Lodgepole pine in the northern Rocky Mountains exhibits two cone types, one that opens at maturity and one that is opened only by heat exceeding 45 C (serotinous). Cone serotiny probably is a genetic adaptation to fire with 1-locus, 2-allele control. Despite a history of repeated fires in the northern Rockies, the frequency of the open-cone phenotype remains high. Simulations suggest that observed phenotype frequencies have been produced by a selection regime in which the average selection coefficients have been 0, or very close to 0 with a high variance. This suggests fire patterns variable in time or in time and space. Certain characteristics of lodgepole pine ecosystems influence these patterns, and thus play a role in maintaining the cone-type polymorphism.