Evolution

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This article is about evolution in biology. For other uses, see Evolution (disambiguation).

"Theory of evolution" redirects here. For more on how evolution is defined, see Evolution as theory and fact.

For a generally accessible and less technical introduction to the topic, see Introduction to evolution.

Part of the Biology series on

Evolution

Mechanisms and processes

Adaptation Genetic drift Gene flow Mutation Natural selection Speciation

Research and history

Evidence Evolutionary history of life History Modern synthesis Social effect / Objections

Evolutionary biology fields

Cladistics Ecological genetics Evolutionary development Human evolution Molecular evolution Phylogenetics Population genetics

Biology Portal · v • d • e

In biology, evolution is the changes seen in the inherited traits of a population from one generation to the next. These changes are relatively minor from one generation to the next, but accumulate with each subsequent generation and can eventually cause substantial changes in the organisms. Inherited traits come from the genes that are passed on to offspring during reproduction. Mutations in genes can produce new or altered traits, resulting in the appearance of heritable differences between organisms, but new traits also come from the transfer of genes between populations, as in migration, or between species, in horizontal gene transfer. Evolution occurs when these heritable differences become more common or rare in a population, either non-randomly through natural selection or randomly through genetic drift.

Natural selection is a process by which heritable traits that are helpful for survival and reproduction become more common in a population, while harmful traits become more rare. This occurs because individuals with advantageous traits are more likely to reproduce successfully, so that more individuals in the next generation inherit these traits.Lande R, Arnold SJ (1983). "The measurement of selection on correlated characters". Evolution 37: 1210–26}. doi:10.2307/2408842.  Over many generations, adaptations occur through a combination of successive, small, random changes in traits, and natural selection of those variants best-suited for their environment.Ayala FJ (2007). "Darwin\'s greatest discovery: design without designer". Proc. Natl. Acad. Sci. U.S.A. 104 Suppl 1: 8567–73. PMID 17494753.  In contrast, genetic drift produces random changes in the frequency of traits in a population. Genetic drift arises from the role chance plays in whether a given individual will survive and reproduce.

One definition of a species is a group of organisms that can reproduce with one another and produce fertile offspring. When a species is separated into populations that are prevented from interbreeding, mutations, genetic drift, and the selection of novel traits cause the accumulation of differences over generations and the emergence of new species.(Gould 2002) The similarities between organisms suggest that all known species are descended from a common ancestor (or ancestral gene pool) through this process of gradual divergence.Futuyma, Douglas J. (2005). Evolution. Sunderland, Massachusetts: Sinauer Associates, Inc. ISBN 0-87893-187-2. 

Studies of the fossil record and the diversity of living organisms had convinced most scientists by the mid-nineteenth century that species changed over time.Ian C. Johnston (1999). History of Science: Early Modern Geology. Malaspina University-College. Retrieved on 2008-01-15.Bowler, Peter J. (2003). Evolution:The History of an Idea. University of California Press. ISBN 0-52023693-9.  However, the mechanism driving these changes remained unclear until the 1859 publication of Charles Darwin\'s On the Origin of Species, detailing the theory of evolution by natural selection.Darwin, Charles (1859). On the Origin of Species, 1st, John Murray, p. 1. . Related earlier ideas were acknowledged in Darwin, Charles (1861). On the Origin of Species, 3rd, John Murray, p. xiii.  Darwin\'s work soon led to overwhelming acceptance of evolution within the scientific community.AAAS Council (December 26, 1922). AAAS Resolution: Present Scientific Status of the Theory of Evolution. American Association for the Advancement of Science.IAP Statement on the Teaching of Evolution. The Interacademy Panel on International Issues (2006). Retrieved on 2007-04-25. Joint statement issued by the national science academies of 67 countries, including the United Kingdom\'s Royal SocietyBoard of Directors, American Association for the Advancement of Science (February 16, 2006). Statement on the Teaching of Evolution. American Association for the Advancement of Science. from the world\'s largest general scientific societyStatements from Scientific and Scholarly Organizations. National Center for Science Education. In the 1930s, Darwinian natural selection was combined with Mendelian inheritance to form the modern evolutionary synthesis, in which the connection between the units of evolution (genes) and the mechanism of evolution (natural selection) was made. This powerful explanatory and predictive theory has become the central organizing principle of modern biology, providing a unifying explanation for the diversity of life on Earth.Special report on evolution. New Scientist (19 Jan 2008).

