If you live where spring beauty, specifically Claytonia virginica, is native, then you may have noticed great variations in flower color. We all know the influence pollinators can have on flower shape and color but how do we explain populations with such a spectrum?
Like me you might be thinking that it is related to its growing conditions. Well, a research paper by Frank M. Frey out of Indiana University would suggest otherwise. He chalks it all up to opposing natural selection from herbivores and pathogens.
Say what now? In a 2 year study, Frey has made some amazing discoveries. First, he made sure that Claytonia flower color is not a result of soil pH or anything like that by growing a ton of them in different conditions. He found that flower color is indeed genetic and is controlled by a couple different compounds. Crimson coloring comes from a compound called "cyanidin" and white colors comes from two flavonols, "guercetin" and "kaempferol". Frey then used spectrometry to analyze flower colors throughout the population and found 4 distinct color morphs ranging from all white to mostly crimson.
As it turns out, the flavonol compounds have pleiotropic effects in Claytonia. While they do produce white pigments, they also help defend the plants against herbivory and pathogens. Frey used a multitude of different analytical methods to assess overall fitness of each color morph and his results are jaw-droppingly cool to say the least.
Fitness of Claytonia was measured as total fruit production and total seed set. Because Claytonia needs a pollinator to visit the plant in order to produce fruit and set seed, reproduction is directly linked to pollinator preference. His research found that pollinators, which for Claytonia are solitary bees, do, in fact, prefer crimson color morphs. This helps to explain the greater number of crimson colored flowers in any given area because the more pollinator visits, the higher overall fitness for that plant. What it does not explain though, is why white morphs exist in the population at all.
As stated above, the flavonols that produce white pigmentation also beef up the plants defenses. Frey found that white colored flowers experienced significantly less predation than crimson flowers. Herbivory has serious consequences for Claytonia and plants that receive high levels of herbivore damage are far more likely to die. Because of this, white morphs, even with significantly less reproductive fitness, are able to maintain themselves in any given population. Wow!
If you're at all like me then you may need to pick you jaw up off the ground at this point. But wait! It gets cooler.... In areas where other white flowering plants like Stellaria pubera abound, white Claytonia morphs are even more rare. Why is this exactly? Well, Frey explains that this is due to a push towards a more pollinator mediated selective pressure. In areas where many plants share the same flower color, it pays to be different. This causes a selective pressure in these Claytonia populations to favor even more crimson color morphs.
Isn't evolution amazing?