When does the exponential growth of a population stop?

Dramatic evolution within human genome may have been caused by malaria parasite. A genetic mutation that protects people from a common form of malaria spread like wildfire in sub-Saharan Africa about 42,000 years ago, according to a new study.

People develop sickle-cell disease, a condition in which the red blood cells are abnormally shaped, if they inherit two faulty copies of the gene for the oxygen-carrying protein haemoglobin. The faulty gene persists because even carrying one copy of it confers some resistance to malaria.05-May-2011

Explanation:

An evolutionary force, called balancing selection, appears to be responsible for maintaining defects in our DNA associated with diseases, such as sickle cell anemia, because the mutation's damaging effects might be offset — in a biological way of thinking — by its potential benefits.