkage occurs when the loci of two alleles are linked, or in close proximity to each other on the chromosome. During the formation of 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. Genetic linkage occurs when particular alleles are inherited jointly. ... Short and long arms Chromosome. ... A scheme of a condensed (metaphase) chromosome. ... Gametes (in Greek: γαμέτες) —also known as sex cells, germ cells, or spores—are the specialized cells that come together during fertilization (conception) in organisms that reproduce sexually. ... Genetic recombination is the process by which a strand of the genetic material (usually DNA; but can also be RNA) is broken and then joined to the end of a different DNA molecule. ...

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 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. A selective sweep is the reduction or elimination of variation among the nucleotides in neighbouring DNA of a mutation as the result of recent and strong natural selection. ... The process by which an evolutionary neutral or in some cases deleterious allele or mutation may spread through the gene pool by virtue of being linked to a beneficial mutation. ... A haplotype is the genetic constitution of an individual chromosome. ...

Whether a selective sweep has occurred or not can be investigated by measuring linkage disequilibrium, or 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. Linkage disequilibrium (LD) is the non-random association of alleles at two or more loci on a chromosome. ... Genetic recombination is the process by which a strand of the genetic material (usually DNA; but can also be RNA) is broken and then joined to the end of a different DNA molecule. ...

Background selection is the opposite of a selective sweep. If a specific site experiences strong and persistent purifying selection, linked variation will tend to be weeded out along with it, producing a region in the genome of low overall variability. Because background selection is a result of deleterious new mutations, which can occur randomly in any haplotype, it produces no linkage disequilibrium. The term background selection refers to the reduction in genetic variation that occurs in the genomic area surrounding a gene that repeatedly mutates to a bad version. ...

Evolution by means of natural selection

Main articles: Evolution and Darwinism

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.^[19][20] Although both mutation rates and average fitness effects of mutations are dependent on the organism, estimates from data in humans have found that a majority of mutations are slightly deleterious.^[21] This article is about evolution in biology. ... Charles Darwin Darwinism is a term for the underlying theory in those ideas of Charles Darwin concerning evolution and natural selection. ... For other uses, see Adaptation (disambiguation). ... Charles Darwins first sketch of an evolutionary tree from his First Notebook on Transmutation of Species (1837) Speciation is the evolutionary process by which new biological species arise. ... Genetic variation is the variation in the genetic material of a population, and includes the nuclear, mitochodrial, ribosomal genomes as well as the genomes of other organelles. ... For linguistic mutation, see Apophony. ... Genetic recombination is the process by which a strand of the genetic material (usually DNA; but can also be RNA) is broken and then joined to the end of a different DNA molecule. ... Karyogram of human male using Giemsa staining. ... A scheme of a condensed (metaphase) chromosome. ... It has been suggested that junk DNA be merged into this article or section. ... A synonymous substitution (also called a silent substitution) is the evolutionary substitution of one base for another in an exon of a gene coding for a protein, such that the amino acid sequence produced is not modified. ... This article is about modern humans. ...

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 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. Image File history File linksMetadata Pavo_cristatus_albino001xx. ... Image File history File linksMetadata Pavo_cristatus_albino001xx. ... Peacock re-directs here; for alternate uses see Peacock (disambiguation). ... Albinism is a genetic condition resulting in a lack of pigmentation in the eyes, skin and hair. ... Broadly, melanin is any of the polyacetylene, polyaniline, and polypyrrole blacks and browns or their mixed copolymers. ...

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. Examples of nonlethal regulatory mutations occur in HOX genes in humans, which can result in a cervical rib^[22] or polydactyly, an increase in the number of fingers or toes.^[23] When such mutations result in a higher fitness, natural selection will favor these phenotypes and the novel trait will spread in the population. Download high resolution version (891x1077, 119 KB)Conversion of a DICOM-format X-ray from a patient of User:Drgnu23, a ten year old male. ... Download high resolution version (891x1077, 119 KB)Conversion of a DICOM-format X-ray from a patient of User:Drgnu23, a ten year old male. ... This article is about the human congenital disorder (disease). ... A regulatory sequence (also called regulatory region or ~ element) is a promoter, enhancer or other segment of DNA where regulatory proteins such as transcription factors bind preferentially. ... For other meanings see Zygote (disambiguation). ... A homeobox is a DNA sequence found within genes that are involved in the regulation of development (morphogenesis) of animals, fungi and plants. ... A cervical rib is a supernumerary (extra) rib which arises from the seventh cervical vertebra. ... This article is about the human congenital disorder (disease). ...

Established traits are not immutable; traits that have high fitness in one environmental context may be much less fit if environmental conditions change. In the absence of natural selection to preserve such a trait, it will become more variable and deteriorate over time, possibly resulting in a vestigial manifestation of the trait. In many circumstances, the apparently vestigial structure may retain a limited functionality, or may be co-opted for other advantageous traits in a phenomenon known as preadaptation. A famous example of a vestigial structure, the eye of the blind mole rat, is believed to retain function in photoperiod perception.^[24] The human vermiform appendix is a vestigial structure; it no longer retains its original function. ... In evolutionary biology, preadaptation describes a situation where an organism uses a preexisting anatomical structure inherited from an ancestor for a potentially unrelated purpose. ... For other uses, see Eye (disambiguation). ... Genera Nannospalax Spalax Blind mole rats are one of many types of rodents that are referred to as mole rats. ... Photoperiodism is the physiological reaction of organisms to the length of day or night. ...

