The Darwin's Finch story I briefly mentioned is a fascinating part of a long term study by husband and wife team Peter and Rosemary Grant. They now have an amazing 37 year data set on these small birds on the Galapagos.
One of the latest parts of the story to be revealed was published in Science in 2006 as Evolution of Character Displacement in Darwin's Finches. It's also included in their news summary as Competition Drives Big Beaks Out of Business.
I think the fascinating part of this story is that it is the interaction of environment and competitors that drives evolution. Whilst resources are plentiful competition is weak but during the drought the effects of competition became severe.
Here we report that a Darwin's finch species (Geospiza fortis) on an undisturbed Galápagos island diverged in beak size from a competitor species (G. magnirostris) 22 years after the competitor's arrival, when they jointly and severely depleted the food supply. The observed evolutionary response to natural selection was the strongest recorded in 33 years of study, and close to the value predicted from the high heritability of beak size. These findings support the role of competition in models of community assembly, speciation, and adaptive radiations.