Selection

Resources

Selection refers to an evolutionary pressure that is the result of a combination of environmental and genetic pressures that affect the ability of an organism to live and, equally importantly, to raise their own reproductively successful offspring.

As implied, natural selection involves the natural (but often complex) pressures present in an organism’s environment. Artificial selection is the conscious manipulation of mating, manipulation, and fusion of genetic material to produce a desired result.

Evolution requires genetic variation, and these variations or changes (mutations) are usually deleterious because environmental factors already support the extent genetic distribution within a population.

Natural selection is based upon expressed differences in the ability of organisms to thrive and produce biologically successful offspring. Importantly, selection can only act to exert influence (drive) on those differences in genotype that appear as phenotypic differences. In a very real sense, evolutionary pressures act blindly.

There are three basic types of natural selection: directional selection favoring an extreme phenotype; stabilizing selection favoring a phenotype with characteristics intermediate to an extreme phenotype (i.e., normalizing selection); and disruptive selection that favors extreme phenotypes over intermediate genotypes.

The evolution of pesticide resistance provides a vivid example of directional selection, wherein the selective agent (in this case DDT) creates an apparent force in one direction, producing a corresponding change (improved resistance) in the affected organisms. Directional selection is also evident in the efforts of human beings to produce desired traits in many kinds of domestic animals and plants. The many breeds of dogs, from dachshunds to shepherds, are all descendants of a single, wolf like ancestor, and are the products of careful selection and breeding for the unique characteristics favored by human breeders.

Not all selective effects are directional, however. Selection can also produce results that are stabilizing or disruptive. Stabilizing selection occurs when significant changes in the traits of organisms are selected against. An example of this is birth weight in humans. Babies that are much heavier or lighter than average do not survive as well as those that are nearer the mean (average) weight.

On the other hand, selection is said to be disruptive if the extremes of some trait become favored over the intermediate values. Perhaps one of the more obvious examples of disruptive selection is sexual dimorphism, wherein males and females of the same species look noticeably different from each other. One sex may be larger, have bright, showy plumage, bear horns, or display some kind of ornament that the other lacks. The male peacock, for instance, has deep green and sapphire blue plumage and an enormous, fanning tail, while the female is a drab brown, with no elaborate tail.

Sexual dimorphism is considered the result of sexual selection, the process in which members of a species compete for access to mates. Sexual selection and natural selection may often operate in opposing directions, producing the two distinct sex phenotypes. Males, who are typically the primary contestants in the competition for mating partners, usually bear the ornaments such as showy plumage in spite of the potential costs of these ornaments, such as increased visibility to predators, and attacks from rival males. Females are less often involved in direct competition for mates, and they are not generally subject to the forces of sexual selection (although there are role reversals in a few species). Females are believed to play a critical role in the evolution of many elaborate male traits, however, because if the female preference has a genetic basis, female choice of particular males as mating partners will cause those male traits to spread in subsequent generations.

Sometimes the fitness of a phenotype in some environment depends on how common (or rare) it is; this is known as frequency-dependent selection. Perhaps an animal enjoys an increased advantage if it conforms to the majority phenotype in the population; this occurs when, for example, predators learn to avoid distasteful butterfly prey, because the butterflies have evolved to advertise their noxious taste by conforming to a particular wing color and pattern. Butterflies that deviate too much from the “warning” pattern are not as easily recognized by their predators, and are eaten in greater numbers. Interestingly, frequency-dependent selection has enabled butterflies who are not distasteful to mimic the appearance of their noxious brethren and thus avoid the same predators. Conversely, a phenotype could be favored if it is rare, and its alternatives are in the majority. Many predators tend to form a “search image” of their prey, favoring the most common phenotypes, and ignoring the rarer phenotypes. Frequency-dependent selection provides an interesting case in which the gene frequency itself alters the selective environment in which the genotype exists.

Many people attribute the phrase “survival of the fittest” to Darwin, but in fact, it originated from another naturalist/philosopher, Herbert Spencer (1820–1903). Recently, many recent evolutionary biologists have asked: Survival of the fittest what? At what organismal level is selection most powerful? What is the biological unit of natural selection-the species, the individual, or even the gene?

