How does crossing over affect evolution?

Crossing over allows genetic variants on the same chromosome to evolve independently, which greatly increases an organism's evolutionary potential.

If there were no crossing over, all genetic variants on a chromosome would be inherited as a block. Image a chromosome copy which contains a good variant--let's say, flu resistance--at one gene, and a bad variant--let's say, tapeworm susceptibility--at a different gene. Without crossing over, the population has to choose between flu and tapeworms. Crossing over can produce a chromosome with the good variant and without the bad one, allowing the population to move toward a better solution. This speeds up the rate of adaptation.