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. A lesser-known mechanism of speciation occurs via hybridization, well-documented in plants and occasionally observed in species-rich groups of animals such as cichlid fishes.^[25] Such mechanisms of rapid speciation can reflect a mechanism of evolutionary change known as punctuated equilibrium, which suggests that evolutionary change and particularly speciation typically happens quickly after interrupting long periods of stasis. Charles Darwins first sketch of an evolutionary tree from his First Notebook on Transmutation of Species (1837) Speciation is the evolutionary process by which new biological species arise. ... In population genetics, gene flow (also known as gene migration) is the transfer of alleles of genes from one population to another. ... Assortative mating (also called Assortative pairing) takes place when sexually reproducing organisms tend to mate with individuals that are like themselves in some respect (positive assortative mating) or dissimilar (negative assortative mating). ... Simple illustration of founder effect. ... This article is about a biological term. ... For other uses, see Plant (disambiguation). ... Subfamilies Astronotinae Cichlasomatinae Cichlinae Etroplinae Geophaginae Heterochromidinae Paratilapiinae Pseudocrenilabrinae Ptychochrominae Retroculinae For genera, see below. ... Punctuated equilibrium (or punctuated equilibria) is a theory in evolutionary biology which states that most sexually reproducing species will show little to no evolutionary change throughout their history. ...

Genetic changes within groups result in increasing incompatibility between the genomes of the two subgroups, thus reducing gene flow between the groups. Gene flow will effectively cease when the distinctive mutations characterizing each subgroup become fixed. 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

Main articles: History of evolutionary thought, Inception of Darwin's theory, and Development of Darwin's theory

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

Evolutionary thought has roots in antiquity as philosophical ideas conceived during the Ancient Greek and Roman eras, but until the 18th century, biological thought was dominated by essentialism, the idea that living forms are static and unchanging in time. ... The inception of Darwins theory began with a search for explanations of contradictions in current Creationist ideas, and led him to formulate his theory of evolution which was eventually published in his book On the Origin of Species. ... The Development of Darwins theory began with a search for explanations of contradictions in current faith based ideas, and led him to formulate his theory of evolution which was eventually published in his book On the Origin of Species, a turning point in the history of evolutionary thought. ... Wikipedia does not yet have an article with this exact name. ... Wikipedia does not yet have an article with this exact name. ... For other people of the same surname, and places and things named after Charles Darwin, see Darwin. ...

Pre-Darwinian theories

Several ancient philosophers expressed the idea that Nature produces a huge variety of creatures, apparently randomly, and that only those creatures survive that manage to provide for themselves and reproduce successfully; well-known examples include Empedocles^[26] and his intellectual successor, Lucretius,^[27] while related ideas were later refined by Aristotle.^[28] The struggle for existence was later described by al-Jahiz in the 9th century.^[29][30] Such classical arguments were reintroduced in the 18th century by Pierre Louis Maupertuis^[31] and others, including Charles Darwin's grandfather Erasmus Darwin. While these forerunners had an influence on Darwinism, they later had little influence on the trajectory of evolutionary thought after Charles Darwin. This article is about the physical universe. ... Empedocles (Greek: , ca. ... Lucretius Titus Lucretius Carus (c. ... For other uses, see Aristotle (disambiguation). ... Al-Jahiz (in Arabic الجاحظ) (real name Abu Uthman Amr Ibn Bahr al-Kinani al-Fuqaimi al-Basri) (born in Basra, 776 - 869) was a famous Arab scholar probably of Abyssinian descent. ... Pierre Louis Maupertuis, here wearing lapmudes or a fur coat from his Lapland expedition. ... For other people of the same surname, and places and things named after Charles Darwin, see Darwin. ... This article is about Erasmus Darwin who lived 1731–1802; for his descendants with the same name see Erasmus Darwin (disambiguation). ... Charles Darwin Darwinism is a term for the underlying theory in those ideas of Charles Darwin concerning evolution and natural selection. ...

Until the early 19th century, the prevailing 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, the theory of uniformitarianism in geology promoted the idea that simple, weak forces could act continuously over long periods of time to produce radical changes in the Earth's landscape. The success of this theory raised awareness of the vast scale of geological time and made plausible the idea that tiny, virtually imperceptible changes in successive generations could produce consequences on the scale of differences between species. Early 19th century evolutionists such as Jean Baptiste Lamarck suggested the inheritance of acquired characteristics as a mechanism for evolutionary change; adaptive traits acquired by an organism during its lifetime could be inherited by that organism's progeny, eventually causing transmutation of species.^[32] This theory has come to be known as Lamarckism and was an influence on the anti-genetic ideas of the Stalinist Soviet biologist Trofim Lysenko.^[33] Evolutionary thought has roots in antiquity as philosophical ideas conceived during the Ancient Greek and Roman eras, but until the 18th century, biological thought was dominated by essentialism, the idea that living forms are static and unchanging in time. ... The history of creationism is tied to the history of religions. ... Platonic idealism is the theory that the substantive reality around us is only a reflection of a higher truth. ... In creation biology, created kinds are b