Directional selection occurs when individuals with traits on one side of the mean in their population survive better or reproduce more than those on the other. ... Directional selection does the “heavy lifting” of evolution by tending to move the trait mean toward the optimum for the environment.
<span>Children who suffer from Tay-Sachs disease have the inability to break down certain lipids in the body. The lipids accumulate in the brain and interfere with normal biological processes.</span>
