Like many other plants, the chili has a strategy for survival: make its fruit, the pepper, so nutritionally desirable that birds and other creatures will eat it and disperse the seeds. But the same things that make a chili pepper attractive to animals also draw bacteria and funguses that can kill the seeds. It has been thought that the chemicals known as capsaicinoids, which surround the seeds and give peppers their characteristic heat, are the chili’s way of deterring microbes. But if so, then microbial infestation should bring selective pressure on chilis — the more bugs, the hotter the peppers...