35th NPS abstract book

Session 2: Evolution
Chair: Andrew Groover
The origins of big: homoplasious evolution of
vascular cambia and arborescence
S2.1
WILLIAM E. FRIEDMAN 13:45–14:15
ned@oeb.harvard.edu
Department of Organismic and Evolutionary Biology, Arnold Arboretum,
Harvard University
The presence of a vascular cambium can be traced to at least 407 million years ago, in plants just
slightly larger than their primary-body-only close relatives. The evolution of true arborescence and
forested ecosystems would wait another 20 million years, when a group of fern-like plants
(cladoxylopsids, now extinct) appear to have gained in height and circumference the key features of
trees. Over the course of an additional roughly 25 million years, a vascular cambium arose
independently among members of an additional four major lineages: progymnosperms,
heterospourous lycophytes, horsetails, and sphenophytes. Despite the multiple origins of a vascular
cambium and arborescence, and the dominance of these linages in ancient forests, only the vascular
cambium of the progymnosperms has persisted through time to the present, as expressed in seed
plants. All other ancient clades that produced a vascular cambium have since gone extinct (with but
one minor exception). The striking homoplasy of vascular cambia in (at least) six distinct clades of
vascular plants over the course of a geological ‘blink of an eye’ suggests that there may have been
compelling adaptive reasons for the evolutionary innovation of plants with larger bodies, although
interestingly, developmental aspects of cambial behavior clearly differ between these different
lineages. Regrettably, the question of whether the independent origins of cambia during the
Paleozoic in different lineages of vascular plants relied upon similar or dissimilar underlying genetic
‘toolkits’ may never be answered; an answer is precluded by the inconvenient fact of extinction.
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