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....
That's evolution. The populations of the two snake species are experiencing selective pressure from the cane toad population. As that pressure continues, the morphology of the snake populations is changing. After 20-25 generations, that change is already noticeable.
This isn't 'evolution' at all--it's patently pro-evolution lies put forth as 'science.' Bigheaded snakes can eat big poisonous things and die. Smaller specimens of the same species then survive--the 'fittest' paradoxically being the smallest and presumably weakest...although presumably also the toads are smaller when young and could be swallowed by smaller snakes. (Read "The Andromeda Strain" ...) Breeders have made miniature horses, miniature pinschers, poodles etc. by similar selective breeding. You show me a snake that can type Othello or sing Sorrento--or a dolphin that's built an underwater hospital--and I'll give a little credence to the hypothesis of evolution.
Red-bellied black snakes live in the southern part of Australia. Cane toads live in the north. Regards, Byron
The distribution of alleles is shifting dramatically in the overall population. Buy a clue, that's evolution is!