The overall change in allelic frequency is a balance between the opposing forces of forward mutation and reverse  mutation:

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#Unit 11. Evolution and Behavior

Fate of duplicate gene 

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#Unit 11. Evolution and Behavior

If the duplicate is redundant and does not provide a fitness benefit, it will be lost by deletion or the accumulation of loss of-function mutations.

A second fate is that a duplicate can simply retain its original function.

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#Unit 11. Evolution and Behavior

A third fate that can befall a duplicated gene is neofunctionalization. Here the duplicate evolves a novel biological function. 

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#Unit 11. Evolution and Behavior

Most protein-encoding genes, the synonymous rate of change is considerably higher than the nonsynonymous rate because synonymous mutations are tolerated by natural selection

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#Unit 11. Evolution and Behavior

Molecular Variation (Rate) - Which is the number of substitutions taking place per nucleotide site per year

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#Unit 11. Evolution and Behavior

Nonsynonymous mutations, on the other hand, alter the amino acid sequence of the protein and in many cases are detrimental to the fitness of the organism, so most of these mutations are eliminated by natural selection.