Contents 1 Heredity 2 Variation 2.1 Mutation 2.2 Sex and recombination 2.3 Population genetics 3 Mechanisms 3.1 Natural selection 3.2 Genetic drift 3.3 Gene flow 4 Outcomes 4.1 Adaptation 4.2 Co-evolution 4.3 Co-operation 4.4 Speciation 4.4.1 Barriers to breeding between species 4.5 Extinction 5 Evolutionary history of life 5.1 Origin of life 5.2 Common descent 5.3 Evolution of life 6 History of evolutionary thought 7 Social and cultural views 8 Applications in technology 9 Further reading 10 References 11 External links

Heredity

For more details on this topic, see Introduction to genetics, Genetics, and Heredity.

DNA structure. Bases are in the center, surrounded by phosphate–sugar chains in a double helix.

Inheritance in organisms occurs through discrete traits – particular characteristics of an organism. In humans, for example, eye color is an inherited characteristic, which individuals can inherit from one of their parents.Sturm RA, Frudakis TN (2004). "Eye colour: portals into pigmentation genes and ancestry". Trends Genet. 20 (8): 327–32. PMID 15262401.  Inherited traits are controlled by genes and the complete set of genes within an organism\'s genome is called its genotype.Pearson H (2006). "Genetics: what is a gene?". Nature 441 (7092): 398–401. PMID 16724031. 

The complete set of observable traits that make up the structure and behavior of an organism is called its phenotype. These traits come from the interaction of its genotype with the environment.Peaston AE, Whitelaw E (2006). "Epigenetics and phenotypic variation in mammals". Mamm. Genome 17 (5): 365–74. PMID 16688527.  As a result, not every aspect of an organism\'s phenotype is inherited. Suntanned skin results from the interaction between a person\'s genotype and sunlight; thus, a suntan is not hereditary. However, people have different responses to sunlight, arising from differences in their genotype; a striking example is individuals with the inherited trait of albinism, who do not tan and are highly sensitive to sunburn.Oetting WS, Brilliant MH, King RA (1996). "The clinical spectrum of albinism in humans". Molecular medicine today 2 (8): 330–35. PMID 8796918. 

Genes are regions within DNA molecules that contain genetic information. DNA is a long molecule with four types of bases attached along its length. Different genes have different sequences of bases; it is the sequence of these bases that encodes genetic information. Within cells, the long strands of DNA associate with proteins to form structures called chromosomes. A specific location within a chromosome is known as a locus. If the DNA sequence at a locus varies between individuals, the different forms of this sequence are called alleles. DNA sequences can change through mutations, producing new alleles. If a mutation occurs within a gene, the new allele may affect the trait that the gene controls, altering the phenotype of the organism. However, while this simple correspondence between an allele and a trait works in some cases, most traits are more complex and are controlled by multiple interacting genes.Mayeux R (2005). "Mapping the new frontier: complex genetic disorders". J. Clin. Invest. 115 (6): 1404–07. PMID 15931374. Wu R, Lin M (2006). "Functional mapping - how to map and study the genetic architecture of dynamic complex traits". Nat. Rev. Genet. 7 (3): 229–37. PMID 16485021. 

Variation

For more details on this topic, see Genetic variation and Population genetics.

Because an individual\'s phenotype results from the interaction of its genotype with the environment, the variation in phenotypes in a population reflects the variation in these organisms\' genotypes. The modern evolutionary synthesis defines evolution as the change over time in this genetic variation. The frequency of one particular allele will fluctuate, becoming more or less prevalent relative to other forms of that gene. Evolutionary forces act by driving these changes in allele frequency in one direction or another. Variation disappears when an allele reaches the point of fixation — when it either disappears from the population or replaces the ancestral allele entirely.Harwood AJ (1998). "Factors affecting levels of genetic diversity in natural populations". Philos. Trans. R. Soc. Lond., B, Biol. Sci. 353 (1366): 177–86. PMID 9533122. 

