I do. Think of Darwin's finches. Some had long beaks, which assisted them in exploiting a new food supply. They flourished, and passed their long beaks on to the next generation. Instead of beak size, substitute lactose tolerance. Some people had it, and found it advantageous to herd dairy animals. They moved where their animals would thrive. Others, without that genetic tweak, didn't. It's the same principle.
"Seeing this gradation and diversity of structure in one small, intimately related group of birds, one might really fancy that from an original paucity of birds in this archipelago, one species had been taken and modified for different ends."
Source: Journal of a Voyage in HMS Beagle.