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Encyclopedia > Selection
Part of the Biology series on
Evolution
Mechanisms and processes

Adaptation
Genetic drift
Gene flow
Mutation
Natural selection
Speciation Look up selection in Wiktionary, the free dictionary. ... This article needs additional references or sources for verification. ... This article is about evolution in biology. ... Image File history File links Tree_of_life. ... A biological adaptation is an anatomical structure, physiological process or behavioral trait of an organism that has evolved over a period of time by the process of natural selection such that it increases the expected long-term reproductive success of the organism. ... In population genetics, genetic drift is the statistical effect that results from the influence that chance has on the success of alleles (variants of a gene). ... In population genetics, gene flow (also known as gene migration) is the transfer of alleles of genes from one population to another. ... It has been suggested that mutant be merged into this article or section. ... Darwins 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. ... Charles Darwins first sketch of an evolutionary tree from his First Notebook on Transmutation of Species (1837) Speciation is the evolutionary process by which new biological species arise. ...

Research and history

Evidence
History
Modern synthesis
Social effect / Objections While on board HMS Beagle, Charles Darwin collected numerous specimens, many new to science, which supported his later theory of evolution by natural selection. ... Evolutionary thought has roots in antiquity as philosophical ideas conceived during the Ancient Greek and Roman eras, but until the 18th century, biological thought was dominated by essentialism, the idea that living forms are static and unchanging in time. ... This article or section does not cite any references or sources. ... The theory of transmutation had early origins in the speculations and hypotheses of Erasmus Darwin, and Jean-Baptiste Lamarck. ... There have been numerous objections to evolution since alternative evolutionary ideas came to be hotly debated around the start of the nineteenth century. ...

Evolutionary biology fields

Ecological genetics
Evolutionary development
Human evolution
Molecular evolution
Evolutionary history of life
Phylogenetics
Population genetics
This article or section does not cite any references or sources. ... Ecological genetics is the study of genetics (itself a field of biology) from an ecological perspective. ... Evolutionary developmental biology (evolution of development or informally, evo-devo) is a field of biology that compares the developmental processes of different animals in an attempt to determine the ancestral relationship between organisms and how developmental processes evolved. ... // For the history of humans on Earth, see History of the world. ... Molecular evolution is the process of the genetic material in populations of organisms changing over time. ... The evolutionary history of life and origin of life are necessary precursor for geological and biological evolution, but understanding that evolution occurred once organisms appeared and investigating how this happens, does not depend on understanding exactly how life began. ... Phylogenetic groups, or taxa, can be monophyletic, paraphyletic, or polyphyletic. ... Population genetics is the study of the distribution of and change in allele frequencies under the influence of the four evolutionary forces: natural selection, genetic drift, mutation, and migration. ...

Biology Portal · v  d  e 

In the context of evolution, certain traits or alleles of a species 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. This article is about evolution in biology. ... For the hard rock band, see Allele (band). ... The hierarchy of scientific classification. ... The eye is an adaptation. ... This article is about the general scientific term. ...

Contents

Overview

Whether or not selection takes place depends on the conditions in which the individuals of a species find themselves. Adults, juveniles, embryos, and even eggs 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. Juvenile (left) and adult (right) leaves of Stone Pine A juvenile is an individual organism that has not yet reached its adult form, sexual maturity or size. ... For other uses, see Embryo (disambiguation). ... A human ovum Sperm cells attempting to fertilize an ovum An ovum (plural ova) is a haploid female reproductive cell or gamete. ... Different types of sperm cells: A) spermatozoon (motile), B) spermatium (non-motile), C) fertilization tube with sperm nuclei The term sperm is derived from the Greek word spermos (Latin: sperma) meaning seed and refers to the male reproductive cells. ...


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). Darwins 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. ... This Chihuahua mix and Great Dane show the wide range of dog breed sizes created using artificial selection. ... Illustration from The Descent of Man and Selection in Relation to Sex by Charles Darwin showing the Tufted Coquette Lophornis ornatus, female on left, ornamented male on right. ... Ecological selection (or environmental selection or survival selection or individual selection or asexual selection) refers to natural selection minus sexual selection, i. ... Stabilizing selection, also known as purifying selection or negative selection, is a type of natural selection in which genetic diversity decreases as the population stabilizes on a particular trait value. ... Disruptive selection is a type of evolution that simultaneously favors individuals at both extremes of the distribution. ... In population genetics, directional selection (sometimes referred to as positive selection) occurs when natural selection favors a single allele and therefore allele frequency continuously shifts in one direction. ...


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 populations. In population genetics, genetic drift is the statistical effect that results from the influence that chance has on the success of alleles (variants of a gene). ... Species with a small population size are subject to a higher chance of extinction because their small population size makes them more vulnerable to genetic drift, resulting in stochastic variation in their gene pool, their demography and their environment. ...


Though deleterious alleles may sometimes become established, selection may act "negatively" as well as "positively." Negative selection decreases the prevalence of traits that enhance individuals' capacity to succeed reproductively (i.e. their fitness), while positive selection increases the prevalence of adaptive traits. Fitness (often denoted in population genetics models) is a central concept in evolutionary theory. ...


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 hierachically classified into natural and artificial selection. Natural selection is further subclassified into ecological and sexual selection
Selection is hierachically classified into natural and artificial selection. Natural selection is further subclassified into ecological and sexual selection

Image File history File links Selection_classification_diagram. ... Darwins 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. ... This Chihuahua mix and Great Dane show the wide range of dog breed sizes created using artificial selection. ... Ecological selection (or environmental selection or survival selection or individual selection or asexual selection) refers to natural selection minus sexual selection, i. ... Illustration from The Descent of Man and Selection in Relation to Sex by Charles Darwin showing the Tufted Coquette Lophornis ornatus, female on left, ornamented male on right. ...

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 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. Stabilizing selection, also known as purifying selection or negative selection, is a type of natural selection in which genetic diversity decreases as the population stabilizes on a particular trait value. ... Disruptive selection is a type of evolution that simultaneously favors individuals at both extremes of the distribution. ... In population genetics, directional selection (sometimes referred to as positive selection) occurs when natural selection favors a single allele and therefore allele frequency continuously shifts in one direction. ... Sickle-shaped red blood cells Balancing selection refers to forms of natural selection which work to maintain genetic polymorphisms (or multiple alleles) within a population. ...


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 overdominance. Disassortative sexual selection is a form of sexual selection in which one sex chooses the other, in such a way that the offspring benefits from the diversity of the parental genotypes. ... Frequency dependent selection is the term given to an evolutionary process where the fitness of a phenotype is dependent on the relative frequency of other phenotypes in a given population. ... Overdominance is an alternate term for heterozygote advantage, a condition in genetics where the phenotype of the heterozygote is fitter than the phenotype of either homozygote. ...


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