Variation comes from mutations in genetic material, migration between populations (gene flow), and the reshuffling of genes through sexual reproduction. Variation also comes from exchanges of genes between different species; for example, through horizontal gene transfer in bacteria, and hybridization in plants.Draghi J, Turner P (2006). "DNA secretion and gene-level selection in bacteria". Microbiology (Reading, Engl.) 152 (Pt 9): 2683–8. PMID 16946263. 
*Mallet J (2007). "Hybrid speciation". Nature 446 (7133): 279–83. PMID 17361174. , Despite the constant introduction of variation through these processes, most of the genome of a species is identical in all individuals of that species.Butlin RK, Tregenza T (1998). "Levels of genetic polymorphism: marker loci versus quantitative traits". Philos. Trans. R. Soc. Lond., B, Biol. Sci. 353 (1366): 187–98. PMID 9533123.  However, even relatively small changes in genotype can lead to dramatic changes in phenotype: chimpanzees and humans differ in only about 5% of their genomes.Wetterbom A, Sevov M, Cavelier L, Bergström TF (2006). "Comparative genomic analysis of human and chimpanzee indicates a key role for indels in primate evolution". J. Mol. Evol. 63 (5): 682–90. PMID 17075697. 

Mutation

For more details on this topic, see Mutation and Molecular evolution.

Duplication of part of a chromosome

Genetic variation comes from random mutations that occur in the genomes of organisms. Mutations are changes in the DNA sequence of a cell\'s genome and are caused by radiation, viruses, transposons and mutagenic chemicals, as well as errors that occur during meiosis or DNA replication.Bertram J (2000). "The molecular biology of cancer". Mol. Aspects Med. 21 (6): 167–223. PMID 11173079. Aminetzach YT, Macpherson JM, Petrov DA (2005). "Pesticide resistance via transposition-mediated adaptive gene truncation in Drosophila". Science 309 (5735): 764–67. doi:10.1126/science.1112699. PMID 16051794. Burrus V, Waldor M (2004). "Shaping bacterial genomes with integrative and conjugative elements". Res. Microbiol. 155 (5): 376–86. PMID 15207870.  These mutagens produce several different types of change in DNA sequences; these can either have no effect, alter the product of a gene, or prevent the gene from functioning. Studies in the fly Drosophila melanogaster suggest that about 70 percent of mutations are harmful, and the remainder are either neutral or have weakly beneficial effects.Sawyer SA, Parsch J, Zhang Z, Hartl DL (2007). "Prevalence of positive selection among nearly neutral amino acid replacements in Drosophila". Proc. Natl. Acad. Sci. U.S.A. 104 (16): 6504-10. PMID 17409186.  Due to the damaging effects that mutations can have on cells, organisms have evolved mechanisms such as DNA repair to remove mutations. Therefore, the optimal mutation rate for a species is a trade-off between short-term costs, such as the risk of cancer, and the long-term benefits of advantageous mutations.Sniegowski P, Gerrish P, Johnson T, Shaver A (2000). "The evolution of mutation rates: separating causes from consequences". Bioessays 22 (12): 1057–66. PMID 11084621.  Some species such as retroviruses have such high mutation rates that most of their offspring will possess a mutated gene.Drake JW, Charlesworth B, Charlesworth D, Crow JF (1998). "Rates of spontaneous mutation". Genetics 148 (4): 1667–86. PMID 9560386.  Such rapid mutation may have been selected so that these viruses can constantly and rapidly evolve, and thus evade the responses of the human immune system.Holland J, Spindler K, Horodyski F, Grabau E, Nichol S, VandePol S (1982). "Rapid evolution of RNA genomes". Science 215 (4540): 1577–85. PMID 7041255. 

