Why, for instance, could he find mild and spicy peppers of the same variety growing in the wild in close proximity to each other? His answer: fungus. A genius of the Fusarium fungus is specific to peppers. In the wild the higher the concentration of capsicum in the peppers, the lower the rate of fungal infection in the pepper plant. And mild peppers often carried heavy loads of the fungus.
The tendency toward high concentrations of capsicum is long lived in the gene pool of the peppers that develop it, which is why healthy plants still produce hot peppers. Then centuries ago spice loving humans came along and selected certain peppers for their heat index to pep up their food, perpetuating those hot genes. Because of some antifungal properties in hot peppers, many cultures have used peppers to preserve food over the centuries.
Chemist Wilber Scoville invented the scale to measure the "heat" in peppers in 1912. The SCU or Scoville Heat Unit measures how hot a pepper is:
Bell Peppers - 0
Poblano - 1,000 - 2,500
Jalapeño - 2,500 - 8,000
Serrano - 7,000 - 25,000
Cayenne - 50,000
Chiltepins - 70,000
Habanero - 200,000 - 300,000
Scotch Bonnet - 200,000 - 300,000
Naga Jolokia - 855,000 - 1,041,427