In the early twentieth century, evolutionary biologists argued whether there was enough genetic ...

In the early twentieth century, evolutionary biologists argued whether there was enough genetic variation in populations to explain the diversity of life by natural selection.
 
  With the advent of protein electrophoresis and DNA-level analysis, the problem turned on its head: the neutralist school arose from the idea that there was too much genetic variation to be maintained by natural selection. What are some of the arguments and observations neutralists cited to support the idea that much variation is neither favored nor disfavored by natural selection?