Under natural conditions, it would take thousands if not millions of years for a change like this to happen. But the act of poaching has changed that. Tusk growth is genetic and those elephants with shorter tusks or in the rare case of no tusks are the ones that are surviving. So the gene pool has changed. Elephants with shorter tusks are passing that gene to the next generation. So the whole process happens much more quickly. Also, remember that elephants have been killed for ivory for hundreds if not thousands of years already.Hopefully, this makes some sense:)