In the context of evolution, certain traits or alleles An allele (pronounced /ˈæliːl/ , /əˈliːl/ (US); from the Greek αλληλος allelos, meaning each other) is one of two or more forms of the DNA sequence of a particular gene of genes A gene is a unit of heredity in a living organism. It is normally a stretch of DNA that codes for a type of protein or for an RNA chain that has a function in the organism. All living things depend on genes, as they specify all proteins and functional RNA chains. Genes hold the information to build and maintain an organism's cells and pass genetic segregating within a population may be subject to selection. Under selection, individuals with advantageous or "adaptive" Adaptation is the process whereby a population becomes better suited to its habitat. This process takes place over many generations, and is one of the basic phenomena of biology 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 , a discipline of biology, is the science of heredity and variation in living organisms. The fact that living things inherit traits from their parents has been used since prehistoric times to improve crop plants and animals through selective breeding. However, the modern science of genetics, which seeks to understand the process of 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.

Contents

Overview

Whether or not selection takes place depends on the conditions in which the individuals of a species find themselves. Adults, juveniles A juvenile is an individual organism that has not yet reached its adult form, sexual maturity or size. Juveniles sometimes look very different from the adult form, particularly in terms of their colour, embryos 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. In humans, it is called an embryo until about eight weeks after fertilization (i.e. ten weeks LMP), and from then it is instead called a fetus, and even eggs 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. In lower plants and algae, the ovum is also often called oosphere 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 Natural selection is a natural law by which genetically heritable traits become more or less common in a population over successive generations. This selection in interaction with the production of variation, the possible genetic fixation process and possibly, in several cases, whith little epigenetic process determine the evolution of the species is the most familiar type of selection by name. The breeding of dogs, cows and horses, however, represents "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 ability (“Darwinian fitness”). As opposed to". Subcategories of natural selection are also sometimes distinguished. These include sexual 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.[citation needed] The variant names describe varying circumstances where sexual selection is wholly suppressed as a mating factor.[, stabilizing selection Stabilizing selection, also referred to as purifying selection or ambidirectional selection, is a type of natural selection in which genetic diversity decreases as the population stabilizes on a particular trait value. Put another way, extreme values of the character are selected against. This is probably the most common mechanism of action for, disruptive selection Disruptive selection, also called diversifying selection, is a descriptive term used to describe changes in population genetics that simultaneously favor individuals at both extremes of the distribution. When disruptive selection operates, individuals at the extremes contribute more offspring than those in the center, producing two peaks in the and directional selection Directional selection is a particular mode or mechanism of natural selection. In population genetics, directional selection occurs when natural selection favors a single phenotype and therefore allele frequency continuously shifts in one direction. Under directional selection, the advantageous allele will increase in frequency independently of its (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 or allelic drift is the change in the relative frequency in which a gene variant occurs in a population due to random sampling and chance: The alleles in offspring are a random sample of those in the parents, and chance has a role in determining whether a given individual survives and reproduces. A population's allele frequency is, 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 populations The influence of stochastic variation in demographic rates is much higher for small populations than large ones. Stochastic variation in demographic rates causes small populations to fluctuate randomly in size. The smaller the population the greater the probability that fluctuations will lead to extinction. They are subject to a higher chance of.

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 is a central idea in evolutionary theory. It describes the capability of an individual of certain genotype to reproduce, and usually is equal to the proportion of the individual's genes in all the genes of the next generation. If differences in individual genotypes affect fitness, then the frequencies of the genotypes will change over), 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

Selection is hierarchically classified into natural Natural selection is a natural law by which genetically heritable traits become more or less common in a population over successive generations. This selection in interaction with the production of variation, the possible genetic fixation process and possibly, in several cases, whith little epigenetic process determine the evolution of the species and 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 ability (“Darwinian fitness”). As opposed to. Natural selection is further subclassified into ecological 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.[citation needed] The variant names describe varying circumstances where sexual selection is wholly suppressed as a mating factor.[ and sexual selection

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 selection Stabilizing selection, also referred to as purifying selection or ambidirectional selection, is a type of natural selection in which genetic diversity decreases as the population stabilizes on a particular trait value. Put another way, extreme values of the character are selected against. This is probably the most common mechanism of action for favors individuals with intermediate characteristics while its opposite, disruptive selection Disruptive selection, also called diversifying selection, is a descriptive term used to describe changes in population genetics that simultaneously favor individuals at both extremes of the distribution. When disruptive selection operates, individuals at the extremes contribute more offspring than those in the center, producing two peaks in the, favors those with extreme characteristics; directional selection Directional selection is a particular mode or mechanism of natural selection. In population genetics, directional selection occurs when natural selection favors a single phenotype and therefore allele frequency continuously shifts in one direction. Under directional selection, the advantageous allele will increase in frequency independently of its occurs when characteristics lie along a phenotypic spectrum and the individuals at one end are more successful; and balancing selection Balancing selection in evolutionary biology refers to a number of processes of natural selection by which multiple alleles are actively maintained in intermediate (balanced) frequencies within the gene pool of a population of a species. In this way genetic polymorphism is conserved 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 Frequency dependent selection is the term given to an evolutionary process where the fitness of a phenotype is dependent on its frequency relative to other phenotypes in a given population. In positive frequency dependent selection, the fitness of a phenotype increases as it becomes more common. In negative frequency dependent selection, the and overdominance Overdominance is a condition in genetics where the phenotype of the heterozygote lies outside of the phenotypical range of both homozygote parents.