Large sections of DNA can also be duplicated, which is a major source of raw material for evolving new genes, with tens to hundreds of genes duplicated in animal genomes every million years.Carroll SB, Grenier J, Weatherbee SD (2005). From DNA to Diversity: Molecular Genetics and the Evolution of Animal Design. Second Edition. Oxford: Blackwell Publishing. ISBN 1-4051-1950-0.  Most genes belong to larger families of genes of shared ancestry.Harrison P, Gerstein M (2002). "Studying genomes through the aeons: protein families, pseudogenes and proteome evolution". J Mol Biol 318 (5): 1155–74. PMID 12083509.  Novel genes are produced either through duplication and mutation of an ancestral gene, or by recombining parts of different genes to form new combinations with new functions.Orengo CA, Thornton JM (2005). "Protein families and their evolution-a structural perspective". Annu. Rev. Biochem. 74: 867–900. PMID 15954844. Pál C, Papp B, Lercher MJ (2006). "An integrated view of protein evolution". Nat. Rev. Genet. 7 (5): 337–48. PMID 16619049.  For example, the human eye uses four genes to make structures that sense light: three for color vision and one for night vision; all four arose from a single ancestral gene.Bowmaker JK (1998). "Evolution of colour vision in vertebrates". Eye (London, England) 12 (Pt 3b): 541–47. PMID 9775215.  An advantage of duplicating a gene (or even an entire genome) is that overlapping or redundant functions in multiple genes allows alleles to be retained that would otherwise be harmful, thus increasing genetic diversity.Gregory TR, Hebert PD (1999). "The modulation of DNA content: proximate causes and ultimate consequences". Genome Res. 9 (4): 317–24. PMID 10207154. 

Changes in chromosome number may also involve the breakage and rearrangement of DNA within chromosomes. For example, two chromosomes in the Homo genus fused to produce human chromosome 2; this fusion did not occur in the lineage of the other apes, and they retain these separate chromosomes. Zhang J, Wang X, Podlaha O (2004). "Testing the chromosomal speciation hypothesis for humans and chimpanzees". Genome Res. 14 (5): 845–51. PMID 15123584.  In evolution, the most important role of such chromosomal rearrangements may be to accelerate the divergence of a population into new species by making populations less likely to interbreed, and thereby preserving genetic differences between these populations.Ayala FJ, Coluzzi M (2005). "Chromosome speciation: humans, Drosophila, and mosquitoes". Proc. Natl. Acad. Sci. U.S.A. 102 Supplement 1: 6535–42. PMID 15851677. 

Sequences of DNA that can move about the genome, such as transposons, make up a major fraction of the genetic material of plants and animals, and may have been important in the evolution of genomes.Hurst GD, Werren JH (2001). "The role of selfish genetic elements in eukaryotic evolution". Nat. Rev. Genet. 2 (8): 597–606. PMID 11483984.  For example, more than a million copies of the Alu sequence are present in the human genome, and these sequences have now been recruited to perform functions such as regulating gene expression.Häsler J, Strub K (2006). "Alu elements as regulators of gene expression". Nucleic Acids Res. 34 (19): 5491–97. PMID 17020921.  Another effect of these mobile DNA sequences is that when they move within a genome, they can mutate or delete existing genes and thereby produce genetic diversity.Aminetzach YT, Macpherson JM, Petrov DA (2005). "Pesticide resistance via transposition-mediated adaptive gene truncation in Drosophila". Science 309 (5735): 764–67. PMID 16051794. 

Sex and recombination

For more details on this topic, see Genetic recombination and Sexual reproduction.

In asexual organisms, genes are inherited together, or linked, as they cannot mix with genes in other organisms during reproduction. However, the offspring of sexual organisms contain random mixtures of their parents\' chromosomes that are produced through independent assortment. In the related process of genetic recombination, sexual organisms can also exchange DNA between two matching chromosomes.Radding C (1982). "Homologous pairing and strand exchange in genetic recombination". Annu. Rev. Genet. 16: 405–37. PMID 6297377.  Recombination and reassortment do not alter allele frequencies, but instead change which alleles are associated with each other, producing offspring with new combinations of alleles.Agrawal AF (2006). "Evolution of sex: why do organisms shuffle their genotypes?". Curr. Biol. 16 (17): R696–704. PMID 16950096.  While this process increases the variation in any individual\'s offspring, genetic mixing can either have no effect, increase, or decrease the genetic variation in the population, depending on how the various alleles in the population are distributed. For example, if two alleles are randomly distributed in a population, then sex will have no effect on variation; however, if two alleles tend to be found as a pair, then genetic mixing will even out this non-random distribution and over time make the organisms in the population more similar to each other.

