In a second study of chilli pungency in the wild, Tewksbury, Levey, and colleagues summarize current views of the evolution of Capsicum species, and report on several years of surveying chilli populations in the semiarid countryside of southeastern Bolivia, the region where their spiciness may have originated. They found mixed populations of pungent and non-pungent plants, and pungency seems to be associated with higher elevations and possibly greater stress.
The scientists studied wild populations of Capsicum chacoense and two other species thought to be among the oldest, C. baccatum (whose domesticated varieties are known as aji) and C. eximium, in the springs of 2002 through 2005, when the fruits were ripe. They tasted the fruits of more than 1,500 plants at elevations over a range of 1,000 meters (3,000 feet), mapped the plants' locations, and analyzed their capsaicinoid contents. The populations were generally mixtures of nonpungent and pungent individuals, with the proportion of pungent plants, and the concentration of capsaicinoids in them, increasing at higher elevations.
So high altitude appears to favor spiciness. Why remains unclear. Tewksbury and Levey found that pungent fruits were less likely to show damage from fungal infection, and less likely to be eaten by animals if they had already fallen to the ground. Maybe the microclimate at higher elevation imposes greater physical stress, which makes the fruits more vulnerable to attack by both molds and animals: and capsaicin is an effective chemical defense against both. In any case, we're getting closer to finding out how nature managed to cook up the world's favorite spice.
Tewksbury, J.J. et al. Where did the chili get its spice? Biogeography of capsaicinoid production in ancestral wild chili species. Journal of Chemical Ecology 2006, 32, 547-64.