Indeed, according to the equations of population genetics, the standard mathematical expression of neo-Darwinian theory, even slight losses in fitness will subject the disadvantageous traits that produce such losses to purifying selection, thus eliminating them. That means that even many protein sequences that retain a significant, though diminished, portion of their original function nevertheless will not survive the winnowing effects of the neo-Darwinian mechanism. Thus, the gradual transformation of one functional fold into another was a complete nonstarter.
