In the context of [[evolution]], certain traits or [[allele]]s of a [[species]] may be subject to '''selection.''' Under selection, individuals with advantageous or [[Adaptation (biology)|"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 [[genetics|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''.
In the context of [[evolution]], certain traits or [[allele]]s of a [[species]] may be subject to '''selection.''' Under selection, individuals with advantageous or [[Adaptation (biology)|"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 [[genetics|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''.
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==Overview==
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==Overview==
Whether or not selection takes place depends on the conditions in which the individuals of a species find themselves. Adults, [[juvenile (animal)|juvenile]]s, [[embryo]]s, and even [[Ovum|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'''.
Whether or not selection takes place depends on the conditions in which the individuals of a species find themselves. Adults, [[juvenile (animal)|juvenile]]s, [[embryo]]s, and even [[Ovum|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'''.
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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.
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.
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Meanwhile, selection does not guarantee that advantageous traits or alleles will become prevalent within a population. Through [[Genetic drift|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 size|small population]]s.
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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 size|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 (biology)|fitness]]), while '''positive selection''' increases the prevalence of adaptive traits.
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 (biology)|fitness]]), while '''positive selection''' increases the prevalence of adaptive traits.
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== Types and subtypes ==
== Types and subtypes ==
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[[Image:Selection classification diagram.png|frame|right|Selection is hierachically classified into [[natural selection|natural]] and [[artificial selection]]. Natural selection is further subclassified into [[ecological selection|ecological]] and [[sexual selection]]]]
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.
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.
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.
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.
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.
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