In the last 25 years, we have witnessed a revolution in the way that ecologists and evolutionary biologists approach their disciplines. This revolution has been fueled by the ability to dissect the genetic and molecular basis of variation that is partly the currency of these disci- plines. Using modern molecular techniques, we have begun to restruc- ture the spectrum of questions that can be addressed in studying the mechanisms and consequences of the ecology and evolution of living organisms. The molecular revolution has co me in waves, so to speak, with three particularly important developments. The first concerns the establish- ment and widespread use of pro tein analysis. Microcomplement fixation and isozyme e1ectrophoresis were the techniques around which much of the genetic work in ecology and evolution were once based. The next wave started with the development of recombinant DNA technology and centered around the use of restriction fragment length polymor- phisms (RFLPs) and sequencing of DNAs cloned in bacteria. This technology was the first to actually examine and accumulate genetic information at the nucleotide level. The most recent wave of technology that we are currently experiencing is based on our ability to amplify DNA sequences enzymatically via the Polymerase Chain Reaction (PCR). Several techniques re1ated to PCR have been developed and used extensively in ecological and evolutionary studies.

Four years ago we edited a volume of 36 papers entitled Molecular Approaches to Ecology and Evolution (Schierwater et ai., 1994), in which we attempted to put to gether a diverse array of papers that demonstrated the impact that the technologi cal revolution ofmolecular biology has had on the field ofevolutionary biologyand ecology. The present volume borrows from that theme but attempts to focus more sharply on the impact that molecular biology has had on our understanding of dif ferent hierarchical levels important in evolutionary and ecological studies. Because DNA sequence variation is at the heart ofeverypaper in the present volume, we feel it necessary to examine how DNA has affected study at various levels of biological organization. The majority of the chapters in the present volume follow themes es tablished in the earlier volume; all chapters by authors in the previous volume are either fully updated or entirely new and expand into areas that we felt were impor tant for a more complete understanding of the impact of DNA technology on ecol ogy and evolution. The collection of papers in this volume cover a diverse array of ecological and evolutionary questions and demonstrates the breadth of coverage molecular tech nology has imparted on modern evolutionary biology. There are also a broad range of hierarchical questions approached by the 17 papers in this volume."