The Beak of the Finch by Jonathan Weiner describes (extremely well, it´s a great book to read btw.) the evolution of Finch beaks on the Galapagos Islands. There

The Beak of the Finch by Jonathan Weiner describes (extremely well, it´s a great book to read btw.) the evolution of Finch beaks on the Galapagos Islands. There was one particular Finch/beak size that had relatively small families and population growth, and was clearly being outcompeted by other, better adapted Finches. Until there was a severe drought, when that type of beak was able to take advantage of a neglected source of foods, which wasn´t accessible to the other Finches - who were more sucessful, but only when there was abundant easy food. After the resultant die-off this particular Finch subtype dominated of course. A lot of the discussion of evolutionary traits implicitly assumes an adaption to an unchanging environment, but that is very unlikely to be the actual situation, especially as in adapting, the adaption itself can change the larger environment.