Although it seems rational that organisms might exhibit parental behavior or other traits “for the good of the species.” In his 1962 book Animal Dispersion in Relation to Social Behaviour, behavioral biologist V. C. Wynne-Edwards proposed that animals would restrain their reproduction in times of resource shortages, so as to avoid extinguishing the local supply, and thus maintain the “balance of nature.” However, Wynne-Edwards was criticized because all such instances of apparent group-level selection can be explained by selection acting at the level of individual organisms. A mother cat who suckles her kittens is not doing so for the benefit of the species; her behavior has evolved because it enhances her kittens” fitness, and ultimately her own as well, since they carry her genes.

Under most conditions, group selection will not be very powerful, because the rate of change in gene frequencies when one individual replaces another in the population is greater than that occurring when one group replaces another group. The number of individuals present is generally greater than the number of groups present in the environment, and individual turnover is greater. In addition, it is difficult to imagine that individuals could evolve to sacrifice their reproduction for the good of the group; a more selfish alternative could easily invade and spread in such a group.

However, there are some possible exceptions; one of these is reduced virulence in parasites, who depend on the survival of their hosts for their own survival. The myxoma virus, introduced in Australia to control imported European rabbits (Oryctolagus cuniculus ), at first caused the deaths of many individuals. However, within a few years, the mortality rate was much lower, partly because the rabbits became resistant to the pathogen, but also partly because the virus had evolved a lower virulence. The reduction in the virulence is thought to have been aided because the virus is transmitted by a mosquito, from one living rabbit to another. The less deadly viral strain is maintained in the rabbit host population because rabbits afflicted with the more virulent strain would die before passing on the virus. Thus, the viral genes for reduced virulence could spread by group selection. Of course, reduced virulence is also in the interest of every individual virus, if it is to persist in its host. Scientists argue that one would not expect to observe evolution by group selection when individual selection is acting strongly in an opposing direction.

Some biologists, most notably Richard Dawkins (1941–), have argued that the gene itself is the true unit of selection. If one genetic alternative, or allele, provides its bearer with an adaptive advantage over some other individual who carries a different allele then the more beneficial allele will be replicated more times, as its bearer enjoys greater fitness. In his book The Selfish Gene, Dawkins argues that genes help to build the bodies that aid in their transmission; individual organisms are merely the “survival machines” that genes require to make more copies of themselves.

This argument has been criticized because natural selection cannot “see” the individual genes that reside in an organism’s genome, but rather selects among phenotypes, the outward manifestation of all the genes that organisms possess. Some genetic combinations may confer very high fitness, but they may reside with genes having negative effects in the same individual. When an individual reproduces, its “bad” genes are replicated along with its “good” genes; if it fails to do so, even its most advantageous genes will not be transmitted into the next generation. Although the focus among most evolutionary biologists has been on selection at the level of the individual, this example raises the possibility that individual genes in genomes are under a kind of group selection. The success of single genes in being transmitted to subsequent generations will depend on their functioning well together, collectively building the best possible organism in a given environment.

When selective change is brought about by human effort, it is known as artificial selection. By allowing only a selected minority of individuals to reproduce, breeders can produce new generations of organisms featuring particular traits, including greater milk production in dairy cows, greater oil content in corn, or a rainbow of colors in commercial flowers. The repeated artificial selection and breeding of individuals with the most extreme values of the desired traits may continue until all the available genetic variation has been exhausted, and no further selection is possible. It is likely that dairy breeders have encountered the limit for milk production in cattle—eventually, a cow’s milk production will increase more slowly for a given increase in feed—but the limit has not yet been reached for corn oil content, which continues to increase under artificial selection.

Seemingly regardless of the trait or characteristic involved (e.g. zygotic selection ), there is almost always a way to construct a selectionist explanation of the manifest phenotype.

Key Terms

Artificial selection —Selective breeding, carried out by humans, to produce desired genetic alterations in domestic animals and plants.

