Evolution works thanks to natural pressures -- in this case, a mutation in the gene gives an elephant no tusks, and thanks to this they are not killed by poachers. That elephant is lucky enough to survive to pass on its DNA. The recessive gene, over many generations, grows more dominant as more and more elephants "benefit" from having no tusks by not being hunted.

So the elephants aren't reacting to poaching -- the "reaction" is a random mutation that happens to align with external pressures, causing a shift in genetics for the population.