Today’s photos feature two sister species of monkeyflowers.
Erythranthe lewisii and Erythranthe cardinalis (formerly members of Mimulus) are rhizomatous perennial herbs that are found along streamsides and in other moist sites in western North America. Erythranthe lewisii occurs in high-elevation sites from 1200-3100 meters. Its pink flowers have a wide corolla, nectar guides, and a small nectar reward. They are bumblebee pollinated. Erythranthe cardinalis occurs on sites ranging from sea level to 2400 meters. Its flowers are pollinated by hummingbirds. They are red, have a narrow corolla tube and offer a large nectar reward. Despite these significant differences, the two species can produce fertile offspring.
Pink and scarlet monkeyflowers offer an interesting study on speciation. Since the two monkeyflowers produce fertile hybrids, it has been suggested that they constitute one single biological species (see Heisey et. al. 1971). Erythranthe lewisii and Erythranthe cardinalis are frequently hybridized in the greenhouse, but despite some overlap in range, they nearly never cross in the wild. Because the two species are reproductively isolated in the wild, we can expect that they will increasingly differ from each other until the point when they can no longer produce fertile hybrids. Erythranthe lewisii and Erythranthe cardinalis are of interest to botanists as they are actively undergoing the process of speciation and can offer insight into how new biological species arise.
Ramsey, Bradshaw and Schemske (2003) studied pink and scarlet monkeyflower in order to understand what factors were preventing them from reproducing in the wild. Ramsey et. al. found that ecogeography played an important role. Each species is less fit outside of its elevational range, resulting in a geographical separation between populations of Erythranthe lewisii and Erythranthe cardinalis. Pollinator preference also maintained a reproductive barrier between the two species. All of the hummingbird visits recorded by the researchers were specific to Erythranthe cardinalis, and 259 of the 262 recorded bee visits were specific to Erythranthe lewisii. A third factor contributing to pink and scarlet monkeyflower speciation was pollen competition. Erythranthe cardinalis produced few hybrid seeds, even when it was pollinated with mostly Erythranthe lewisii pollen. In either species, pollination with its sister species produced only about half as many seeds as usual. Using these factors, Ramsey and his colleagues were able to calculate that the degree of reproductive isolation between Erythranthe lewisii and Erythranthe cardinalis is 99.87%. Despite the ease of hybridization in cultivation, and the fact that wild hybrids are able to grow and survive well in the wild, Ramsey et. al. suggest that the high rate of reproductive isolation warrants the classification of Erythranthe lewisii and Erythranthe cardinalis as different biological species.