Final answer:
John Garcia discovered taste aversion in rats, where the rats learned to avoid flavors associated with illness, demonstrating a biological adaptation aiding in the avoidance of harmful foods.
Step-by-step explanation:
John Garcia discovered that rats developed an aversion to certain flavors associated with illness, even with an extended time lapse between tasting the flavor and the onset of illness. This phenomenon, called taste aversion, suggests an evolutionary adaptation where animals learn to avoid foods that may have harmful effects on their health. Garcia and Koelling's experiments highlighted that this form of conditioning is more likely to occur with flavors than with visual or auditory cues, adding to the understanding that there are biological constraints to learning.