What to grow after the tree of life?

(crocuses at the Missouri Botanical Garden)

I had a fun conversation with Peter Raven of the Missouri Botanical Garden the other day. In essence, the topic that we discussed is what happens after the tree of life has been constructed? Today, thousands of researchers are working with a primary goal to reconstruct the phylogenetic relationships among organisms. But what happens after the tree is largely reconstructed? Yes, the tree of life will never entirely be reconstructed. One can argue that we can work on finer and finer branches, for example, trying to understand relationships among populations. 

Yet, I can't help but look at the tree and begin to ask why. Not why we are reconstructing it, but why it is shaped the way it is. What ecological forces might have been behind so many of the radiations? For example, we know about 10% of the world's flora either use the C4 or CAM photosynthetic pathways. Broadly speaking, these pathways are an evolutionary response to low atmospheric CO2 concentrations. These species are not just adapted to low CO2, nor have the other species not been influenced by low CO2, but what about the other 90% of the world's species? How many are a result of selection for success in shade? Or competition for nutrients? Or the ability to tolerate drought? Moreover, can we begin to compare the regions of the world and better understand the biogeography of species? Sure, we can see who is where, but do we know why? 

Evolutionary ecology is more than microevolutionary examination of processes that affect selection. There are broad questions about the evolution of the world's that cannot be answered by small-scale manipulations. One of the hurdles we need to get over is bias against what before have been called screening experiments. Hundreds of species are grown in common conditions and their traits measured to better understand the fundamental differences among species. These experiments are not trivial, but it seems clear that if we are going to answer these broad questions about the evolution of the world's flora, we need to screen more species. Not tens of species, but tens of thousands. Imagine a single greenhouse with all 10,000 of the world's grass species growing side by side. As far as I know, the largest ecological screening project measuring ecological traits of plant species included 640 species. After that, few experiments measured a hundred.

So what do we grow after the tree of life? If you ask me, it would be a garden.