If your looking for a good example of how evolution works then look no further than out of your kitchen window! Two populations of Great Tits have been studied since the 70s one in the UK and the other in Holland, turns out that the UK population has a (significantly) higher percentage of birds with slightly longer beaks than their Dutch cousins. One possible cause of this difference is the higher propensity for people in the UK to use bird feeders in their gardens, leading to the longer billed variants successfully raising roughly one more chick in every five. Of course over time this difference would lead to the longer billed variants coming to (eventually) dominate the population over here.
Unlike Darwin, who also noticed variations in the beak shapes of similar birds, scientists today now understand the mechanism for these differences. The specific genes that control facial shape have been isolated and do indeed show evidence of selective changes in the the two populations. The hypothesis regarding the bird feeders being the driver for this difference is, as yet, unproven, but never the less it's certainly credible and fascinating if true.
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