Natural selection is the process by which individual organisms with favorable traits are more likely to survive and reproduce. Natural selection works on the whole individual, but only the heritable component of a trait will be passed on to the offspring, with the result that favorable, heritable traits become more common in the next generation. Given enough time, this passive process can result in adaptations and speciation.
Natural selection is a cornerstone of modern biology. The term was introduced by Charles Darwin in his 1859 book The Origin of Species, by analogy with 'artificial selection', by which a farmer selects his breeding stock.
An example: antibiotic resistance
Figure 1: Schematic representation of how antibiotic resistance is enhanced by natural selection. The top section represents a population of bacteria before exposure to an antibiotic. The middle section shows the population directly after exposure, the phase in which selection took place. The last section shows the distribution of resistance in a new generation of bacteria. The legend indicates the resistance levels of individuals.
Natural selection helps the organisms aquire new traits in order to survive to the changes in the environment. Those new traits help the populations evolve.
A well-known 'example' of natural selection in action is the development of antibiotic resistance in microorganisms. Antibiotics have been used to fight bacterial diseases since the discovery of penicillin in 1928 by Alexander Fleming. However, the widespread use and especially misuse of antibiotics has led to increased microbial resistance against antibiotics, to the point that the methicillin-resistant Staphylococcus aureus (MRSA ) has been described as a 'superbug' because of the threat it poses to health and its relative invulnerability to existing drugs.
Natural populations of bacteria contain, among their vast numbers of individual members, considerable variation in their genetic material, primarily as the result of mutations. When exposed to antibiotics, most bacteria die quickly, but some may have mutations that make them a little less susceptible. If the exposure to antibiotics is short, these individuals will survive the treatment. This selective elimination of 'maladapted' individuals from a population is natural selection in action.
These surviving bacteria will then reproduce again, producing the next generation. Due to the elimination of the maladapted individuals in the past generation, this population contains more bacteria that have some resistance against the antibiotic. At the same time, new mutations occur, contributing new genetic variation to the existing genetic variation. Spontaneous mutations are very rare, very few have any effect at all, and usually any effect is deleterious. However, populations of bacteria are enormous, and so a few individuals will have beneficial mutations. If a new mutation reduces their susceptibility to an antibiotic, these individuals are more likely to survive when next confronted with that antibiotic. Given enough time, and repeated exposure to the antibiotic, a population of antibiotic-resistant bacteria will emerge.
A similar situation occurs with pesticide resistance in plants and insects.
Background and context
Until the early 19th century, the established view was that differences between individuals of a species were uninteresting departures from their Platonic ideal (or typus) of created kinds. However, growing awareness of the fossil record led to the recognition that species that lived in the distant past were often very different from those that exist today. Naturalists of the time tried to reconcile this with the emerging ideas of uniformitarianism in geology - the notion that simple, weak forces, acting continuously over very long periods of time could have radical consequences, shaping the landscape as we know it today. Most importantly perhaps, these notions led to the awareness of the immensity of geological time, which makes it possible for slight causes to produce dramatic consequences. This opened the door to the notion that species might have arisen by descent with modification from ancestor species.
In the early years of the 19th century, radical evolutionists such as Jean Baptiste Lamarck had proposed that characteristics (adaptations) acquired by individuals might be inherited by their progeny, causing, in enough time, transmutation of species. Even today, many people believe that adaptations are actively aquired, often learned, though some intentional process. By contrast, Darwin postulated that adaptation is a passive process in which the selective culling by nature of maladapted individuals results in an increase of the fittest individuals. In other words, Darwin suggested that adaptation may be an unintended effect, rather than an intended cause. He realised that this simple and apparently inevitable process might be powerful enough to explain the evolution of the astounding ways in which organisms are adapted to their environments and the origins of the millions of species that exist.
Evolutionary change can also happen without any selection, as a result of genetic drift or gene flow. However, adaptive change needs more than this, because it is very unlikely that favorable characteristics will consistently become more common in successive generations simply as a result of random fluctuations in occurrence. Favorable characteristics that can be attributed to genes that become more common through evolution by natural selection are called adaptations.
For Darwin, 'natural selection' was synonymous with 'evolution by natural selection'; other mechanisms of evolution such as 'evolution by genetic drift' were not explicitly formulated at that time, and Darwin realised that: "I am convinced that [it] has been the main, but not exclusive means of modification." Now, scientists use 'natural selection' mainly to describe the mechanism. In this sense, natural selection includes any selection by a natural agent, including sexual selection and kin selection. Sometimes, sexual selection is distinguished from natural selection, but a more useful distinction is between sexual selection and ecological selection.
Definitions of natural selection
Scientists use several, slightly different definitions of natural selection. This section explains the different uses.
