I am interested in the evolution of morphological diversity. Even the most cursory view of the living world reveals a remarkable diveristy of shapes, sizes, and lifestyles. Where did this diversity come from? To answer this question requires two major groups of questions. First, in terms of ultimate causation, what are the evolutionary forces responsible for pushing populations from one part of morphopace to another? The force most commonly invoked is natural selection, and the changes between morphologies are often explained by adaptive evolution. The second major category of questions are those relating to proximal causes, which I like to call the mechanisms of evolutionary change. That is, what evolutionary and ontogenetic processes permit movement through morphospace if it is favored by natural selection?
In my laboratory, we are employing a comparative strategy to address both questions of adaptation and evolutionary mechanisms. We are identifying clades of woody plants of extraordinary morphological diversity to provide long-term comparative study systems. These systems include Moringa (Moringaceae), the simaruba clade of Bursera (Burseraceae), the Mexican clade of Manihot (Euphorbiaceae), and the Beaucarnea- Calibanus clade of Nolinaceae.
Major questions we are examining in these clades include: To what extent do traits covary and why? How are traits constrained in their variation? How are morhpological differences associated with functional differences? What is the potential role of adaptive evolution in these changes? We use the stems of woody plants as a study system because they have relatively few cell types (~20, as compared to the more than 200 cell types in humans), and serve relatively few functions; the chief functions of all woody stems are mechanical support, conduction of water, and storage of photosythates. Moreover, because wood cells remain permanently in place, a woody plant is a record of its own ontogeny. Our results are suggesting that covariation between anatomical, allometric, biomechanical, and hydraulic features of stems is due to functional and physical constraints, and not due to arbitrary limits of ontogenetic systems (developmental constraints).
We are also taking a complementary approach that examines the ways that entire communities fill morphological and functional space. For example, the plants of the Pedregal de San Ángel, which I can see from my office here in Mexico, span a range greater than that seen by most anatomists in a lifetime: Sedum oxypetalum, whose wood is composed only of vessels and parenchyma, grows adjacent to Dodonaea viscosa, which has extremely reinforced wood that withstands extremely negative xylem pressures. How do the species of a community partition morphological and functional space? Does each species occupy a distinct region? Can guilds of species of similar structure and function be distinguished? We are currently generating phylogenetic, morphological, and functional data for this and other communities to examine these questions.