When One Becomes Two

By far one of the most beautiful spring flowering plants in the eastern forests has to be blue cohosh (Caulophyllum spp.). Around this time of year they begin poking up through the leaf litter. The deep purple stems gradually give way to shades of blue and green as the leaves and flowers expand into the springtime sun. They seem otherworldly and finding them among the speckled leaves of trout lily is a sight I could never tire of.

For as long as it has been known, North America's Caulophyllum has been considered a single species, Caulophyllum thalictroides. The specific epithet hints at how similar this species can look to the meadow rues (Thalictrum spp.). However, a keen observer could tell you that there are apparent differences between some blue cohosh populations, especially in the northeast. Some cohosh flower much earlier than others. Also, there are differences in flower color. Some plants sport flowers decked in deep maroon whereas others are pale green. These differences have led some authors to list the purple flowering variety as a subspecies, Caulophyllum thalictroides giganteum.

Caulophyllum thalictroides

Caulophyllum thalictroides

More recently, however, it has become apparent that these two varieties may actually be separate species. Though their ranges overlap, what is now being called Caulophyllum giganteum is distributed much farther north than C. thalictroides. The key differences between these two has to do with flowering time. If these two species become reproductively active at different times, then they are in fact reproductively isolated from one another. Though they can hybridize, the resulting seeds experience reduced viability. By studying somewhat cryptic species such as these, researchers are gaining a better understanding of just how speciation works.

Photo Credit: Tom Potterfield (http://bit.ly/1E0JcQ5)

Further Reading:
http://www.jstor.org/discover/10.2307/2426731?uid=2&uid=4&sid=21106474718243

http://plants.usda.gov/core/profile?symbol=CAULO