Fitness —The average number of offspring produced by individuals with a certain genotype, relative to that of individuals with a different genotype.

Gene frequency —The relative fraction of a particular gene in the population, compared to its alternatives.

Genome —The complete set of genes an organism carries.

Genotype —The full set of paired genetic elements carried by each individual, representing the its genetic blueprint; also used to refer to such a pair at a single genetic locus.

Group selection —The replacement by natural selection of one or more groups of organisms in favor of other groups.

Natural selection —The differential survival and reproduction of organisms, producing evolutionary change in populations.

Phenotype —The outward manifestation of the genotype; the physical, morphological, and behavioral traits of an organism.

Sexual selection —This is a type of natural selection in which anatomical or behavioral traits may be favored because they confer some advantage in courtship or another aspect of breeding. For example, the bright coloration, long tail, and elaborate displays of male pheasants have resulted from sexual selection by females, who apparently favor extreme expressions of these traits in their mates.

See also Adaptation; Evolution, convergent; Evolution, divergent; Evolution, evidence of; Evolution, parallel; Evolutionary change, rate of; Evolutionary mechanisms; Genetics.

Resources

BOOKS

Darwin, Charles R. On Natural Selection. Toronto, ON: Penguin, September 2005.

Darwin, Charles R. The Origin of the Species by Means of Natural Selection. Boston: Adamant Media Corporation, May 2001.

Finley, Robert B., Jr. Intermittent Frontiers : On How Changing Ecological Factors Control Natural Selection. Santa Fe, NM: Pilgrim’s Progress, July 2005.

Gould, Stephen Jay. The Structure of Evolutionary Theory. Cambridge, MA: Harvard University Press, 2002.

Mayer, Ernest. What Evolution Is. Basic Books, 2001.

Sideris, Lisa H., Lisa Sideris. Environmental Ethics, Ecological Theology and Natural Selection. New York: Columbia University Press, August 2003.

Wallace, Alfred Russell. Natural Selection and Tropical Nature. Boston: Adamant Media Corporation, November 2005.

K. Lee Lerner Susan Andrew

Selection

Selection refers to an evolutionary pressure that is the result of a combination of environmental and genetic pressures that affect the ability of an organism to live and, equally importantly, to raise their own reproductively successful offspring.

As implied, natural selection involves the natural (but often complex) pressures present in an organism's environment. Artificial selection is the conscious manipulation of mating, manipulation, and fusion of genetic material to produce a desired result.

Evolution requires genetic variation, and these variations or changes (mutations) are usually deleterious because environmental factors already support the extent genetic distribution within a population.

Natural selection is based upon expressed differences in the ability of organisms to thrive and produce biologically successful offspring. Importantly, selection can only act to exert influence (drive) on those differences in genotype that appear as phenotypic differences. In a very real sense, evolutionary pressures act blindly.

There are three basic types of natural selection: directional selection favoring an extreme phenotype; stabilizing selection favoring a phenotype with characteristics intermediate to an extreme phenotype (i.e., normalizing selection); and disruptive selection that favors extreme phenotypes over intermediate genotypes.

The evolution of pesticide resistance provides a vivid example of directional selection, wherein the selective agent (in this case DDT) creates an apparent force in one direction, producing a corresponding change (improved resistance) in the affected organisms. Directional selection is also evident in the efforts of human beings to produce desired traits in many kinds of domestic animals and plants. The many breeds of dogs, from dachshunds to shepherds, are all descendants of a single, wolf-like ancestor, and are the products of careful selection and breeding for the unique characteristics favored by human breeders.

Not all selective effects are directional, however. Selection can also produce results that are stabilizing or disruptive. Stabilizing selection occurs when significant changes in the traits of organisms are selected against. An example of this is birth weight in humans. Babies that are much heavier or lighter than average do not survive as well as those that are nearer the mean (average) weight.

