Selection
Encyclopedia
In the context of evolution
, certain traits or allele
s of genes
segregating within a population may be subject to selection. Under selection, individuals with advantageous or "adaptive" traits tend to be more successful than their peers reproductively—meaning they contribute more offspring to the succeeding generation than others do. When these traits have a genetic basis, selection can increase the prevalence of those traits, because offspring will inherit
those traits from their parents. When selection is intense and persistent, adaptive traits become universal to the population or species, which may then be said to have evolved. Scientists who do experimental genetics employ artificial selection experiments that permit the survival of organisms with user-defined phenotypes. Artificial selection is widely used in the field of microbial genetics
, especially molecular cloning
.
s, and even egg
s and sperm
may undergo selection. Factors fostering selection include limits on resources (nourishment, habitat space, mates) and the existence of threats (predators, disease, adverse weather). Biologists often refer to such factors as selective pressures.
Natural selection
is the most familiar type of selection by name. The breeding of dogs, cows and horses, however, represents "artificial selection
". Subcategories of natural selection are also sometimes distinguished. These include sexual selection
, ecological selection
, stabilizing selection
, disruptive selection
and directional selection
(more on these below).
Selection occurs only when the individuals of a population are diverse in their characteristics—or more specifically when the traits of individuals differ with respect to how well they equip them to survive or exploit a particular pressure. In the absence of individual variation, or when variations are selectively neutral, selection does not occur.
Meanwhile, selection does not guarantee that advantageous traits or alleles will become prevalent within a population. Through genetic drift
, such traits may become less common or disappear. In the face of selection even a so-called deleterious allele may become universal to the members of a species. This is a risk primarily in the case of "weak" selection (e.g. an infectious disease with only a low mortality rate) or small population
s.
Though deleterious alleles may sometimes become established, selection may act "negatively" as well as "positively." Negative selection decreases the prevalence of traits that diminish individuals' capacity to succeed reproductively (i.e. their fitness
), while positive selection increases the prevalence of adaptive traits.
In biological discussions, traits subject to negative selection are sometimes said to be "selected against," while those under positive selection are said to be "selected for," as in the sentence Desert conditions select for drought tolerance in plants and select against shallow root architectures.
favors individuals with intermediate characteristics while its opposite, disruptive selection
, favors those with extreme characteristics; directional selection
occurs when characteristics lie along a phenotypic spectrum and the individuals at one end are more successful; and balancing selection
is a pattern in which multiple characteristics may be favored.
.
Evolution
Evolution is any change across successive generations in the heritable characteristics of biological populations. Evolutionary processes give rise to diversity at every level of biological organisation, including species, individual organisms and molecules such as DNA and proteins.Life on Earth...
, certain traits or allele
Allele
An allele is one of two or more forms of a gene or a genetic locus . "Allel" is an abbreviation of allelomorph. Sometimes, different alleles can result in different observable phenotypic traits, such as different pigmentation...
s of genes
Gênes
Gênes is the name of a département of the First French Empire in present Italy, named after the city of Genoa. It was formed in 1805, when Napoleon Bonaparte occupied the Republic of Genoa. Its capital was Genoa, and it was divided in the arrondissements of Genoa, Bobbio, Novi Ligure, Tortona and...
segregating within a population may be subject to selection. Under selection, individuals with advantageous or "adaptive" traits tend to be more successful than their peers reproductively—meaning they contribute more offspring to the succeeding generation than others do. When these traits have a genetic basis, selection can increase the prevalence of those traits, because offspring will inherit
Genetics
Genetics , a discipline of biology, is the science of genes, heredity, and variation in living organisms....
those traits from their parents. When selection is intense and persistent, adaptive traits become universal to the population or species, which may then be said to have evolved. Scientists who do experimental genetics employ artificial selection experiments that permit the survival of organisms with user-defined phenotypes. Artificial selection is widely used in the field of microbial genetics
Microbial genetics
Microbial genetics is a subject area within microbiology and genetic engineering. It studies the genetics of very small organisms. This involves the study of the genotype of microbial species and also the expression system in the form of phenotypes.It also involves the study of genetic processes...
, especially molecular cloning
Molecular cloning
Molecular cloning refers to a set of experimental methods in molecular biology that are used to assemble recombinant DNA molecules and to direct their replication within host organisms...
.
Overview
Whether or not selection takes place depends on the conditions in which the individuals of a species find themselves. Adults, juveniles, embryoEmbryo
An embryo is a multicellular diploid eukaryote in its earliest stage of development, from the time of first cell division until birth, hatching, or germination...
s, and even egg
Ovum
An ovum is a haploid female reproductive cell or gamete. Both animals and embryophytes have ova. The term ovule is used for the young ovum of an animal, as well as the plant structure that carries the female gametophyte and egg cell and develops into a seed after fertilization...
s and sperm
Sperm
The term sperm is derived from the Greek word sperma and refers to the male reproductive cells. In the types of sexual reproduction known as anisogamy and oogamy, there is a marked difference in the size of the gametes with the smaller one being termed the "male" or sperm cell...
may undergo selection. Factors fostering selection include limits on resources (nourishment, habitat space, mates) and the existence of threats (predators, disease, adverse weather). Biologists often refer to such factors as selective pressures.
