Not quite evolution. The red-bellied black snake and the green tree snake with larger heads have killed themselves off through eating cane toads; therefore, the snakes with the smaller heads, that can't eat the toads, are going to be the ones left to reproduce. The snakes aren't evolving. The individuals with certain characteristics are just be handed a reproductive advantage.... The distribution of alleles is shifting dramatically in the overall population. Buy a clue, that's evolution is!