How does natural selection affect undesirable traits?
Natural selection is an aspect of Darwin's theory of Evolution which dictates that traits more favorable for survival in a particular habitat will become predominant over less favorable traits in a gene pool over long periods of time.
A gene pool is the collection of genes possible for an species to acquire in a region through sexual breeding.
Natural selection will eventually wipe off an undesirable trait off the gene pool because organisms with a more favorable trait will breed more, and generate more offspring, whereas an organism without this trait would be at a disadvantage, and would die off.
Below is an example that fits for any given trait.