On the other hand, selection is said to be disruptive if the extremes of some trait become favored over the intermediate values. Perhaps one of the more obvious examples of disruptive selection is sexual dimorphism, wherein males and females of the same species look noticeably different from each other. One sex may be larger, have bright, showy plumage, bear horns, or display some kind of ornament that the other lacks. The male peacock, for instance, has deep green and sapphire blue plumage and an enormous, fanning tail, while the female is a drab brown, with no elaborate tail.

Sexual dimorphism is considered the result of sexual selection, the process in which members of a species compete for access to mates. Sexual selection and natural selection may often operate in opposing directions, producing the two distinct sex phenotypes. Males, who are typically the primary contestants in the competition for mating partners, usually bear the ornaments such as showy plumage in spite of the potential costs of these ornaments, such as increased visibility to predators, and attacks from rival males. Females are less often involved in direct competition for mates, and they are not generally subject to the forces of sexual selection (although there are role reversals in a few species). Females are believed to play a critical role in the evolution of many elaborate male traits, however, because if the female preference has a genetic basis, female choice of particular males as mating partners will cause those male traits to spread in subsequent generations.

Sometimes the fitness of a phenotype in some environment depends on how common (or rare) it is; this is known as frequency-dependent selection. Perhaps an animal enjoys an increased advantage if it conforms to the majority phenotype in the population; this occurs when, for example, predators learn to avoid distasteful butterfly prey , because the butterflies have evolved to advertise their noxious taste by conforming to a particular wing color and pattern. Butterflies that deviate too much from the "warning" pattern are not as easily recognized by their predators, and are eaten in greater numbers. Interestingly, frequency-dependent selection has enabled butterflies who are not distasteful to mimic the appearance of their noxious brethren and thus avoid the same predators. Conversely, a phenotype could be favored if it is rare, and its alternatives are in the majority. Many predators tend to form a "search image" of their prey, favoring the most common phenotypes, and ignoring the rarer phenotypes. Frequency-dependent selection provides an interesting case in which the gene frequency itself alters the selective environment in which the genotype exists.

Many people attribute the phrase "survival of the fittest" to Darwin, but in fact, it originated from another naturalist/philosopher, Herbert Spencer (1820–1903). Recently, many recent evolutionary biologists have asked: Survival of the fittest what? At what organismal level is selection most powerful? What is the biological unit of natural selection-the species, the individual , or even the gene?

Although it seems rational that organisms might exhibit parental behavior or other traits "for the good of the species." In his 1962 book Animal Dispersion in Relation to Social Behaviour, behavioral biologist V. C. Wynne-Edwards proposed that animals would restrain their reproduction in times of resource shortages, so as to avoid extinguishing the local supply, and thus maintain the "balance of nature." However, Wynne-Edwards was criticized because all such instances of apparent group-level selection can be explained by selection acting at the level of individual organisms. A mother cat who suckles her kittens is not doing so for the benefit of the species; her behavior has evolved because it enhances her kittens' fitness, and ultimately her own as well, since they carry her genes.

Under most conditions, group selection will not be very powerful, because the rate of change in gene frequencies when one individual replaces another in the population is greater than that occurring when one group replaces another group. The number of individuals present is generally greater than the number of groups present in the environment, and individual turnover is greater. In addition, it is difficult to imagine that individuals could evolve to sacrifice their reproduction for the good of the group; a more selfish alternative could easily invade and spread in such a group.

However, there are some possible exceptions; one of these is reduced virulence in parasites , who depend on the survival of their hosts for their own survival. The myxomavirus , introduced in Australia to control imported European rabbits (Oryctolagus cuniculus), at first caused the deaths of many individuals. However, within a few years, the mortality rate was much lower, partly because the rabbits became resistant to the pathogen, but also partly because the virus had evolved a lower virulence. The reduction in the virulence is thought to have been aided because the virus is transmitted by a mosquito, from one living rabbit to another. The less deadly viral strain is maintained in the rabbit host population because rabbits afflicted with the more virulent strain would die before passing on the virus. Thus, the viral genes for reduced virulence could spread by group selection. Of course, reduced virulence is also in the interest of every individual virus, if it is to persist in its host. Scientists argue that one would not expect to observe evolution by group selection when individual selection is acting strongly in an opposing direction.