In each generation, only some individuals will produce offspring themselves, and of those that reproduce, some will leave more offspring than others. We can think of this as the "natural" process of selection of individuals to reproduce. Individuals with beneficial traits are more likely to be 'selected' than individuals with other traits. When those traits have a heritable component, they tend to become more common in the next generation. The mechanism of selection of individuals in a population does not 'know' which traits are heritable; in this sense, the mechanisms of selection are 'blind'.
However, the term "natural selection" is often used to encompass the consequence of blind selection as well as the mechanisms to describe the complete process that leads to the enrichment of the beneficial characteristics in the next generation.
Nevertheless, it is sometimes helpful to distinguish clearly between the mechanisms of selection and the effects of selection. When this distinction is important, scientists define "natural selection" specifically as --those mechanisms that contribute to the selection of individuals that reproduce--, without regard to whether the basis of the selection is heritable. This is sometimes referred to as 'phenotypic natural selection.'
This article discusses natural selection in this sense of being the mechanisms of selection of individuals to reproduce. Of particular importance is selection according to traits by which individuals differ from each other. It discusses the effects of this selection on the genetic characteristics of a population when some components of beneficial traits are heritable. Finally, the article ventures into the consequences of these effects for evolution.
Some traits are determined by just a single gene, but most are affected by many different genes. Variation in most of these genes has only a small effect on the phenotypic value of a trait, and the study of the genetics of these quantitative traits is called quantitative genetics.
Natural selection acts on the phenotype. The phenotype is the overall result of an individual's genetic make-up (genotype), the environment, and the interactions between genes and between genes and the environment. Often, natural selection acts on specific traits of an individual, and the terms phenotype and genotype are sometimes used narrowly to indicate these specific traits.
The key element in understanding natural selection is the concept of fitness. Natural selection acts on individuals, but its average effect on all individuals with a particular genotype is the fitness of that genotype. Fitness is measured as the proportion of progeny that survives, multiplied by the average fecundity, and it is equivalent to the reproductive success of a genotype. A fitness value of greater than one indicates that the frequency of that genotype in the population increases, while a value of less than one indicates that it decreases. The relative fitness of a genotype is estimated as the proportion of the fitness of a reference genotype. Related to relative fitness is the selection coefficient, which is the difference between the relative fitness of two genotypes. The larger the selection coefficient, the stronger natural selection will act against the genotype with the lowest fitness.
Natural selection can act on any phenotypic trait, and any aspect of the environment, including mates and conspecifics, can produce selective pressure. However, this does not imply that natural selection is always directional and results in adaptive evolution; natural selection often results in the maintenance of the status quo through purifying selection. The 'unit of selection' is not limited to the level of individuals, but includes other levels within the hierarchy of biological organisation, such as genes, cells and relatives. There is still debate, however, about whether natural selection acts at the level of groups or species, (i.e. selection for adaptations that benefit the group or species, rather than the individual). Selection at a different level than the individual, for example the gene, can result in an increase in fitness for that gene, while at the same time reducing the fitness of the individuals carrying that gene. 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.
Natural selection occurs at every life stage of an individual, and selection at any of these stages can affect the likelihood that an individual will survive and reproduce. After an individual is born, it has to survive until adulthood before it can reproduce, and selection of those that reach this stage is called viability selection. In many species, adults must compete with each other for mates (sexual selection), and success in this competition determines who will parent the next generation. When species reproduce more than once, a longer survival in the reproductive phase increases the number of offspring (survival selection). The fecundity of both females (e.g. how many eggs a female bird produces) and males (e.g. giant sperm in certain species of Drosophila) can be limited (fecundity selection). The viability of produced gametes can differ, while intragenomic conflict (meiotic drive) between the haploid gametes can result in gametic or genic selection. Finally, the union of some combinations of eggs and sperm might be more compatible that others (compatibility selection).
"Ecological selection" and "sexual selection"
It is also useful to make a mechanistic distinction between ecological selection and sexual selection. Ecological selection covers any mechanism of selection as a result of the environment (including relatives (e.g. kin selection) and conspecifics (e.g. competition, infanticide)), while sexual selection refers specifically to competition between conspecifics for mates. Sexual selection includes mechanisms such as mate choice and male-male competition although the two forms can act in combination in some species, when females choose the winners of the male-male competition. Mate choice, or 'intersexual selection', typically involves 'female choice', as it is usually the females who are most choosy, but in some sex-role reversed species it is the males that choose. Some features that are confined to one sex only of a particular species can be explained by selection exercised by the other sex in the choice of a mate, e.g. the extravagant plumage of some male birds. Aggression between members of the same sex (intrasexual selection) is typically referred to as 'male-male competition', and is sometimes associated with very distinctive features, such as the antlers of stags, which are used in combat with other stags. More generally, intrasexual selection is often associated with sexual dimorphism, including differences in body size between males and females of a species.
