Structure trees and species trees: what they say about morphological development and evolution

The evolutionary history of morphological structures generally is equated with that of the taxa that carry them. It is argued here that, analogous to genes, developmental genetic pathways underlying morphological structures may be subject to developmental evolutionary changes that result, for instance, in duplication (serial homology analogous to gene duplication and paralogy). Entities that undergo evolution are expected to be related to each other as a tree. Just as with molecular evolution, "structure trees" and species trees sometimes may be incongruent, with implications for morphological homology concepts. Detection of structure trees through morphological evolutionary analyses may point to an entity that is maintained through evolution, possibly in part because it is a developmentally integrated structure ("individualized"). This idea is illustrated in a morphological evolutionary analysis of leaf primordia. These analyses suggest that leaf primordia in monocots and close relatives are related to each other as a tree and, therefore, are developmentally integrated, evolving entities. Among monocot primordia this tree structure breaks down, and it is concluded that there is no entity, the "monocot leaf primordium." However, one group of primordia is identified within monocots that have uniform characteristics and that are well represented by model species maize and rice. Such analyses of structure trees can facilitate the extrapolation and interpretation of results from molecular developmental and other comparative studies.