Some biologists, most notably Richard Dawkins (1941–), have argued that the gene itself is the true unit of selection. If one genetic alternative, or allele, provides its bearer with an adaptive advantage over some other individual who carries a different allele then the more beneficial allele will be replicated more times, as its bearer enjoys greater fitness. In his book The Selfish Gene, Dawkins argues that genes help to build the bodies that aid in their transmission; individual organisms are merely the "survival machines" that genes require to make more copies of themselves.

This argument has been criticized because natural selection cannot "see" the individual genes that reside in an organism's genome , but rather selects among phenotypes, the outward manifestation of all the genes that organisms possess. Some genetic combinations may confer very high fitness, but they may reside with genes having negative effects in the same individual. When an individual reproduces, its "bad" genes are replicated along with its "good" genes; if it fails to do so, even its most advantageous genes will not be transmitted into the next generation. Although the focus among most evolutionary biologists has been on selection at the level of the individual, this example raises the possibility that individual genes in genomes are under a kind of group selection. The success of single genes in being transmitted to subsequent generations will depend on their functioning well together, collectively building the best possible organism in a given environment.

When selective change is brought about by human effort, it is known as artificial selection. By allowing only a selected minority of individuals to reproduce, breeders can produce new generations of organisms featuring particular traits, including greater milk production in dairy cows, greater oil content in corn, or a rainbow of colors in commercial flowers. The repeated artificial selection and breeding of individuals with the most extreme values of the desired traits may continue until all the available genetic variation has been exhausted, and no further selection is possible. It is likely that dairy breeders have encountered the limit for milk production in cattle—eventually, a cow's milk production will increase more slowly for a given increase in feed-but the limit has not yet been reached for corn oil content, which continues to increase under artificial selection.

Seemingly regardless of the trait or characteristic involved (e.g. zygotic selection), there is almost always a way to construct a selectionist explanation of the manifest phenotype.

See also Adaptation; Evolution, convergent; Evolution, divergent; Evolution, evidence of; Evolution, parallel; Evolutionary change, rate of; Evolutionary mechanisms; Genetics.

Resources

books

Darwin, Charles R. On the Origin of Species. London: John Murray, 1859.

Dawkins, Richard. The Selfish Gene. Oxford: Oxford University Press, 1976.

Futuyma, Douglas J. Evolutionary Biology. 2nd ed. Sunderland, MA: Sinauer, 1986.

Gould, Stephen Jay. The Structure of Evolutionary Theory. Cambridge, MA: Harvard University Press, 2002.

Harvey, Paul H., and M.D. Pagel. The Comparative Method in Evolutionary Biology. Oxford: Oxford University Press, 1991.

Mayer, Ernst. What Evolution Is. Basic Books, 2001.

Mayr, Ernst. The Growth of Biological Thought: Diversity, Evolution, and Inheritance. Cambridge, MA: Harvard University Press, 1982.

K. Lee Lerner Susan Andrew

KEY TERMS

Artificial selection

—Selective breeding, carried out by humans, to produce desired genetic alterations in domestic animals and plants.

Fitness

—The average number of offspring produced by individuals with a certain genotype, relative to that of individuals with a different genotype.

Gene frequency

—The relative fraction of a particular gene in the population, compared to its alternatives.

Genome

—The complete set of genes an organism carries.

Genotype

—The full set of paired genetic elements carried by each individual, representing the its genetic blueprint; also used to refer to such a pair at a single genetic locus.

Group selection

—The replacement by natural selection of one or more groups of organisms in favor of other groups.

Natural selection

—The differential survival and reproduction of organisms, producing evolutionary change in populations.

Phenotype

—The outward manifestation of the genotype; the physical, morphological, and behavioral traits of an organism.

Sexual selection

—This is a type of natural selection in which anatomical or behavioral traits may be favored because they confer some advantage in courtship or another aspect of breeding. For example, the bright coloration, long tail, and elaborate displays of male pheasants have resulted from sexual selection by females, who apparently favor extreme expressions of these traits in their mates.

