Found surviving in a garden by two little girls in Ontario, Canada, this one-of-a-kind toad was likely affected by a genetic phenomenon that biologists call a macromutation.
A macromutation is a mutation that has made a significant impact on an organism, caused by a change in a regulatory gene that's responsible for the expression of an array of structural genes. While the process of adaptation is widely regarded to be driven by an accumulation of tiny genetic changes, biologists have suggested that macromutations could be the cause of certain adaptations.
According to Princeton University, macromutations appear to be the only explanation for differences in body segment numbers among arthropods, because very few changes to genes actually need to occur in order to cause such a huge physical change. Polydactyl cats are another example where a significant physical change - extra toes - can result from a small change to an individual's phenotype, as are these flies, with double or missing wings. But it's thought that, generally, macromutations don't have much influence on the process of evolution.
The undated photograph was taken by Scott Gardner of The Hamilton Spectator - a local newspaper in the Canadian town of Hamilton. It appears in the 1996 Richard Dawkins Book, Climbing Mount Improbable, with the following caption: "Macromutations do happen. This freak toad with eyes in the roof of its mouth is said to have been found surviving wild in a Canadian garden. This photograph was originally published in a local newspaper, The Hamilton Spectator."
Further on in Dawkins's book is a more detailed explanation for the photograph:
"The toad in Figure 3.2 is said by the photographer, Scott Gardner of The Hamilton Spectator, to have been found by two girls in their garden in Hamilton, Ontario. He says that they put it on the kitchen table for him to photograph. It had no eyes at all on the outside of its head. When it opened its mouth, Mr. Gardner said, it seemed to become more aware of its surroundings."
It's been suggested that the cause of the mutation was the result of a parasitic infection by a trematode worm (Ribeiroia ondatrae). Trematode infections have reportedly been linked to an increasing number of amphibian limb mutations, particularly missing, malformed, and extra hind legs. A 2002 study led by Geoffrey Stopper from the Department of Biology at the US Sacred Heart University reported that, "trematode cysts cause massive disruption and abnormal cellular growth involving the limb buds of infected individuals."
There's been no confirmation that trematode infections can cause the shifting of eyes, so for now, this little Canadian toad will have to remain a bizarre mystery.