'Selection for' versus 'selection of'
Selection targets specific traits of an individual, and if such a trait has a heritable component, that trait will become more common in the next generation. So selection for a specific trait results in selection of certain individuals. This distinction is important, because an individual is more than the trait it is selected for. For example, sometimes two or more traits are genetically linked through mechanisms such as pleiotropy (single gene that affects multiple traits) and linkage disequilibrium (non-random association of two genes). Sometimes, selection of a trait relates to a specific function of that trait, while that trait has also other functions that are not affected by natural selection. In either case, direct selection for specific traits or functions results in indirect selection of other traits or functions.
The genetical theory of natural selection
Natural selection by itself is a simple concept, in which fitness differences between phenotype play a crucial role. However, the interplay of the actual selection mechanism with the underlying genetics is where the explanatory power of natural selection comes from.
Directionality of selection
When some component of a trait is heritable, selection will alter the frequencies of the different alleles (variants of a gene) involved. Selection can be divided into three classes, on the basis of their effect on the allele frequencies.
Positive or directional selection occurs when a certain allele has a greater fitness than others, resulting in an increase in frequency of that allele until it is fixed and the entire population expresses the fitter phenotype.
Far more common is purifying or stabilizing selection, which lowers the frequency of alleles which have a deleterious effect on the phenotype (that is, lower fitness), until they are eliminated from the population. Purifying selection results in functional genetic features (e.g. protein-coding sequences or regulatory sequences) being conserved over time because of selective pressure against deleterious variants.
Finally, a number of forms of balancing selection exist, which do not result in fixation, but maintain an allele at intermediate frequencies in a population. This can occur in diploid species (with two pair of chromosomes) when individuals with a combination of two different alleles at a single position at the chomosome (heterozygote) have a higher fitness than individuals that have two of the same alleles (homozygote). This is called heterozygote advantage or overdominance. Maintenance of allelic variation can also occur through disruptive or diversifying selection, which favors genotypes that depart from the average in either direction (that is, the opposite of overdominance), and can result in a bimodal distribution of trait values. Finally, it can occur through frequency-dependent selection, where the fitness of one particular phenotype depends on the distribution of other phenotypes in the population.
Selection and genetic variation
A portion of all genetic variation is functionally neutral; i.e., it produces no phenotypic effect or significant differences in fitness. Previously, this was though to encompass most of the genetic variation in non-coding DNA, but recent studies have shown that large parts of those sequences are highly conserved and under strong purifying selection; i.e. they do not vary as much from individual to individual, indicating that mutations in these regions have deleterious consequences). When genetic variation does not result in differences in fitness, selection cannot directly affect the frequency of such variation. As a result, the genetic variation at those sites will be higher than at sides where selection does have a result.
Genetic linkage occurs when two alleles are in close proximity to each other. During the formation of the gametes, recombination of the genetic material results in reshuffling of the alleles. However, the chance that such a reshuffle occurs between two alleles depends on the distance between those alleles; the closer the alleles are to each other, the less likely it is that such a reshuffle will occur. Consequently, when selection targets one allele, this automatically results in selection of the other allele as well; through this mechanism, selection can have a strong influence on patterns of variation in the genome.
Natural selection results in the reduction of genetic variation through the elimination of maladapted individuals and, through that, of the mutations that causes the maladaptation. At the same time, new mutations occur, resulting in a mutation-selection balance. The exact outcome of the two processes depends both on the rate at which new mutations occurs and on the strength of the natural selection. Consequently, changes in the mutation rate or the selection pressure will result in a different mutation-selection balance.
Selective sweeps occur when an allele becomes more common in a population as a result of positive selection. As the prevalence of one allele increases, linked alleles (those nearby on the chromosome) can also become more common, whether they are neutral or even slightly deleterious. This is called genetic hitchhiking. A strong selective sweep results in a region of the genome where the positively selected haplotype (the allele and its neighbours) are essentially the only ones that exist in the population.
Whether a selective sweep has occurred or not can be investigated by measuring linkage disequilibrium, i.e., whether a given haplotype is overrepresented in the population. Normally, genetic recombination results in a reshuffling of the different alleles within a haplotype, and none of the haplotypes will dominate the population. However, during a selective sweep, selection for a specific allele will also result in selection of neighbouring alleles. Therefore, the presence of strong linkage disequilibrium might indicate that there has been a 'recent' selective sweep, and this can be used to identify sites recently under selection.
Background selection is the opposite of a selective sweep. If a specific site experiences strong and persistent purifying selection (perhaps as a result of mutation-selection balance), linked variation will tend to be weeded out along with it. Background selection, however, acts as a result of new mutations, which can occur randomly in any haplotype. It therefore produces no linkage disequilibrium, though it reduces the amount of variation in the region.
evolution darwin haplotype