Recombination allows even alleles that are close together in a strand of DNA to be inherited independently. However, the rate of recombination is low, since in humans in stretch of DNA one million base pairs long there is about a one in a hundred chance of a recombination event occurring per generation. As a result, genes close together on a chromosome may not always be shuffled away from each other, and genes that are close together tend to be inherited together.Lien S, Szyda J, Schechinger B, Rappold G, Arnheim N (2000). "Evidence for heterogeneity in recombination in the human pseudoautosomal region: high resolution analysis by sperm typing and radiation-hybrid mapping". Am. J. Hum. Genet. 66 (2): 557–66. PMID 10677316.  This tendency is measured by finding how often two alleles occur together, which is called their linkage disequilibrium. A set of alleles that is usually inherited in a group is called a haplotype, and this co-inheritance can indicate that inheriting this group of alleles confers an advantage to an organism (see positive selection below).Sabeti P, Schaffner S, Fry B, Lohmueller J, Varilly P, Shamovsky O, Palma A, Mikkelsen T, Altshuler D, Lander E (2006). "Positive natural selection in the human lineage". Science 312 (5780): 1614–20. PMID 16778047. 

Sexual reproduction helps to remove harmful mutations and retain beneficial mutations.Otto S (2003). "The advantages of segregation and the evolution of sex". Genetics 164 (3): 1099–118. PMID 12871918.  Consequently, when alleles cannot be separated by recombination – such as in mammalian Y chromosomes, which pass intact from fathers to sons – harmful mutations accumulate.Muller H (1964). "The relation of recombination to mutational advance". Mutat. Res. 106: 2–9. PMID 14195748. Charlesworth B, Charlesworth D (2000). "The degeneration of Y chromosomes". Philos. Trans. R. Soc. Lond., B, Biol. Sci. 355 (1403): 1563–72. PMID 11127901.  In addition, recombination and reassortment can produce individuals with new and advantageous gene combinations. These positive effects are balanced by the fact that this process can cause mutations and separate beneficial combinations of genes.

Population genetics

White peppered moth

 

Black morph in peppered moth evolution

From a genetic viewpoint, evolution is a generation-to-generation change in the frequencies of alleles within a population that shares a common gene pool.Stoltzfus A (2006). "Mutationism and the dual causation of evolutionary change". Evol. Dev. 8 (3): 304–17. PMID 16686641.  A population is a localized group of individuals belonging to the same species. For example, all of the moths of the same species living in an isolated forest represent a population. A single gene in this population may have several alternate forms, which account for variations between the phenotypes of the organisms. An example might be a gene for coloration in moths that has two alleles: black and white. A gene pool is the complete set of alleles in a single population, so each allele occurs a certain number of times in a gene pool. The fraction of genes within the gene pool that are a particular allele is called the allele frequency. Evolution occurs when there are changes in the frequencies of alleles within a population of interbreeding organisms; for example the allele for black color in a population of moths becoming more common.

To understand the mechanisms that cause a population to evolve, it is useful to consider what conditions are required for a population not to evolve. The Hardy-Weinberg principle states that the frequencies of alleles (variations in a gene) in a sufficiently large population will remain constant if the only forces acting on that population are the random reshuffling of alleles during the formation of the sperm or egg, and the random combination of the alleles in these sex cells during fertilization.O\'Neil, Dennis (2008). Hardy-Weinberg Equilibrium Model. The synthetic theory of evolution: An introduction to modern evolutionary concepts and theories. Behavioral Sciences Department, Palomar College. Retrieved on 2008-01-06. Such a population is said to be in Hardy-Weinberg equilibrium - it is not evolving.Bright, Kerry (2006). Causes of evolution. Teach Evolution and Make It Relevant. National Science Foundation. Retrieved on 2007-12-30.

Mechanisms

There are three basic mechanisms of evolutionary change: natural selection, genetic drift, and gene flow. Natural selection favors genes that improve capacity for survival and reproduction. Genetic drift is random change in the frequency of alleles, caused by the random sampling of a generation\'s genes during reproduction, and gene flow is the transfer of genes within and between populations. The relative importance of natural selection and genetic drift in a population varies depending on the strength of the selection and the effective population size, which is the number of individuals capable of breeding.Whitlock M (2003). "Fixation probability and time in subdivided populations". Genetics 164 (2): 767–79. PMID 12807795.  Natural selection usually predominates in large populations, while genetic drift dominates in small populations. The dominance of genetic drift in small populations can even lead to the fixation of slightly deleterious mutations.Ohta T (2002). "Near-neutrality in evolution of genes and gene regulation". PNAS 99 (25): 16134–37.  As a result, changing population size can dramatically influence the course of evolution. Population bottlenecks, where the population shrinks temporarily and therefore loses genetic variation, result in a more uniform population. Bottlenecks also result from alterations in gene flow such as decreased migration, expansions into new habitats, or population subdivision.

