Duplicate gene and its parental copy can subfunctionalize so that each carries out only some of the roles that the ancestral gene performed. 

Ex- a duplicate of the β-hemoglobin locus is expressed in the fetus. 

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The allelic frequency at equilibrium (qˆ) depends on the relative fitnesses (usually expressed as selection coefficients) of the two homozygotes:

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Hitchhiking

An important consequence of linkage disequilibrium is hitchhiking. This happens when an allele at  one locus spreads by natural selection acting on a second locus that is in linkage disequilibrium with  the first.

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Type of natural selection

1. Stabilized natural selection

2. Directional Natural selection

3. Disruptive Natural selection

4. Balance natural selection

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Stabilized natural selection

Favor intermediate phenotype but eliminate lower and higher-grade phenotype

It introduced homogeneity in the population

Exp - clutch size of birds that lay too many or too few eggs per nest have lower fitness values than do those that lay an intermediate number of eggs