Genetic and evolutionary factors by effect.

Persistence of Deleterious Traits

Heterozygote superiority: A phenomenon in which heterozygotes for a Mendelian gene are selected for more than either homozygote. If the gene for a recessive deleterious allele displays heterozygote superiority, the allele will persist in heterozygotes which may have homozygote recessive offspring.

Hitchiking: A phenomenon in which a neutral or slightly deleterious allele is swept to high frequency as a result of having high linkage with a beneficial allele.

Mutation-selection balance: A phenomenon in which the rate of an allele reentering the population is equal to the rate at which negative selection removes it, resulting in an equilibrium in its frequency.

Pleiotropy: A phenomenon in which a single gene simultaneously encodes multiple traits. If one of the effects is slightly deleterious while another is beneficial enough to compensate, the deleterious trait will likely persist.

Increasing Variation

Allelic Variation

Disruptive selection: A pattern of selection in which traits at either extreme are favoured while middle values are disfavored. Note that this will increase the range of variation but decrease the total variation.

Heterozygote superiority: A phenomenon in which heterozygotes for a Mendelian gene are selected for more than either homozygotes. If the gene for a recessive allele displays heterozygote superiority, the allele will persist in heterozygotes which may have homozygote recessive offspring.

Karyotypic Variation

Migration:

Decreasing Variation

Allelic Variation

Heterozygote inferiority:

Neutral evolution:

Stabilizing selection:

Karyotypic Variation

Bottleneck event

Founder effect

Inbreeding

Speciation