Everything about Selection totally explained
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.
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.
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 doesn't occur.
Meanwhile, selection doesn't 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 (for example 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 (for example 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.
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 don't 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.
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.
Further Information
Get more info on 'Selection'.
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