Wheat - Early Agricultural Selection
Early agriculture's shift from hunter-gathering to domestication cannot pinpoint the first instance of domestication, but cereals such as wheat and barley are often referenced as some of our earliest crops. According to Jared Diamond in Guns, Germs, and Steel, early domestication was made possible in species such as einkorn wheat as a result of a simple genetic mutation that prevented some wheat stalks from shattering and spreading their seeds. While this may normally have been an unfit mutation for wheat spreading its own seed, wheat that remained on its stalk could all of a sudden become more available to humans to gather. By eating and defecating the wheat's seeds, humans fertilized the mutated wheat and allowed it to further propagate. As this wheat became more readily available, it was further selected for (however unwittingly) by humans and eventually came to be domesticated.
Corn - Selective Breeding
Corn is one of the best examples of the impacts that humans' selective breeding can have on crops. While scientists believe corn production could have started as much as 10,000 years ago by Native Americans, 7,000 year old fossils show corn cobs were approximately one inch long. Not only has corn's zone of growth expanded dramatically (thanks, in part to its C4 photosynthetic abilities to withstand harsher climate conditions), but today's corn cobs are around a foot long as a result of continuous human selection for larger yield.
Similar examples can be seen every day - wild versus cultivated strawberies, bananas, etc.
Below: Fossilized Corn
Below: A Poor Baby Dressed as Baby Corn for Halloween