Evolutionary changes over the fifty-year history of a hybrid population of sunflowers (Helianthus)

Although there are many studies of the evolution and ecology of hybrid zones, few extend over long enough time periods to track evolutionary changes in the zones or assess the ultimate outcome of hybridization. Here we describe the current genetic and morphological composition of a hybrid sunflower population relative to its initial makeup 50 years ago. It appears that few genetically pure parental plants remain in the hybrid population and the average phenotype has shifted from an initial bias toward Helianthus bolanderi to a predominance of H. annuus-like plants. The similarity to H. annuus is more pronounced for morphology than for neutral genetic markers. In contrast to the shift in morphology that occurred primarily in the past 40 years, overall pollen viability increased to its current level during the first 10 to 15 years of hybridization, indicating the presence of strong fertility selection. Dramatic differences are seen in morphology, genetics, and pollen viability between the eastern and western halves of the population, thus confirming observations by previous authors and suggesting that selection pressures in the two halves differ. The trends seen in this hybrid population over the past 50 years suggest that H. bolanderi is undergoing genetic assimilation, and this trend may be representative of its fate throughout its range.