(This is just my theory)

I think that over time, it will; however, it won't happen abruptly. Eventually it will occur, as soon as instinct overcomes the habit. Otherwise, all of the people of South Africa will die out. If they linger in adolescence/childhood and cannot start families and have children before their death, the population will decrease. Nature will most likely prevent that before it happens.

But also, South Africa isn't isolated from the rest of the world. People migrate in and out. Either advances in medicine can prevent certain illnesses from claiming the people, or if what is killing them is gentetic, with the new people migrating to the area, the gene pool can be altered.