Bah, an elephant with no tusks is still an elephant.
If you shoot all the ones that spend most of their time on dry land, instead of in shallow water, they'll still be elephants.
Then if you shoot all the ones with long trunks, so the only ones that grow up have a short snout instead of a trunk, it will still be an elephant.
Then if you shoot all the tall ones, they'll still be elephants.
Then if you shoot all the ones with small heads, so that only ones with big flat heads are left, they'll still be elephants, even if they look and act like hippos by this point.
When the elephants take on the characteristics of hippos, they have become hippos. Yo're trying to refute speciation by changing the definition of species.