Tewksbury et al. examine variation in "pungency" (that is, concentration of capsaicin) in wild populations of the chili Capsicum chacoense and compare it to rates of fungal infection in the fruit. The result is interesting, and not necessarily clear-cut: more-pungent fruits are less frequently attacked by an assortment of true bugs, and when these bugs attack, they can introduce fungal spores into the fruit, which ultimately destroys the seeds inside. So more pungency means less bug damage, and lower rates of fungal infection, and potentially more seeds.
Photo by bleu celt.
Photo by bleu celt.
But the story of chili pungency is more complicated than that. Back in 2001, Tewksbury and Nabhan showed that capsaicin helps ensure that chilis are eaten by birds instead of mammals [$-a]. Birds make good seed dispersers - they eat a fruit, then, um, pass the seeds on undigested; mammals, on the other hand, like to eat the seeds specifically. Capsaicin irritates mammals, but doesn't bother birds.
To complicate things still further, there's a downside to producing capsaicin. In this January's issue of the journal Ecology, Tewksbury and his coauthors showed that Capsicum chacoense seeds from more-pungent fruits also had thinner seed coats, which meant they were more likely to suffer damage in birds' digestive tracts [$-a].
So why are chilies spicy? The answer is, probably for all these reasons, and maybe more that haven't been discovered yet. This is a common situation in evolutionary biology - in many organisms, the traits that scientists find interesting may be useful in several different ways, and unhelpful in others. Very few traits experience natural selection in only one direction, as it turns out. The traits that we observe in nature are usually compromises between many different, sometimes directly conflicting, sources of natural selection.
J. J. Tewksbury, D. J. Levey, M. Huizinga, D. C. Haak, A. Traveset (2008). Costs and benefits of capsaicin-mediated control of gut retention in dispersers of wild chilies Ecology, 89 (1), 107-17 DOI: 10.1890/07-0445.1
J. J. Tewksbury, G. P. Nabhan (2001). Seed dispersal: Directed deterrence by capsaicin in chilies. Nature, 412 (6845), 403-4 DOI: 10.1038/35086653
J. J. Tewksbury, K. M. Reagan, N. J. Machnicki, T. A. Carlo, D. C. Haak, A. L. C. Penaloza, D. J. Levey (2008). Evolutionary ecology of pungency in wild chilies PNAS, 105 (33), 11808-11 DOI: 10.1073/pnas.0802691105