Tiger snakes on some small islands off southern Australia depend on a single brief food source: the chicks of burrow-nesting shearwaters, available for a few weeks each year. Swallowing a fat chick whole demands a wider gape than swallowing a frog, and island populations show correspondingly larger relative head dimensions.
What makes the case interesting is that the difference is not purely inherited. Feeding experiments have shown that tiger snakes raised on larger prey items develop proportionally larger jaw structures, so part of the island pattern is developmental plasticity expressed within a lifetime, on top of any genetic divergence between populations.
These island snakes also endure a feast and famine cycle unusual for a temperate snake, gorging in the chick season and fasting for much of the rest of the year. How the genetic and plastic contributions divide, and how quickly the trait can shift when prey changes, are still active questions, and results from one island do not necessarily generalise to others.