Human civilization could be changing the course of Evolution. Technological advances are adding new selective pressures and neutralizing old ones.
Because of advances in medicine, people with asthma can live with and pass on the gene to future generations. Thus what used to be a disability that would kill someone and eliminate the gene in the 13th century is now neutral. With planes in the air more now than ever before, birds with favorable genes are randomly killed in flight.
These new selective pressures allow asthma to spread, aided with advances in medicine, but we can eliminate this problem with stem cell research.