The frequency of an allele in a gene pool of a population depends on many factors and may be stable or unstable over time.
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Natural selection acts on the phenotype rather than the genotype of an organism.
Alleles that are lethal in a homozygous individual may be carried in a heterozygote and thus maintained in a gene pool.
New mutations are constantly being generated in a gene pool.
Variation within a species increases the likelihood that at least some members of a species will survive under changed environmental conditions.