When is the last time you saw a purple organism? Green, brown, grey, these are common – but purple is reserved for an odd leaf, a strange fruit (eggplant, maybe), some algae and bacteria, and the odd marine animal. That’s one of the reasons I so enjoy spending time in the subalpine zone. Purple flowers are readily seen. Delphinium, Gentiana, Geranium, and of course, Castilleja – the paintbrush.

My favorite is Castilleja rhexifolia, the alpine paintbrush. I took a photo of one two summers ago, and just found out it won the Colorado Native Plant Society‘s annual photo contest.

The image is a simple way to share the joy in one beautiful part of the world, but also a chance to explore the origin of purple coloration, and why it is so rare. It’s a topic I’ve never thought about much before, and the first thing I learned is that it isn’t actually so rare. Flower coloration primarily comes through the synthesis of anthocyanin pigments. Before the evolution of flowers, anthocyanins had already been evolved in other species and functioned to deter insect herbivores and prevent sun damage in young leaves. Anthocyanins are all synthesized through a core set of pathways (precursors including DH-kaempferol, DH-quercitin, and DH-myricetin) that can yield red, blue or purple coloration. Blue/purple is the most complex to synthesize, but also the most common and the most evolutionarily basal within the flowering plants. The short version of this biochemical digression is that I should actually be more surprised by red flowers than purple ones.

That answer is only part of the mystery, however. What determines the color of any given species? There are two plausible possibilities. The first is pleiotropic selection – if flower color is genetically linked to other traits, then selection on those other traits can lead to inadvertent changes in flower color. The second, and more popular, is pollination syndromes. If certain species are more sensitive to some wavelengths of light, then species should evolve flowers that are easily seen by these pollinators. In this theory, the flower’s color indicates the evolutionary ‘best guess’ of what the plant ‘thinks’ the pollinator wants to see. This idea was first proposed by Raven, and later modified by Possingham and Rodriguez-Girones.

The evidence for what drives flower coloration is unfortunately less clear. A recent review of the topic by Rausher found no clear support for any of the two possibilities. On the other hand, there is more recent evidence from Shrestha et al. in Australia that birds and bees see blue and red differently, consistent with the species they pollinate. Rausher did document an intriguing pattern – evolution often transitions from blue flowers to red flowers, but not the other way round. This should make sense – red flowers involve loss of function in the anthocyanin pathway, and it’s hard to recover function once it’s been lost. (There are a small quantity of possible mutations that can recover function, while there are a much larger number that can make the problem worse. Think of trying to fix your car.) But what this means, overall, is that coloration may be somewhat random over long evolutionary times. Genetic drift – that is, changes in phenotypes simply due to sampling effects and random mutations – may be a sufficient explanation for the whole problem, with pollinators being an effect rather than a cause of the pattern.

This answer may not be satisfying. It depends entirely on us assuming the structure of the anthocyanin synthesis pathway, and of the vision systems and very existence of birds and insects. Asking why questions in evolutionary biology can take one down a long tunnel from which there is no easy exit. It is a lot to think about when just appreciating an image of a purple flower on a midsummer’s afternoon.

N.B. – Nature’s Palette by David Lee is an excellent popular book that touches on this topic and many others.