These Elephant species live in Savannahs, Safaris, or Jungles. Most elephants have long ivory tusks and are hunted by poachers that want to take their tusks to sell. There are mostly female elephants that are tuskless (they don't have tusks).
A lot of elephants are killed by poachers because of their tusks. Some females are tuskless, not a lot of males are tuskless. The elephants with tusks are more likely to be killed and have a lower fitness than those without tusks.
The surviving elephants have offspring that have a chance of being tuskless and a higher fitness in order to survive and reproduce.
Over generations more and more elephants are born tuskless and have a higher fitness, in order to avoid being killed by poachers.
Eventually, there will be more tuskless elephants than elephants with tusks because most of the time the tuskless survive and are able to reproduce more elephants without tusks.