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Artificial selection  -  Selection by humans for desired traits. Also called domestication or breeding.
Choice  -  In sexual selection, the competition for attention from the opposite sex.
Contest  -  In sexual selection, fights or aggressive displays between same-sex individuals of the same species; contests usually determine possession of territory or mates.
Directional Selection  -  When selection pressures favor one extreme of the trait distribution.
Disruptive Selection  -  When the average form of the trait is selected against while either extreme is unaffected.
Fitness  -  The probability that an individual will contribute its genes to the next generation.
Morphs  -  The different physical forms a trait may have. Long necks and short necks are examples of morphs.
Natural Selection  -  The theory holding that competition exists within species, determining which species live to have offspring, and pass their traits on to those offspring.
Polymorphic  -  A trait that exists in several different morphs is polymorphic.
Stabilizing Selection  -  When selection pressures favor the average form of the trait.
Sexual Selection  -  The selection of traits based on their role in courtship and mating.
Trait  -  A particular characteristic of an individual plant or animal.