The imagery of nature is woven into our language and culture. The moral significance of “going green” or the fear expressed by the term “running wild” are two examples. It is ironic when we turn our nature-derived concepts to the task of describing nature. I have been talking about branching rates in many of my posts on evolution. Today, I will use this image of branching evolutionary trees on trees themselves. Not just on trees but on the flowering plants. This is a bit of a homecoming for me, having decided in first year university that plants would be my organism of choice as I studied ecology.
The radiation of flowering plants into a wide variety of colors and forms is not well documented in the fossil record, nor are there many consistent traits to group the hundreds of plant families into orders. DNA evidence has clarified many of the relationships between long recognized plant families. I am going to focus on the dicots or broad-leaved plants, being the most diverse group in the Plant Kingdom. The Species 2000 initiative identifies 48 orders of dicots, leaving out their narrow-leaved cousins (grasses and lillies) but including the most ancient groups of flowering plants like the magnolias and the waterlillies. I will be comparing this branching pattern of orders to families with the branching within one particular order of plants, the Lamiales or Mints. It is a broad assortment of plants including mangroves, african violets and the mint family itself, from which many of our spices come. As before, our question will be whether the pattern of branching among the early flowers sustains itself into more recent branching patterns.
Most botanists divide the dicots into two kinds: those that resemble roses (or rosids, green in the attached diagram) and those that resemble sunflowers (or asterids, blue in the attached diagram). The Lamiales belong to the latter group.
When we compare the branching of the early dicots to a model based on the branching within the Mints, we find that dicots were slow to get started. They rank dead last behind 99 simulations of early branching with an average of 5.4 families per order and a maximum of 28 families in a single order. The Mint- based model predicts an average of 26 branches with a maximum of 150.There is less than a 1% chance that these two branching patterns are the same.
Back in June 2012, I wrote, “There is no correlation between the branching rates of classes, orders or families, not even of their rank order. You simply cannot predict how often a family splits into species from the number of times its class splits into orders”. Closer examination of the birds, insects and plants has confirmed this conclusion with the notable exception of the beetles continuing the pattern set by their insect forefathers. Still, the general pattern of Haldane’s rule holds true. A very small number of life forms command the vast majority of life’s diversity. Possibly, this arises not from inherited branching patterns but from the consistently lop-sided distributions of branching that occurs among all evolving life.