One Gene or Many? Different Genetic Mechanisms Drive Convergent Evolution in Monkey Flowers

Abstract

Evolutionary history is riddled with examples of convergent evolution, in which the same adaptation appears independently across multiple populations as a result of similar selective pressures. Convergent evolution can proceed by identical, similar, or unrelated genetic mechanisms. The relative frequencies of these scenarios and the evolutionary constraints that produce them are poorly understood, partly due to a lack of diverse comparative models. One such promising model is repeated adaptation to serpentine soil, a soil environment characterized by abundant heavy metals, low nutrient content, and poor water retention. Many species of Mimulus monkey flowers have populations that can tolerate these soils, but most Mimulus populations cannot. I compared the genetic signatures of serpentine tolerance across the genomes of four tolerant Mimulus populations, in order to determine whether these different species employed similar genetic mechanisms. Previous work has identified a single locus strongly connected with serpentine tolerance in M. guttatus tolerant populations. I found that M. glaucescens also likely has a single or small number of loci responsible for this adaptation. However, serpentine tolerance in M. nudatus appears to be controlled by many genes of smaller effect, rather than a single locus. This vastly different genetic mechanism is surprising given the close evolutionary and ecological relationships of these species. The results of this study show that, even under similar circumstances, evolution can employ very different genetic strategies.