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{{details|Genetic variation}}
The heritable portion of an individual's apparent traits, or [[phenotype]], is primarily the result of the specific genetic makeup, or [[genotype]], encoded on [[DNA]] molecules called [[chromosome]]s. Thus, the variation in heritable traits within a population reflects the variation in genetic makeup. A specific ___location on a chromosome is known as a [[locus (genetics)|locus]]; a variant of a DNA sequence at a given locus is an [[allele]]. The [[modern evolutionary synthesis]] defines evolution as the change over time in the relative [[allele frequency|frequencies]] of alleles in a population
Genetic variation is often the result of a new mutation in a single individual (usually point mutations, insertions, or deletions); in subsequent generations, the frequency of that variant may fluctuate in the population, becoming more or less prevalent relative to other alleles at the site. All evolutionary forces act by driving this change in allele frequency in one direction or another. Variation disappears when an allele reaches the point of [[fixation (population genetics)|fixation]] — when it either reaches a frequency of zero and disappears from the population, or reaches a frequency of one and replaces the ancestral allele entirely. Most sites in the complete DNA sequence, or [[genome]], of a species are identical in all individuals in the population. Consequently, relatively small genotypic changes can lead to dramatic phenotypic ones. Sites with more than one allele are called [[polymorphism (genetics)|polymorphic]], or segregating, sites. Polymorphism leads to distinct groups of traits arising within the same species, such as different [[hair color]]s or [[sex]]es. Interactions between a genotype and the environment may also affect the phenotype, as reflected in developmental and [[phenotypic plasticity]].
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