Natural selection
Natural selection
Natural selection is the nonrandom process by which biologic traits become either more or less common in a population as a function of differential reproduction of their bearers. It is a key mechanism of evolution....
is the most familiar type of selection by name. The breeding of dogs, cows and horses, however, represents "artificial selection
Artificial selection
Artificial selection describes intentional breeding for certain traits, or combination of traits. The term was utilized by Charles Darwin in contrast to natural selection, in which the differential reproduction of organisms with certain traits is attributed to improved survival or reproductive...
". Subcategories of natural selection are also sometimes distinguished. These include sexual selection
Sexual selection
Sexual selection, a concept introduced by Charles Darwin in his 1859 book On the Origin of Species, is a significant element of his theory of natural selection...
, ecological selection
Ecological selection
Ecological selection refers to natural selection minus sexual selection, i.e. strictly ecological processes that operate on a species' inherited traits without reference to mating or secondary sex characteristics...
, stabilizing selection
Stabilizing selection
-Description:Stabilizing or ambidirectional selection, , is a type of natural selection in which genetic diversity decreases as the population stabilizes on a particular trait value. This is probably the most common mechanism of action for natural selection...
, disruptive selection
Disruptive selection
Disruptive selection, also called diversifying selection, describes changes in population genetics in which extreme values for a trait are favored over intermediate values. In this case, the variance of the trait increases and the population is divided into two distinct groups...
and directional selection
Directional selection
In population genetics, directional selection is a mode of natural selection in which a single phenotype is favored, causing the allele frequency to continuously shift in one direction...
(more on these below).
Selection occurs only when the individuals of a population are diverse in their characteristics—or more specifically when the traits of individuals differ with respect to how well they equip them to survive or exploit a particular pressure. In the absence of individual variation, or when variations are selectively neutral, selection does not occur.
Meanwhile, selection does not guarantee that advantageous traits or alleles will become prevalent within a population. Through genetic drift
Genetic drift
Genetic drift or allelic drift is the change in the frequency of a gene variant in a population due to random sampling.The alleles in the offspring are a sample of those in the parents, and chance has a role in determining whether a given individual survives and reproduces...
, such traits may become less common or disappear. In the face of selection even a so-called deleterious allele may become universal to the members of a species. This is a risk primarily in the case of "weak" selection (e.g. an infectious disease with only a low mortality rate) or small population
Small population size
Small populations behave differently from larger populations. They often result in population bottlenecks, which have harmful consequences for the survival of that population.-Demographic effects:...
s.
Though deleterious alleles may sometimes become established, selection may act "negatively" as well as "positively." Negative selection decreases the prevalence of traits that diminish individuals' capacity to succeed reproductively (i.e. their fitness
Fitness (biology)
Fitness is a central idea in evolutionary theory. It can be defined either with respect to a genotype or to a phenotype in a given environment...
), while positive selection increases the prevalence of adaptive traits.
In biological discussions, traits subject to negative selection are sometimes said to be "selected against," while those under positive selection are said to be "selected for," as in the sentence Desert conditions select for drought tolerance in plants and select against shallow root architectures.
Types and subtypes
Patterns of selection
Aspects of selection may be divided into effects on a phenotype and their causes. The effects are called patterns of selection, and do not necessarily result from particular causes (mechanisms); in fact each pattern can arise from a number of different mechanisms. Stabilizing selectionStabilizing selection
-Description:Stabilizing or ambidirectional selection, , is a type of natural selection in which genetic diversity decreases as the population stabilizes on a particular trait value. This is probably the most common mechanism of action for natural selection...
favors individuals with intermediate characteristics while its opposite, disruptive selection
Disruptive selection
Disruptive selection, also called diversifying selection, describes changes in population genetics in which extreme values for a trait are favored over intermediate values. In this case, the variance of the trait increases and the population is divided into two distinct groups...
, favors those with extreme characteristics; directional selection
Directional selection
In population genetics, directional selection is a mode of natural selection in which a single phenotype is favored, causing the allele frequency to continuously shift in one direction...
occurs when characteristics lie along a phenotypic spectrum and the individuals at one end are more successful; and balancing selection
Balancing selection
Balancing selection refers to a number of selective processes by which multiple alleles are actively maintained in the gene pool of a population at frequencies above that of gene mutation. This usually happens when the heterozygotes for the alleles under consideration have a higher adaptive value...
is a pattern in which multiple characteristics may be favored.
Mechanisms of selection
Distinct from patterns of selection are mechanisms of selection; for example, disruptive selection often is the result of disassortative sexual selection, and balancing selection may result from frequency-dependent selection and overdominanceOverdominance
Overdominance is a condition in genetics where the phenotype of the heterozygote lies outside of the phenotypical range of both homozygote parents. Overdominance can also be described as heterozygote advantage, wherein heterozygous individuals have a higher fitness than homozygous individuals.An...
.
See also
- Natural selectionNatural selectionNatural selection is the nonrandom process by which biologic traits become either more or less common in a population as a function of differential reproduction of their bearers. It is a key mechanism of evolution....
- Sexual selectionSexual selectionSexual selection, a concept introduced by Charles Darwin in his 1859 book On the Origin of Species, is a significant element of his theory of natural selection...
- Artificial selectionArtificial selectionArtificial selection describes intentional breeding for certain traits, or combination of traits. The term was utilized by Charles Darwin in contrast to natural selection, in which the differential reproduction of organisms with certain traits is attributed to improved survival or reproductive...
- EvolutionEvolutionEvolution is any change across successive generations in the heritable characteristics of biological populations. Evolutionary processes give rise to diversity at every level of biological organisation, including species, individual organisms and molecules such as DNA and proteins.Life on Earth...