The process
The process of Neofunctionalization begins with aSelective constraints
Neofunctionalization is also commonly referred to as "mutation during non-functionality" or "mutation during redundancy". Regardless of if the mutation arises after non-functionality of a gene or due to redundant gene copies, the important aspect is that in both scenarios one copy of the duplicated gene is freed from selective constraints and by chance acquires a new function which is then improved by natural selection. This process is thought to occur very rarely in evolution for two major reasons. The first reason is that functional changes typically require a large number of amino acid changes; which has a low probability of occurrence. Secondly, because deleterious mutations occur much more frequently than advantageous mutations in evolution. This makes the likelihood that gene function is lost over time (i.e. pseudogenization) far greater than the likelihood of the emergence of a new gene function. Walsh discovered that the relative probability of Neofunctionalization is determined by the selective advantage and the relative rate of advantageous mutations. This was proven in his derivation of the relative probability of Neofunctionalization to pseudogenization, which is given by: where ρ is the ratio of advantageous mutation rate to null mutation rate and S is the population selection 4NeS (Ne: effective population size S: selection intensity).Classical model
In 1936, Muller originally proposed Neofunctionalization as a possible outcome of a gene duplication event. In 1970, Ohno suggested that Neofunctionalization was the only evolutionary mechanism that gave rise to new gene functions in a population. He also believed that Neofunctionalization was the only alternative to pseudogenization. Ohta (1987) was among the first to suggest that other mechanisms may exist for the preservation of duplicated genes in the population. Today, subfunctionalization is a widely accepted alternative fixation process for gene duplicates in the population and is currently the only other possible outcome of functional divergence.Neosubfunctionalization
Neosubfunctionalization occurs when Neofunctionalization is the end result ofExamples
The evolution of the antifreeze protein in the Antarctic zoarcid fish ''L. dearborni'' provides a prime example of Neofunctionalization after gene duplication. In the case of the Antarctic zoarcid fish type III antifreeze protein gene (AFPIII; ) diverged from a paralogous copy of sialic acid synthase (SAS) gene. The ancestral SAS gene was found to have both sialic acid synthase and rudimentary ice-binding functionalities. After duplication one of the paralogs began to accumulate mutations that lead to the replacement of SAS domains of the gene allowing for further development and optimization of the antifreeze functionality. The new gene is now capable of noncolligative freezing-point depression, and thus is neofunctionalized. This specialization allows Antarctic zoarcid fish to survive in the frigid temperatures of the Antarctic Seas.Model limitations
Limitations exist in Neofunctionalization as model for functional divergence primarily because: #the amount of nucleotide changes giving rise to a new function must be very minimal; making the probability for pseudogenization much higher than neofunctionalization after a gene duplication event. #After a gene duplication event both copies may be subjected to selective pressure equivalent to that constraining the ancestral gene; meaning that neither copy is available for Neofunctionalization. #In many cases positive Darwinian selection presents a more parsimonious explanation for the divergence of multigene families.See also
*References
{{Reflist Genetics