This is a rather interesting. Basically people have been hunting and killing elephants for their tusks since ages. Ivory is then carved from the tusks and sold. Elephants with larger tusks are hunted first and therefore have less chance to survive and breed. The ones that do remain have smaller tusks and according to ENN, the average African elephant’s tusk size has gone down by half in the last century and a half.
I would guess we will eventually end up with mostly tusk-less elephants since there is (hopefully) no use for hunting them. Fascinating simulation of evolution.