2.8 Natural Selection
Natural selection is the process where heritable traits that make it more likely for an organism to survive long enough to reproduce become more common over successive generations of a population. It is a key mechanism of evolution.
The natural variation within a population of animals, plants, bacteria, etc. means that some individuals will survive better than others in their current environment. For example, the peppered moth exists in both light and dark colours in the United Kingdom, but during the industrial revolution many of the trees on which the moths rested became blackened by soot, giving the dark-collared moths an advantage in hiding from predators. This gave dark-coloured moths a better chance of surviving to produce dark-coloured offspring, and in just a few generations the majority of the moths were dark.
Natural selection is one of the cornerstones of modern biology. The term was introduced by Charles Darwin in his 1859 book “On the Origin of Species” 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 favoured for reproduction. The union of traditional Darwinian evolution with subsequent discoveries in classical and molecular genetics is termed the modern evolutionary synthesis. Natural selection remains the primary explanation for adaptive evolution.
2.8.1 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.
There is natural variation among the individuals of any population of organisms. Many of these differences do not matter, such as differences in eye colour. However, some differences, or traits, may improve the chances of survival of a particular individual. A rabbit which runs faster than others may be more likely to escape from predators, and an algae which is more efficient at extracting the energy from sunlight will grow faster. Individuals that have better odds for survival also have better odds for reproduction. If the traits which give these individuals a reproductive advantage are also heritable then there will be a slightly higher proportion of fast rabbits or efficient algae in the next generation. This is known as “differential reproduction.” Even if the reproductive advantage is very slight, over many generations any heritable advantage will become dominant in the population, due to exponential growth. In this way the natural environment of an organism “selects” for traits that confer a reproductive advantage, causing gradual changes or evolution of life.
Although a complete theory of evolution requires an account of how genetic variation arises in the first place (such as by mutation and sexual reproduction) and includes other evolutionary mechanisms (such as gene flow), natural selection is still understood as a fundamental mechanism for evolution.
2.8.2 Nomenclature and usage
The term “natural selection” has slightly different definitions in different contexts. It is most often defined to operate on heritable traits, because these are the traits that directly participate in evolution. However, natural selection is “blind” in the sense that changes in phenotype (physical and behavioural characteristics) can give a reproductive advantage regardless of whether or not the phenotype is heritable.
Following Darwin’s primary usage the term is often used to refer to both the evolutionary consequence of blind selection and to its mechanisms. 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.”
Traits that cause greater reproductive success of an organism are said to be selected for, whereas those that reduce success are selected against.
The concept of fitness is central to natural selection. Broadly, individuals which are more “fit” have better potential for survival, as in the well-known phrase “survival of the fittest.” Modern evolutionary theory defines fitness not by how long an organism lives, but by how successful it is at reproducing. If an organism lives half as long as others of its species, but has twice as many offspring surviving to adulthood, its genes will become more common in the adult population of the next generation. This is known as differential reproduction.
Though natural selection acts on individuals, the effects of chance mean that fitness can only really be defined “on average” for the individuals within a population. Very low-fitness genotypes cause their bearers to have few or no offspring on average. Since fitness is an averaged quantity, it is also possible that a favourable mutation arises in an individual that does not survive to adulthood for unrelated reasons. Fitness also depends crucially upon the environment.
2.8.4 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 competition with 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.
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. 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 intra-genomic 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.
The figure to the right shows the life cycle of a sexually reproducing organism. Various components of natural selection are indicated for each life stage.
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 offsprings, called survival selection. The fecundity of both females and males can be limited via fecundity selection. Finally, the union of some combinations of eggs and sperm might be more compatible than others; this is termed compatibility selection.
2.8.5 Sexual selection
It is also useful to distinguish between ecological selection and the narrower term sexual selection. Ecological selection covers any mechanism of selection as a result of the environment, while sexual selection refers specifically to competition for mates. Sexual selection can be intra-sexual, 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, intra-sexual selection involves male-male competition and intersexual selection involves female choice of suitable males. However, some species exhibit sex-role reversed behaviour in which it is males that are most selective in mate choice. 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, intra-sexual selection is often associated with sexual dimorphism, including differences in body size between males and females of a species.
2.8.6 Examples of Natural Selection
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.
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.
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. 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. 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.
2.8.7 Evolution by means of natural selection
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. 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 non-coding DNA or resulted in a synonymous substitution. However, recent research suggests that many mutations in non-coding DNA do have slight deleterious effects.
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.
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. When such mutations result in a higher fitness, natural selection will favour these phenotypes and the novel trait will spread in the population.