Selection

Selection is a process in which members of a population reproduce at different rates, due to either natural or human-influenced factors. The result of selection is that some characteristic is found in increasing numbers of organisms within the population as time goes on.

Types of Selection

Artificial selection, which is even older than agriculture, refers to a conscious effort to use for future breeding those varieties of a plant or animal that are most useful, attractive, or interesting to the breeder. Artificial selection is responsible for creating the enormous number of breeds of domestic dogs, for instance, as well as high-yielding varieties of corn and other agricultural crops.

Selection also occurs in nature, but it is not conscious. Charles Darwin called this natural selection. Darwin saw that organisms constantly vary in a population from generation to generation. He proposed that some variations allow an organism to be better adapted to a given environment than others in the population, allowing them to live and reproduce while others are forced out of reproduction by death, sterility, or isolation. These genetic variations gradually replace the ones that fail to survive or to reproduce. This gradual adjustment of the genotype to the environment is called adaptation. Natural selection was not only Darwin's key mechanism of evolution for the origin of species, it is also the key mechanism today for understanding the evolutionary biology of organisms from viruses to humans. Natural selection leads to evolution, which is the change in gene frequencies in a population over time.

The concept of selection plays an increasingly important role in biological theory. New fields such as evolutionary psychology rely heavily on natural selection to explain the evolution of human behavioral traits, such as mate choice, aggression, and other types of social behavior. A great difficulty in such a theoretically based science is the paucity of experimental or direct evidence for presumed past environments and presumed behavioral responses that were genetically adaptive.

Variation

The variation that selection requires arises from two distinct sources. The ultimate sources of variation are gene mutation , gene duplication and disruption, and chromosome rearrangements. Gene mutations are randomly occurring events that at a molecular level consist mostly of substitutions or small losses or gains of nucleotides within genes. Gene duplication makes new copies of existing genes, while gene disruptions destroy functional copies of genes, often through insertion of a mobile genetic element. Chromosome rearrangements are much larger changes in chromosome structure, in which large pieces of chromosomes break off, join up, or invert. Individually, such mutations are rare. Most small mutations are either harmful or have no effect, and they may persist in a population for dozens or hundreds of generations before their advantages or disadvantages are evident.

The second source of variation arises from the shuffling processes undergone by genes and chromosomes during reproduction. During meiosis , maternally and paternally derived chromosome pairs are separated randomly, so that each sperm or egg contains a randomly chosen member of each of the twenty-three pairs. The number of possible combinations is over eight billion. Even more variation arises when pair members exchange segments before separating, in the process known as crossing over. The extraordinary variety in form exhibited even by two siblings is due primarily to the shuffling of existing genes, rather than to new mutations.

The Importance of the Environment

A disadvantageous trait in one environment may be advantageous in a very different environment. A classic example of this is sickle cell disease in regions where malaria is common. Individuals who inherit a copy of the sickle cell gene from both of their parents (homozygotes ) die early from the disease, whereas heterozygotes (individuals who inherit only one copy of the gene) are favored in malarial areas (including equatorial Africa) over those without any copies, because they contract milder cases of malaria and thus are more likely to survive it.

Even though homozygotes rarely pass on their genes, because of their low likelihood of surviving to reproduce, the advantage of having one copy is high enough that natural selection continues to favor presence of the gene in these populations. Thus a malarial environment can keep the gene frequency high. However, in temperate regions where malaria is absent (such as North America), there is no heterozygote advantage to the sickle cell gene. Because heterozygotes still suffer from the disease, they are less likely to survive and reproduce. Thus, selection is gradually depleting the gene from the African American population that harbors it.

Artificial Selection

One of the first uses of genetic knowledge to improve yields and the quality of plant products was applied to hybrid seed production at the start of the twentieth century by George Shull. Artificial selection today is still done by hobbyists who garden or raise domestic animals. It is done on a more professional level in agriculture and animal breeding. The benefits are enormous. Virtually all commercial animal and plant breeding uses selection to isolate new combinations of traits to meet consumer needs. In these organisms, most of the variation is preexisting in the population or in related populations in the wild. The breeder's task is to combine (hybridize) the right organisms and select offspring with the desired traits.

