Phylogenetic patterns from morphogenetic processes
Thomas Stach, Institut für Zoologie, Freie Universität Berlin, Germany
The so-called biogenetic law “ontogeny recapitulates phylogeny” is currently reanimated in form of the “molecular phylotypic stage.” I argue that both concepts are empirically questionable and logically untenable. Instead, I propose to include ontogenetic characters from all available organismic levels in formal cladistic analyses in order to properly evaluate potential evolutionary information and to infer phylogenetic hypotheses. Using examples from comparative developmental morphology of deuterostomes, I suggest how embryological data could be profitably utilized in a phylogenetic context. Simultaneously, I demonstrate the recent progress in microscopical techniques and its effect on embryological research. Computer assisted three-dimensional reconstructions allow detailed comparisons that can be used to propose hypotheses of primary homology. Adding the fourth ontogenetic dimension, time, considerably increases the number of characters that can be investigated and reveals similarities in embryological processes, such as between neurulation in chordates and neurulation of the collar cord in enteropneusts. Similarly, 4D-microscopy can reveal correspondences between morphogenetic events on a cellular level. Moreover, this latter technique elegantly reveals precise cell-lineages at a level unattainable for traditional methods. These advances together with the current explosion of molecular data and in combination with internet-based research tools such as databases and interactive collaborative phylogenetic matrices hold great potential for comparative morphology.