Tuesday, March 15, 2016

From Weeds to Trees and Back Again (and Again)

In a post a few weeks ago, I discussed the evolution of trees and gave a few examples illustrating how simple it is (evolutionarily) to transition from a tree to a weed or back. "Evolutionary time" is generally interpreted to mean "a very long time" and probably "a long time ago". Most of the interesting evolutionary transitions people think about took place long in the past, so these interpretations aren't entirely without cause.

However, evolution definitely happens over very short time frames, we just have to pay attention for long enough to notice it. The image at right is a composite of a the flowers from a bunch of different individual specimens of the California Wild Radish. This population is derived from hybridization between feral Garden Radish (Raphanus sativus) and wild Jointed Charlock (Raphanus raphanistrum) in the central valley of California.

During the process of forming this hybrid population, the ancestral species were absorbed and eradicated. (Well...  garden radishes still exist just fine, but they're not actively reproducing in the wild of California any more.) The population is full of plants with all sorts of lovely shades of color (including nice combinations with both pink and yellow pigment) because the genetics of the population is still sorting itself out.

One specific plant caught my interest. Its flowers were pretty, but didn't stand out from the many others I'd already seen that day. What did stand out... was that this plant was a woody shrub which has been growing for several years. Both R. sativus and R. raphanistrum (the parent species) are annual weedy plants.

How long did it take for this [small] tree to have evolved from the weedy ancestors? The weedy parents merged together over just an estimated hundred years. So, it took something less than a hundred years for an albeit small tree to have evolved from its herbaceous weedy ancestors. I think that's a pretty quick transition.

Almost every reference I can find talking about the California Wild Radish only talk about herbaceous weedy forms. At best, there is the occasional reference to some plants being short-lived perennials. I haven't found any descriptions of woody perennial California Wild Radish plants growing as shrubs. The next time I visit this part of the country, I hope to spend some time looking for more specimens like this. If I'm lucky, I'll be able to collect some seeds and then grow out such a plant for a more detailed examination in the lab.