A very cool example of mutation and natural selection at work--in just the last few decades. Basically, a small group of lizards got transferred to a new island (by scientists) and were then totally ignored for 30 years. Their new island was a bit different than their old one. The lizards had survived on an omnivorous diet, but had eaten mostly meat, on their old island. On the new one, meat was rare, and they had to eat more plants to get by.
Head size changed a bit because of the diet change--but that wasn't all that changed. The lizards had evolved new structures in their digestive tract to help them digest plants. These structures are rare in lizards, and entirely unknown in this species of lizard...until now.
This is an example of evolution at its finest. A new environment with less available meat. The lizards are getting by, surviving but not thriving, and the lizards with larger heads tend to survive better and reproduce more (they have an easier time chewing plants), so eventually only large-headed lizards are left. And then, a mutation in the digestive tract--a mutation (or possibly a number of mutations) that are a big advantage to lizards, but only if they're eating lots of plants. The mutation wouldn't have been an advantage on the old island, where they ate mainly meat, but here on the new island, the mutation allows the lizard to better digest food, and with that comes a longer life, more offspring, etc. Lizards without this mutation can't compete and eventually die out, and so the lizards on this island are now very different from the lizard on the old island.
A beautiful, real-life demonstration of natural selection.