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.
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, 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.
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.
2.8.9 Historical Development
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. The struggle for existence was later described by al-Jahiz in the 9th century. Such classical arguments were reintroduced in the 18th century by Pierre Louis Maupertuis 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.
In 1859, Charles Darwin set out his theory of evolution by natural selection as an explanation for adaptation and speciation. He defined natural selection as the “principle by which each slight variation [of a trait], if useful, is preserved”. The concept was simple but powerful: individuals best adapted to their environments are more likely to survive and reproduce. As long as there is some variation between them, there will be an inevitable selection of individuals with the most advantageous variations. If the variations are inherited, then differential reproductive success will lead to a progressive evolution of particular populations of a species, and populations that evolve to be sufficiently different eventually become different species.
Darwin’s ideas were inspired by the observations that he had made on the Beagle voyage, and by the work of two political economists. The first was the Reverend Thomas Malthus, who in “An Essay on the Principle of Population”, noted that population (if unchecked) increases exponentially whereas the food supply grows only arithmetically; thus inevitable limitations of resources would have demographic implications, leading to a “struggle for existence”. The second was Adam Smith who, in “The Wealth of Nations”, identified a regulating mechanism in free markets, which he referred to as the “invisible hand“, which suggests that prices self-adjust according to supplies and demand. When Darwin read Malthus in 1838 it struck him that as population outgrew resources, “favourable variations would tend to be preserved and unfavourable ones to be destroyed. The result of this would be the formation of new species.”
Darwin thought of natural selection by analogy to how farmers select crops or livestock for breeding, which he called artificial selection. At the time, other mechanisms of evolution such as evolution by genetic drift were not yet explicitly formulated, and Darwin believed that selection was likely only part of the story. In a letter to Charles Lyell in September 1860, Darwin regretted the use of the term ‘Natural Selection’, preferring the term ‘Natural Preservation’. For Darwin and his contemporaries, natural selection was essentially synonymous with evolution by natural selection. After the publication of “The Origin of Species”, educated people generally accepted that evolution had occurred in some form. However, natural selection remained controversial as a mechanism, partly because it was perceived to be too weak to explain the range of observed characteristics of living organisms, and partly because even supporters of evolution balked at its ‘unguided’ and non-progressive nature. Herbert Spencer introduced the term survival of the fittest, which became a popular summary of the theory. The fifth edition of “On the Origin of Species” published in 1869 included Spencer’s phrase as an alternative to natural selection, with credit given.
Modern evolutionary synthesis
Natural selection relies crucially on the idea of heredity, but it was developed long before the basic concepts of genetics. Although Austrian monk Gregor Mendel, the father of modern genetics, was a contemporary of Darwin’s, his work would lie in obscurity until the early 20th century. Only after the integration of Darwin’s theory of evolution with a complex statistical appreciation of Gregor Mendel‘s ‘re-discovered’ laws of inheritance did natural selection become generally accepted by scientists.
Impact of the idea
Darwin‘s ideas, along with those of Adam Smith and Karl Marx, had a profound influence on 19th century thought. Perhaps the most radical claim of the theory of evolution through natural selection is that “elaborately constructed forms, so different from each other, and dependent on each other in so complex a manner” evolved from the simplest forms of life by a few simple principles. The radicalism of natural selection, according to Stephen Jay Gould, challenged long-standing beliefs in such concepts as a special and exalted place for humans in the natural world and a benevolent creator whose intentions were reflected in nature’s order and design.
2.8.10 Social and psychological theory
The social implications of the theory of evolution by natural selection also became the source of continuing controversy.
More recently, work among anthropologists and psychologists has led to the development of socio-biology and later evolutionary psychology, a field that attempts to explain features of human psychology in terms of adaptation to the ancestral environment. Extensions of the theory of natural selection to a wide range of cultural phenomena have been distinctly controversial and are not widely accepted.
2.8.11 Information and systems theory
In 1922, Alfred Lotka proposed that natural selection might be understood as a physical principle which could be described in terms of the use of energy by a system, a concept that was later developed by Howard Odum as the maximum power principle whereby evolutionary systems with selective advantage maximise the rate of useful energy transformation.
The principles of natural selection have inspired a variety of computational techniques, such as “soft” artificial life.
2.8.12 Genetic basis of natural selection
2.8.13 Genotype and Phenotype
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.
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. 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.
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.
2.8.14 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.
1- 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.
2- 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.
3- 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.
2.8.15 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. 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; 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.
2.8.16 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 unfavourable the mutation proves to be.
2.8.17 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.
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.
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.