Natural selection

For more details on this topic, see Natural selection and Fitness (biology).

Natural selection of a population for dark coloration.

Natural selection is the process by which genetic mutations that enhance reproduction become, and remain, more common in successive generations of a population. It has often been called a "self-evident" mechanism because it necessarily follows from three simple facts:

• Heritable variation exists within populations of organisms.
• Organisms produce more offspring than can survive.
• These offspring vary in their ability to survive and reproduce.

These conditions produce competition between organisms for survival and reproduction. Consequently, organisms with traits that give them an advantage over their competitors pass these advantageous traits on, while traits that do not confer an advantage are not passed on to the next generation.

The central concept of natural selection is the evolutionary fitness of an organism. This measures the organism\'s genetic contribution to the next generation. However, this is not the same as the total number of offspring: instead fitness measures the proportion of subsequent generations that carry an organism\'s genes.Haldane J (1959). "The theory of natural selection today". Nature 183 (4663): 710–13. PMID 13644170.  Consequently, if an allele increases fitness more than the other alleles of that gene, then with each generation this allele will become more common within the population. These traits are said to be "selected for". Examples of traits that can increase fitness are enhanced survival, and increased fecundity. Conversely, the lower fitness caused by having a less beneficial or deleterious allele results in this allele becoming rarer — they are "selected against". Importantly, the fitness of an allele is not a fixed characteristic, if the environment changes, previously neutral or harmful traits may become beneficial and previously beneficial traits become harmful..

Natural selection within a population for a trait that can vary across a range of values, such as height, can be categorized into three different types. The first is directional selection, which is a shift in the average value of a trait over time — for example organisms slowly getting taller.Hoekstra H, Hoekstra J, Berrigan D, Vignieri S, Hoang A, Hill C, Beerli P, Kingsolver J (2001). "Strength and tempo of directional selection in the wild". Proc. Natl. Acad. Sci. U.S.A. 98 (16): 9157–60. PMID 11470913.  Secondly, disruptive selection is selection for extreme trait values and often results in two different values becoming most common, with selection against the average value. This would be when either short or tall organisms had an advantage, but not those of medium height. Finally, in stabilizing selection there is selection against extreme trait values on both ends, which causes a decrease in variance around the average value.Felsenstein (1979). "Excursions along the Interface between Disruptive and Stabilizing Selection". Genetics 93 (3): 773–95. PMID 17248980.  This would, for example, cause organisms to slowly become all the same height.

A special case of natural selection is sexual selection, which is selection for any trait that increases mating success by increasing the attractiveness of an organism to potential mates.Andersson M, Simmons L (2006). "Sexual selection and mate choice". Trends Ecol. Evol. (Amst.) 21 (6): 296–302. PMID 16769428.  Traits that evolved through sexual selection are particularly prominent in males of some animal species, despite traits such as cumbersome antlers, mating calls or bright colors that attract predators, decreasing the survival of individual males.Kokko H, Brooks R, McNamara J, Houston A (2002). "The sexual selection continuum". Proc. Biol. Sci. 269 (1498): 1331–40. PMID 12079655.  This survival disadvantage is balanced by higher reproductive success in males that show these hard to fake, sexually selected traits.Hunt J, Brooks R, Jennions M, Smith M, Bentsen C, Bussière L (2004). "High-quality male field crickets invest heavily in sexual display but die young". Nature 432 (7020): 1024–27. PMID 15616562. 

