What is "neofunctionalization"

Neofunctionalization, one of the possible outcomes of functional divergence, occurs when one gene copy, or paralog, takes on a totally new function after a gene duplication event. Neofunctionalization is an adaptive mutation process; meaning one of the gene copies must mutate to develop a function that was not present in the ancestral gene. In other words, one of the duplicates retains its original function, while the other accumulates molecular changes such that, in time, it can perform a different task. This process is thought to be free of selective pressure because one gene copy can mutate without adversely affecting the fitness of the organism since ancestral function is retained in the other copy.