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In biology, evolution is the change in the inherited traits of a population from one generation to the next. These traits are the expression of genes that are copied and passed on to offspring during reproduction. Mutations in these genes can produce new or altered traits, resulting in heritable differences between organisms. New traits can also come from 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 that causes heritable traits that are helpful for survival and reproduction to become more common, and harmful traits to become more rare. This occurs because organisms with advantageous traits pass on more copies of these heritable traits to the next generation.[1][2] 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.[3] 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. However, 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.[4] 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.[1]
The theory of evolution by natural selection was proposed at about the same time by both Charles Darwin and Alfred Russel Wallace, and was set out in detail in Darwin's 1859 book On the Origin of Species.[5] It encountered initial resistance from religious authorities who believed humans were divinely set apart from the animal kingdom. In the 1930s, Darwinian natural selection was combined with Mendelian inheritance to form the modern evolutionary synthesis,[6] 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.[7]
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.[8] Inherited traits are controlled by genes and the complete set of genes within an organism's genome is called its genotype.[9]
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.[10] 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.[11]
Genes are regions within DNA molecules that contain genetic information.[9] 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.[12][13]
Variation
Because an individual's phenotype results from the interaction of their genotype with the environment, the variation in phenotypes in a population reflects the variation in these organisms' genotypes.[13] The modern evolutionary synthesis defines evolution as the change over time in this genetic variation.[14] 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.[15]
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.[16] Despite the constant introduction of variation through these processes, most of the genome of a species is identical in all individuals of that species.[17] 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.[18]
Mutation
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.[19][20][21] 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 deleterious, and the remainder are either neutral or have a weak beneficial effect.[22] Due to the damaging effects that mutations can have on cells, organisms have evolved mechanisms such as DNA repair to remove mutations.[19] 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.[23]
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.[24] Most genes belong to larger families of genes of shared ancestry.[25] 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.[26][27] 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.[28] 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.[29]
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. [30] The completion of both the human genome project and the chimpanzee genome project has allowed the identification of the relevant chromosomes. In evolution, the most important role of such chromosomal rearrangements may be to accelerate the divergence of a population into new species by preserving genetic differences within populations.[31]
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.[32] 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.[33] 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.[34]
Recombination
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 a random mixture of their parents' chromosomes that is produced through independent assortment. In the related process of genetic recombination, sexual organisms can also exchange DNA between two matching chromosomes.[35] These shuffling processes can allow even alleles that are close together in a strand of DNA to be inherited independently. However, as only about one recombination event occurs per million base pairs in humans, genes close together on a chromosome may not be shuffled away from each other, and tend to be inherited together.[36] 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 the locus is under positive selection (see below).[37]
Recombination in sexual organisms helps to remove harmful mutations and retain beneficial mutations.[38] Consequently, when alleles cannot be separated by recombination – such as in mammalian Y chromosomes, which pass intact from fathers to sons – harmful mutations accumulate.[39][40] In addition, recombination can produce individuals with new and advantageous gene combinations. These positive effects of recombination are balanced by the fact that this process can cause mutations and separate beneficial combinations of genes.[38] The optimal rate of recombination for a species is therefore a trade-off between conflicting factors.
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.[41] 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.[42] 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.[15] Bottlenecks also result from alterations in gene flow such as decreased migration, expansions into new habitats, or population subdivision.[41]
Natural selection
Natural selection is the process by which genetic mutations that enhance reproduction become, and remain, more common in successive generations of a population. It has often been called a "self-evident" mechanism because it necessarily follows from three simple facts:
-
- Heritable variation exists within populations of organisms.
- Organisms produce more offspring than can survive.
- These offspring vary in their ability to survive and reproduce.
These conditions produce competition between organisms for survival and reproduction. Consequently, organisms with traits that give them an advantage over their competitors pass these advantageous traits on, while traits that do not confer an advantage are not passed on to the next generation.
The central concept of natural selection is the evolutionary fitness of an organism. 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.[43] 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".[2] 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.[1].
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.[44] 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.[45] 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.[46] 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.[47] This survival disadvantage is balanced by higher reproductive success in males that show these hard to fake, sexually selected traits.[48]
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.[49][50] None of these models are mutually-exclusive and selection may act on multiple levels simultaneously.[51] Below the level of the individual, genes called transposons try to copy themselves throughout the genome.[52] Selection at a level above the individual, such as group selection, may allow the evolution of co-operation, as discussed below.[53]
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
Genetic drift is the change in allele frequency from one generation to the next that occurs because alleles in the offspring generation are a random sample of those in the parent generation, and are thus subject to sampling error.[15] As a result, when selective forces are absent or relatively weak, allele frequencies tend to "drift" upward or downward 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 by random fluctuation into two divergent populations with different sets of alleles.[54] The time for an allele to become fixed by genetic drift depends on population size, with fixation occurring more rapidly in smaller populations.[55]
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.[56] These investigations were prompted by the neutral theory of molecular evolution, which proposed that most evolutionary changes are the result the fixation of
Classical
from 