In the antibiotic industry selection is used to identify new antibiotics. Usually, microorganisms are intentionally mutated to produce variation. Mutations can be induced with a variety of physical and chemical agents called mutagens, which randomly alter genes. Some early strains of penicillin-producing molds were x-rayed and their mutations selected for higher yields.

Biologists also make use of selection in the process called molecular cloning. Here, a new gene is inserted into a host along with a marker gene. The marker is typically a gene for antibiotic resistance. To determine if the host has taken up the new genes, it is exposed to antibiotics. The ones who survive are those that took up the resistance gene, and so also have the gene of interest. This selection process allows the researcher to quickly isolate only those organisms with the new gene.

Selection in Humans

Both natural and artificial selection occur in human beings. If a trait is lethal and kills before reproductive maturity, then that gene mutation is gradually depleted from the population. Mutations with milder effects persist longer and are more common than very severe mutations, and recessive mutations persist for much longer than dominant ones. With a recessive trait, such as albinism, the parents are usually both carriers of a single copy of the gene and may not know that they carry it. If a child receives a copy of this gene from both of the carrier parents, the albino child may die young, may find it difficult to find a partner, or may end up marrying much later in life. This is usually considered a form of natural selection.

Considerable abuse of genetic knowledge in the first half of the twentieth century led to the eugenics movement. Advocates of eugenics claimed some people were more fit and others less fit (or unfit), and argued that the least fit should be persuaded or forced not to reproduce. Eugenicists typically defined as unfit those who were "feeble-minded, criminal, socially deviant, or otherwise undesirable." Coerced sterilization, a form of artificial selection, was practiced on some of these individuals.

see also Cloning Genes; Eugenics; Hardy-Weinburg Equilibrium; Muller, Hermann; Mutagenesis; Mutation.

Elof Carlson

Bibliography

Huxley, Julian. "Adaptation and Selection." In Evolution: The Modern Synthesis. New York: Harper and Brothers, 1942.

Pianka, Eric. Evolutionary Biology, 6th ed. San Francisco: Addison-Wesley-Longman, 2000.

Selection

Evolutionary selection pressures act on all living organisms, regardless whether they are prokaryotic or higher eukaryotes . Selection refers to an evolutionary pressure that is the result of a combination of environmental and genetic pressures that affect the ability of an organism to live and, equally importantly, to produce reproductively successful offspring (including prokaryotic strains of cells).

As implied, natural selection involves the natural (but often complex) pressures present in an organism's environment. Artificial selection is the conscious manipulation of mating, manipulation, and fusion of genetic material to produce a desired result.

Evolution requires genetic variation, and these variations or changes (mutations ) are usually deleterious because environmental factors already support the extent genetic distribution within a population.

Natural selection is based upon expressed differences in the ability of organisms to thrive and produce biologically successful offspring. Importantly, selection can only act to exert influence (drive) on those differences in genotype that appear as phenotypic differences. In a very real sense, evolutionary pressures act blindly.

There are three basic types of natural selection: directional selection favoring an extreme phenotype ; stabilizing selection favoring a phenotype with characteristics intermediate to an extreme phenotype (i.e., normalizing selection); and disruptive selection that favors extreme phenotypes over intermediate genotypes.

The evolution of pesticide resistance provides a vivid example of directional selection, wherein the selective agent (in this case DDT) creates an apparent force in one direction, producing a corresponding change (improved resistance) in the affected organisms. Directional selection is also evident in the efforts of human beings to produce desired traits in many organisms ranging from bacteria to plants and animals.

Not all selective effects are directional, however. Selection can also produce results that are stabilizing or disruptive. Stabilizing selection occurs when significant changes in the traits of organisms are selected against. An example of this is birth weight in humans. Babies that are much heavier or lighter than average do not survive as well as those that are nearer the mean (average) weight.

