Or they may have unusually effective repair mechanisms. That may not be true. Developmental mutations occur all the time in living systems. For those who might not know polyploidy is the process whereby a plant doubles its chromosome number and becomes a new species overnight. Wheat and corn are polyploids.
Natural selection works on changes that have already occurred.
Natural selection is not the cause of the changes but the arbitor of the value of the previously made changes. The changes do not have to be gradual. Polyploidy isn't gradual. That toad with his eyes on the roof of his mouth that I mentioned in the last post did not get there gradually. His parents did not have eyes on their noses.