Further reading

Topics in population genetics Population genetics is the study of allele frequency distribution and change under the influence of the four main evolutionary processes: natural selection, genetic drift, mutation and gene flow. It also takes into account the factors of population subdivision and population structure. It attempts to explain such phenomena as adaptation and
Key concepts Hardy-Weinberg law · Genetic linkage Genetic linkage is a term which describes the tendency of certain loci or alleles to be inherited together. Genetic loci on the same chromosome are physically close to one another and tend to stay together during meiosis, and are thus genetically linked · Linkage disequilibrium In population genetics, linkage disequilibrium is the non-random association of alleles at two or more loci, not necessarily on the same chromosome. It is not the same as linkage, which describes the association of two or more loci on a chromosome with limited recombination between them. Linkage disequilibrium describes a situation in which some · Fisher's fundamental theorem In population genetics, R. A. Fisher's fundamental theorem of natural selection was originally stated as: · Neutral theory The neutral theory of molecular evolution was introduced by Motoo Kimura in the late 1960s and early 1970s. The theory states that the vast majority of evolutionary changes at the molecular level are caused by random drift of selectively neutral mutants. Although the theory was received by some as an argument against Darwin's theory of evolution · Price equation The Price equation is a covariance equation which is a mathematical description of evolution and natural selection. The Price equation was derived by George R. Price, working in London to rederive W.D. Hamilton's work on kin selection
Selection Natural Natural selection is a natural law by which genetically heritable traits become more or less common in a population over successive generations. This selection in interaction with the production of variation, the possible genetic fixation process and possibly, in several cases, whith little epigenetic process determine the evolution of the species · Sexual · Artificial 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 ability (“Darwinian fitness”). As opposed to · Ecological 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.[citation needed] The variant names describe varying circumstances where sexual selection is wholly suppressed as a mating factor.[
Effects of selection on genomic variation Genetic hitchhiking Genetic hitchhiking is the process by which an evolutionarily neutral or in some cases deleterious allele or mutation may spread through the gene pool by virtue of being linked to a gene that is positvely selected. More generally, genetic hitchhiking can refer to the process by which a gene's frequency changes due to selection operating upon · Background selection Negative selection, in natural selection, is the selective removal of alleles that are deleterious. This can result in stabilizing selection through the purging of deleterious variations that arise. It is also known as purifying selection
Genetic drift Genetic drift or allelic drift is the change in the relative frequency in which a gene variant occurs in a population due to random sampling and chance: The alleles in offspring are a random sample of those in the parents, and chance has a role in determining whether a given individual survives and reproduces. A population's allele frequency is Small population size The influence of stochastic variation in demographic rates is much higher for small populations than large ones. Stochastic variation in demographic rates causes small populations to fluctuate randomly in size. The smaller the population the greater the probability that fluctuations will lead to extinction. They are subject to a higher chance of · Population bottleneck A population bottleneck is an evolutionary event in which a significant percentage of a population or species is killed or otherwise prevented from reproducing · Founder effect In population genetics, the founder effect is the loss of genetic variation that occurs when a new population is established by a very small number of individuals from a larger population. It was first fully outlined by Ernst Mayr in 1952, using existing theoretical work by those such as Sewall Wright. As a result of the loss of genetic variation, · Coalescence In genetics, coalescent theory is a retrospective model of population genetics. It employs a sample of individuals from a population to trace all alleles of a gene shared by all members of the population to a single ancestral copy, known as the most recent common ancestor . The inheritance relationships between alleles are typically represented as · Balding–Nichols model
Founders R. A. Fisher Sir Ronald Aylmer Fisher FRS was an English statistician, evolutionary biologist, eugenicist and geneticist. He was described by Anders Hald as "a genius who almost single-handedly created the foundations for modern statistical science," and Richard Dawkins described him as "the greatest of Darwin's successors" · J. B. S. Haldane John Burdon Sanderson Haldane FRS , known as Jack (but who used 'J.B.S.' in his printed works), was a British-born geneticist and evolutionary biologist. He was one of the founders (along with Ronald Fisher and Sewall Wright) of population genetics · Sewall Wright Sewall Green Wright was an American geneticist known for his influential work on evolutionary theory and also for his work on path analysis. With R. A. Fisher and J.B.S. Haldane, he was a founder of theoretical population genetics. He is the discoverer of the inbreeding coefficient and of methods of computing it in pedigrees. He extended this work
Related topics Evolution · Microevolution Microevolution is the occurrence of small-scale changes in allele frequencies in a population, over a few generations, also known as "change below the species level". These changes are due to four different processes: mutation, selection , gene flow and genetic drift · Evolutionary game theory Evolutionary game theory is the application of game theory to interaction dependent strategy evolution in populations. EGT is useful in a biological context by defining a framework of strategies in which adaptive features can be modeled. It originated in 1973 with John Maynard Smith and George R. Price's formalization of evolutionarily stable · Fitness landscape In evolutionary biology, fitness landscapes or adaptive landscapes are used to visualize the relationship between genotypes and reproductive success. It is assumed that every genotype has a well defined replication rate (often referred to as fitness). This fitness is the "height" of the landscape. Genotypes which are very similar are · Genetic genealogy Genetic genealogy is the application of genetics to traditional genealogy. Genetic genealogy involves the use of genealogical DNA testing to determine the level of genetic relationship between individuals
List of evolutionary biology topics

Categories: Evolutionary biology | Population genetics | Selection

 

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