Paleontological Context for the Developmental Mechanisms of Evolution

Abstract

The emerging field of evolutionary developmental biology (evo‐devo) seeks to identify the specific genetic basis for plant evolution. Whereas molecular biological studies are rapidly exposing the developmental mechanisms of evolution, paleontological studies reveal the sequence in which evolutionary changes have occurred. Aspects of both fields are reviewed in this article. Plant structure develops along genetically patterned pathways, some of which produce distinctive morphological features. Such features serve as “fingerprints” for regulatory mechanisms, thus providing a record of growth regulation for both living species and plant fossils. Transformational series of structure through geological time form the contextual framework for inferring evolution from the fossil record, but the most widely practiced application of transformational series for plants, Telome Theory, has no developmental basis. By contrast, the fossil record of leaf evolution in seed plants documents morphological changes to which a growing understanding of genetic pathways can be correlated. Leaves are defined by a number of features, including determinacy and adaxial/abaxial identity. Fossils of early seed plants show stepwise acquisition of these characters, from which we can infer developmental modifications leading to the evolution of the characters. Previous work has revealed that an interregulated network of genes is involved in the specification of these features. The genes identified as being involved in various aspects of leaf development provide clues to the evolution of each character. In particular, expression of the AS1 and AS2 genes appears to facilitate both determinacy and bilateral patterning. The novel expression pattern of these genes may have been the key innovation that resulted in the evolution of leaves in seed plants. Combining molecular biological studies with knowledge of evolutionary patterns gleaned from the fossil record brings a new depth of understanding to the evolution of fundamental plant form.