Small populations, non-random breeding, and mutations can change allele frequencies in populations {population genetics}|.
In populations, heterozygous and homozygous proportion tends to stay constant {balanced polymorphism}. Balanced polymorphism uses habitat efficiently and preserves variation. In isolated groups, gene ratios stay constant even if environment favors one allele.
For genes, populations have number of one allele divided by number of all alleles {allele frequency} {gene frequency}.
Small populations can have chance allele-frequency changes {genetic drift}|.
If individuals have extreme genetic traits, general trait is in species {principle of genetic gradients} {genetic gradient principle}.
In geographic areas, allele frequencies are constant for species with stable populations {Hardy-Weinberg law}.
Allele-proportion change rate depends on allele reproductive advantage {selection pressure}| and on whether allele is recessive or dominant.
variation
Population allele ratio changes by heterozygote superiority, environmental heterogeneity, cycles, agonistic or antagonistic gene linkages, and homozygote selection.
balancing
Natural selection can maintain allele ratios {stabilizing selection} {balancing selection} to maintain variation. Balancing selection happens in large, non-isolated populations with alleles that are neither dominant nor recessive.
directional
Natural selection can change allele ratios {directional selection} {purifying selection} to reduce variation. Directional selection can happen by genetic drift or inbreeding in small isolated populations.
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Date Modified: 2022.0225