An active area of research is the unit of selection, with natural selection being proposed to work at the level of genes, cells, individual organisms, groups of organisms and even species.Gould SJ (1998). "Gulliver\'s further travels: the necessity and difficulty of a hierarchical theory of selection". Philos. Trans. R. Soc. Lond., B, Biol. Sci. 353 (1366): 307–14. PMID 9533127. Mayr E (1997). "The objects of selection". Proc. Natl. Acad. Sci. U.S.A. 94 (6): 2091–94. PMID 9122151.  None of these models are mutually-exclusive and selection may act on multiple levels simultaneously.Maynard Smith J (1998). "The units of selection". Novartis Found. Symp. 213: 203–11; discussion 211–17. PMID 9653725.  Below the level of the individual, genes called transposons try to copy themselves throughout the genome.Hickey DA (1992). "Evolutionary dynamics of transposable elements in prokaryotes and eukaryotes". Genetica 86 (1–3): 269–74. PMID 1334911.  Selection at a level above the individual, such as group selection, may allow the evolution of co-operation, as discussed below.Gould SJ, Lloyd EA (1999). "Individuality and adaptation across levels of selection: how shall we name and generalize the unit of Darwinism?". Proc. Natl. Acad. Sci. U.S.A. 96 (21): 11904–09. PMID 10518549. 

Genetic drift

For more details on this topic, see Genetic drift and Effective population size.

Simulation of genetic drift of 20 unlinked alleles in populations of 10 (top) and 100 (bottom). Drift is more rapid in the smaller population.

Genetic drift is the change in allele frequency from one generation to the next that occurs because alleles in offspring are a random sample of those in the parents. In mathematical terms, alleles are subject to sampling error. As a result, when selective forces are absent or relatively weak, allele frequencies tend to "drift" upward or downward randomly (in a random walk). This drift halts when an allele eventually becomes fixed, either by disappearing from the population, or replacing the other alleles entirely. Genetic drift may therefore eliminate some alleles from a population due to chance alone, and two separate populations that began with the same genetic structure can drift apart into two divergent populations with different sets of alleles.Lande R (1989). "Fisherian and Wrightian theories of speciation". Genome 31 (1): 221–27. PMID 2687093. 

The time for an allele to become fixed by genetic drift depends on population size, with fixation occurring more rapidly in smaller populations.Otto S, Whitlock M (1997). "The probability of fixation in populations of changing size". Genetics 146 (2): 723–33. PMID 9178020.  The precise measure of populations that is important here is called the effective population size, which was defined by Sewall Wright as a theoretical number representing the number of breeding individuals that would exhibit the same observed degree of inbreeding.

Although natural selection is responsible for adaptation, the relative importance of the two forces of natural selection and genetic drift in driving evolutionary change in general is an area of current research in evolutionary biology.Nei M (2005). "Selectionism and neutralism in molecular evolution". Mol. Biol. Evol. 22 (12): 2318–42. PMID 16120807.  These investigations were prompted by the neutral theory of molecular evolution, which proposed that most evolutionary changes are the result of the fixation of neutral mutations that do not have any immediate effects on the fitness of an organism.Kimura M (1991). "The neutral theory of molecular evolution: a review of recent evidence". Jpn. J. Genet. 66 (4): 367–86. PMID 1954033.  Hence, in this model, most genetic changes in a population are the result of constant mutation pressure and genetic drift.Kimura M (1989). "The neutral theory of molecular evolution and the world view of the neutralists". Genome 31 (1): 24–31. PMID 2687096. 

Gene flow

For more details on this topic, see Gene flow, Hybrid, and Horizontal gene transfer.

Male lions leave the pride where they are born and take over a new pride to mate. This results in gene flow between prides.

Gene flow is the exchange of genes between populations, which are usually of the same species.Morjan C, Rieseberg L (2004). "How species evolve collectively: implications of gene flow and selection for the spread of advantageous alleles". Mol. Ecol. 13 (6): 1341–56. PMID 15140081.  Examples of gene flow within a species include the migration and then breeding of organisms, or the exchange of pollen. Gene transfer between species includes the formation of hybrid organisms and horizontal gene transfer.

Migration into or out of a population can change allele frequencies, as well as introducing genetic variation into a population. Immigration may add new genetic material to the established gene pool of a population. Conversely, emigration may remove genetic material. As barriers to reproduction between two diverging populations are required for the populations to become new species, gene flow may slow this process by spreading genetic differences between the populations. Gene flow is hindered by mountain ranges, oceans and deserts or even man-made structures such as the Great Wall of China, which has hindered the flow of plant genes.Su H, Qu L, He K, Zhang Z, Wang J, Chen Z, Gu H (2003). "The Great Wall of China: a physical barrier to gene flow?". Heredity 90 (3): 212–19. PMID 12634804.