5.3  Sunday, Jan. 4  Environmental Heterogeneity as an Agent of Selection: Why Aren’t All Chilies Hot? HAAK, DC*; MCGINNIS, L; LEVEY, DJ; TEWKSBURY, JJ; University of Washington haakd@u.washington.edu

Polymorphisms are a source of the variation that constitiutes evolutionary change and as such offer a window for viewing contemporary evolutionary processes. Polymorphisms can be maintained within a population through both neutral and selective processes. Natural selection can maintain polymorphisms as hybrids (heterozygote advantage), as rare/common alleles in a population (negative/positive frequency dependent selection) or through a less well documented process environmental heterogeneity. Environmental heterogeneity can impose natural selection on adaptive traits particularly under conditions of limiting resources. Here we report empirical evidence for selection maintaining a stable polymorphism via spatial heterogeneity in available moisture.
Chili peppers get their "heat" or pungency from a class of chemicals known as capsaicinoids. The relative abundance of two primary capsaicinoids capsaicin and dihydrocapsaicin explain over 95 percent of the pungency found in all chilies. The degree of pungency, or how hot the individual, is subject to environmental influence, ca. 40% genetically determined. However, the state of pungency (hot or not) is entirely genetic. Pungency appears to be a derived character as the most derived chilies (Capsicum annuum) are all pungent whereas basal chilies (Capsaicum cilliatum) all lack pungency. We have previously reported a species of chili (Capsicum chacoense) which occurs in natural populations in a putative center of origin for the chili pepper, Bolivia, that is polymorphic for pungency. If pungency is strictly adaptive then all chilies should be pungent across all environments. Herein we have identified an evolutionary tradeoff and mechanism which may offer an answer to the question, "Why aren’t all chilies hot?"