The finches started of with all the same genes however as more of the fiches reproduced variation occurred wether it be environmental or genetical. Some of these variations proved helpful, for example a thicker layer of fat to survive in the cold weather or longer and thinner beak to make it easier to catch prey in the wild. Therefore the finches which don't have the variation won't win in competition with other finches that have variations and so the finches which have the variation will survive as they beat all competition and therefore reproduce with other finches that have the variation passing as a gene which makes the finches more varied as new variations are always been created and followed through