What is the connection between variation and natural selection?
If the variation of an individual is positive, then its traits will most likely be passed on to its offspring. If natural selection favours those with the positive variation, the variation will become more frequent within the population and will eventually be considered a trait.
However, if the variation of an individual is negative, then its traits will most likely not be passed on to its offspring due to a decreased chance of survival. Natural selection will not favour those with the negative variation, further decreasing the chance of survival for those who inherit the trait.