Damouse writes:
To get a positive change, any of those authors must find an improvement. Any negative changes will die off.
Actually negative changes will tend to impair the carriers, but they will not necessarily die off. That is very context sensitive, and assuming the negative aspect is not immediately life threatening, it could be some generations before any effect was seen.
The point being that neutral and mildly negative mutations can and will exist in the population, adding to the variability.