What molecules do (and don't) say about angiosperm evolution

Abstract

Phylogeny and Evolution of Angiosperms is one of the more important books about the evolution of flowering plants that has been published during the past quarter century. It would be a worthy addition to the literature if it were nothing more than a comprehensive summary of the contributions of DNA sequences to the production of phylogenetic hypotheses for flowering plants. However, Phylogeny and Evolution of Angiosperms represents more than just an overview of molecular phylogenies.For those who have not kept abreast of how molecular data have revolutionized our understanding of flowering plant phylogeny, Phylogeny and Evolution of Angiosperms is a great place to get up to speed, from the major clades to relationships among families. The authors discuss clades of plants within the context of the ‘pre-molecular’ and ‘pre-cladistic’ systems of giants such as Cronquist, Takhtajan and Thorne, who erected systems from their amazing knowledge of plants. Placing molecular phylogenies within the historical context of earlier classification systems enhances the value of the discussions to readers who might not otherwise be interested in the nuts-and-bolts of producing a three-gene phylogeny or the lessons learned from the analysis of large data sets. The depth with which the authors compare and contrast concepts of relationships demonstrates their commitment to making Phylogeny and Evolution of Angiosperms a scholarly synthesis rather than a collection of simple statements that, for example, the molecular data proved Cronquist wrong on a particular point.Although Phylogeny and Evolution of Angiosperms is a testament to the contributions of molecular data to our understanding of the pattern of angiosperm evolution, it is also an elegant and troubling statement of how little is known about such basic information as the distribution of anatomical and morphological characters within the flowering plants. As impressive as the production of phylogenetic hypotheses from DNA sequences has been, that task is, in reality, easy compared with documenting and understanding in some depth the distribution of characters and character states beyond rather simple assessments (as a general example, ‘herbaceous’ versus ‘woody’ without even the most rudimentary anatomical study of the origin of secondary xylem). The authors provide a nice example of this problem with their discussion of coding the presence of vessel members (water-conducting elements). The success of molecular approaches to reconstructing phylogeny has, in fact, exacerbated the problem of the lack of more ‘traditional’ kinds of data for flowering plants, and the authors acknowledge the problem when they state ‘It is probably fair to say that attention to formidable problems of morphological character analysis, as well as to other non-DNA characters, has tended to wane during the past decade in the understandable enthusiasm for molecular systematics.’ The authors then call for a swing back to broader training for aspiring plant systematists. This call is laudable on their part and echoes what others have been saying for several years.Most chapters end with a ‘Future Research’ section. Such sections in books often consist of nothing more than calls for ‘more of the same’, and there is a bit of that in some chapters of this volume. Overall, however, the authors present candid, insightful assessments of what is required to understand angiosperm phylogeny above and beyond sequencing more regions for more taxa. It will come as no surprise to those plant systematists trained before the molecular revolution hit that future research outlined in the sections addresses some of the same questions that have been around for many decades (e.g. need more fossils, etc.). Although these sections have a sort of ‘back to the future’ flavor, it must not be forgotten that, thanks to DNA sequencing, the old questions can now be investigated and interpreted within the framework of robust phylogenetic hypotheses that were not available to earlier botanists.No doubt those who read Phylogeny and Evolution of Angiosperms will find various issues with which they could quibble; this reviewer found several. However, the importance of the book transcends the quibbles. It is a statement of where the field of plant systematics finds itself after undergoing a revolution for more than a decade. The volume also offers a vision of where the discipline should be moving. To their credit, the authors do not pretend to have easy solutions to the many long-standing questions that must be answered to achieve their vision. But, Phylogeny and Evolution of Angiosperms will prompt thought, discussion and debate. Is that not what any good book should do?