A couple of years ago we had a visiting class of students from Lewis & Clark College in Oregon for a semester of New Zealand studies. They were bright, friendly, and fun to be with as we travelled the length of New Zealand and even to the Subantarctic islands. I’ve enjoyed keeping in touch with them through Facebook as they graduate and spread out over the USA beginning their careers and further study.
|Lewis & Clark group on Campbell Island|
Last week one of these students posted a photo on Facebook of this amazing little flower, the steer’s head (Dicentra uniflora). It’s a spring-flowering plant of British Columbia and western USA, often seen in Yosemite and such places. She asked, “How can a flower like this exist?” and I thought, “What a great question for a blog entry”.
There are many answers to the question, depending on what is really being asked. First, these students aren’t credulous creationists, so this wasn’t the standard statement disguised as a question, suggesting that something so wonderful couldn’t possibly have evolved.
A first answer might look at the question of how the flower functions. We can be pretty sure it’s not actually mimicking a steers head, but in that part of the world, it’s a natural thing to see the likeness, a kind of pareidolia again. The key to the function question is, “What pollinates it?” There’s a thesis by Julia Rayl at Washington State University on just this topic, but it appears not to be available on line. However, a published guide to selecting plants for pollinators lists bees as the pollinators of the steer's head. That’s consistent with other American species of Dicentra, which are pollinated by queen bumble bees (Bombus). It’s also what I would have guessed from its morphology: a closed flower that needs to be forced open by a visitor, with a nectar-spur. So the form of this flower works to attract bees and probably to make it difficult for other insects to steal nectar or pollen.
|Corn poppy (Papaver rhoeas), ramping fumitory (Fumaria capreolata), wall fumitory (F. muralis)|
Another way to answer the question is to ask how it evolved. Dicentra is a member of the family Fumariaceae, which all have flowers a bit like this. Fumariaceae are close relatives of the poppies, Papaveraceae; in fact many botanists treat these two as a single large family. The key differences are in the flowers and the sap. Poppies have a milky latex that becomes sticky and finally solid as it dries; the dried latex of Papaver somniferum is opium. In Fumariaceae the juice is watery. Poppy flowers (above, left) are open and symmetrical with many separate stamens, whereas most Fumariaceae flowers (above, centre) are closed to form a tube-like structure, with a pair of branched stamens (that look like bundles of three) on each side of the flower. Both families have flowers with four petals—not a common number in the flowering plants—considered to be presented in two opposite pairs. In poppies the outer pair and the inner pair are much the same, whereas in Fumariaceae they're different and one or both outer petals has a spur. In Dicentra both outer petals are spurred, and in Fumaria, only one is. The evolutionary history of the poppies and fumitories can be shown in a branching family tree diagram, or cladogram:
This shows the common ancestor of Dicentra and Fumaria, and a bunch of other genera, was the sister species to the common ancestor of the poppies (represented here by Eschscholzia and Chelidonium). The diagram was developed by comparing the changes in DNA sequences among these plants, and it's a bit simplified from the original. We can use the order of branching to make some inferences about the likely order of change in flower features, by mapping the flower features onto this diagram.
|From top, Californian poppy (Eschscholzia californica), steer's head (Dicentra uniflora), ramping fumitory (Fumaria capreolata)|
It looks like the first evolutionary step was the development of an asymmetrical flower with two short nectar spurs, like that found in Hypecoum, sister to the rest of the Fumariaceae. Next, the flower has become closed in the ancestor of Dicentra, Fumaria, and their relatives. The curling back of the outer petals is accentuated in the steer’s head flower, Dicentra uniflora, but it’s seen in other species of Dicentra too, and related flowers, like Lamprocapnos spectabilis. A later step, after the Dicentra lineage diverged, involved loss of one nectar spur to make a flower with only one plane of symmetry, typical of Corydalis and Fumaria.
The key to understanding this is the realization that evolution of complex structures occurs step-wise, with each step being adaptive. Richard Dawkins explains it well in Climbing Mount Improbable.
A third way to answer is to ask about what’s going on as the flower develops. In other words, how do the evolved differences at the gene level affect flower development to actually change the flowers as they grow? Nowadays, botanists are homing in on the actual genes that are responsible for features like floral asymmetry and changes in the timing of development, and early results in Dicentra and Fumaria are promising, even though they haven’t yet fully answered these questions (Kölsch & Gleissberg 2006).
In short, we can explain how the flowers work to attract their pollinators, the series of evolutionary steps that led to this flower as well as the order they occurred in, and I’m confident that very soon the mechanisms of development of the steer’s head flower will be known.
Kölsch A; Gleissberg S (2006). Diversification of CYCLOIDEA-like TCP Genes in the Basal Eudicot Families Fumariaceae and Papaveraceae s.str. Plant Biology 8: 680 – 687