On the other hand, selection is said to be disruptive if the extremes of some trait become favored over the intermediate values. Although not a factor for microorganisms , sexual selection and sexual dimorphism can influence the immunologic traits and capacity of a population.

Sometimes the fitness of a phenotype in some environment depends on how common (or rare) it is; this is known as frequency-dependent selection. Perhaps an animal enjoys an increased advantage if it conforms to the majority phenotype in the population. Conversely, a phenotype could be favored if it is rare, and its alternatives are in the majority. Frequency-dependent selection provides an interesting case in which the gene frequency itself alters the selective environment in which the genotype exists.

Many people attribute the phrase "survival of the fittest" to Darwin, but in fact, it originated from another naturalist/philosopher, Herbert Spencer (1820–1903). Recently, many recent evolutionary biologists have asked: Survival of the fittest what? At what organismal level is selection most powerful? What is the biological unit of natural selection-the species, the individual, or even the gene?

Selection can provide interesting consequences for bacteria and viruses . For example, reduced virulence in parasites , who depend on the survival of their hosts for their own survival may increase the reproductive success of the invading parasite. The myxoma virus, introduced in Australia to control imported European rabbits (Oryctolagus cuniculus ), at first caused the deaths of many individuals. However, within a few years, the mortality rate was much lower, partly because the rabbits became resistant to the pathogen, but also partly because the virus had evolved a lower virulence. The reduction in the virulence is thought to have been aided because the virus is transmitted by a mosquito, from one living rabbit to another. The less deadly viral strain is maintained in the rabbit host population because rabbits afflicted with the more virulent strain would die before passing on the virus. Thus, the viral genes for reduced virulence could spread by group selection. Of course, reduced virulence is also in the interest of every individual virus, if it is to persist in its host. Scientists argue that one would not expect to observe evolution by group selection when individual selection is acting strongly in an opposing direction.

Some biologists, most notably Richard Dawkins (1941–), have argued that the gene itself is the true unit of selection. If one genetic alternative, or allele, provides its bearer with an adaptive advantage over some other individual who carries a different allele then the more beneficial allele will be replicated more times, as its bearer enjoys greater fitness. In his book The Selfish Gene, Dawkins argues that genes help to build the bodies that aid in their transmission; individual organisms are merely the "survival machines" that genes require to make more copies of themselves.

This argument has been criticized because natural selection cannot "see" the individual genes that reside in an organism's genome, but rather selects among phenotypes, the outward manifestation of all the genes that organisms possess. Some genetic combinations may confer very high fitness, but they may reside with genes having negative effects in the same individual. When an individual reproduces, its "bad" genes are replicated along with its "good" genes; if it fails to do so, even its most advantageous genes will not be transmitted into the next generation. Although the focus among most evolutionary biologists has been on selection at the level of the individual, this example raises the possibility that individual genes in genomes are under a kind of group selection. The success of single genes in being transmitted to subsequent generations will depend on their functioning well together, collectively building the best possible organism in a given environment.

When selective change is brought about by human effort, it is known as artificial selection. By allowing only a selected minority of individuals or specimen to reproduce, breeders can produce new generations of organisms (e.g. a particular virus or bacterium) that feature desired traits.

See also Epidemiology; Evolution and evolutionary mechanisms; Evolutionary origin of bacteria and viruses; Rare genotype advantage

se·lec·tion / səˈlekshən/ • n. 1. the action or fact of carefully choosing someone or something as being the best or most suitable: such men decided the selection of candidates they objected to his selection. ∎  a number of carefully chosen things: the publication of a selection of his poems. ∎  a range of things from which a choice may be made: the restaurant offers a wide selection of hot and cold dishes. ∎  a horse or horses tipped as worth bets in a race or meeting.2. Comput. data highlighted on a computer screen that is a target for various manipulations: your selection may not contain two different data types. ∎  the action or capability of selecting data in this way. 3. Biol. a process in which environmental or genetic influences determine which types of organism thrive better than others, regarded as a factor in evolution.See also natural selection.