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In the context of evolution, factors that organisms face during their lives and in their development are liable to "select" ( or "cause selection of" ) particular traits. These traits will be those that confer an advantage to the individuals who possess them, in particular with regard to the selective factor. These factors, which biologists more commonly refer to as selective forces or pressures, include potential resources (nourishment or habitat space) and threats (such as predators or adverse climatic conditions). In the presence of selection, individuals with advantageous traits are more successful, on average, than their peers--meaning they contribute more offspring to the succeeding generation. This leads to what is regarded as the primary effect of selection: An increase in the proportion of a population that possesses the advantageous or "adaptive" trait. Under persistent and intense selection, a trait will become predominant or universal.

Biologists divide selection into various categories and subcategories. Natural selection is the most familiar by name. The breeding of dogs, cows and horses, for example, represents "artificial selection." Other categories 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 traits, or more specifically when the traits of individuals differ with respect to how well they equip the individuals to survive or exploit a particular selective pressure. In the absence of individual variation, or when individual variations are "selectively neutral," selection does not occur. Also, selection does not guarantee that advantageous traits or alleles will come to predominate within a population. Through genetic drift, such traits may become less common or disappear. In the face of selection, in fact, even a so-called "deleterious" allele may become universal among 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.

Despite the occasional rare establishment of deleterious alleles, selection may act "negatively" as well as "positively." While positive selection tends to increase the commonness of certain traits, negative selection tends to decrease the commonness of traits that confer a diminished fitness.

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 of Selection

Stabilizing selection[?] favors individuals with intermediate characteristics while its opposite, disruptive selection[?], favors those with extreme characteristics. Directional selection[?] occurs when characteristic lie along a phenotypic spectrum and the individuals at one end are more successful.

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