Royal Society Publishing   David C. Haak, Leslie A. McGinnis,
Douglas J. Levey, Joshua J. Tewksbury   Evolutionary biologists
increasingly recognize that evolution can be constrained by
trade-offs, yet our understanding of how and when such
constraints are manifested and whether they restrict adaptive
divergence in populations remains limited. Here, we show that
spatial heterogeneity in moisture maintains a polymorphism for
pungency (heat) among natural populations of wild chilies
(Capsicum chacoense) because traits influencing water-use
efficiency are functionally integrated with traits controlling
pungency (the production of capsaicinoids). Pungent and non-
pungent chilies occur along a cline in moisture that spans their
native range in Bolivia, and the proportion of pungent plants in
populations increases with greater moisture availability. In
high moisture environments, pungency is beneficial because
capsaicinoids protect the fruit from pathogenic fungi, and is
not costly because pungent and non-pungent ch