Gene duplications and subsequent mutations also led to the evolution of numerous sister nonapeptides across vertebrates, including isotocin in teleost fish and oxytocin in mammals. (bowdoin.edu)
There is significant evidence to suggest that at least some complex features of organisms result from what biologists call exadaptation, a phenomenon in which one genetic mutation confers a certain kind of survival advantage on an organism, and later, after subsequent mutations have occurred, the original mutation combines with the subsequent ones to confer a wholly different advantage. (firstthings.com)
This expansion may have involved a kind of snowball effect, in which initial mutations caused changes that were not only beneficial in themselves but also allowed subsequent mutations that enhanced the brain still further. (newscientist.com)