All organisms - from the AIDS virus, to bacteria, fish, and humans - must evolve to survive. Despite the central place of evolution within biology, there are many things that are still poorly understood. For Charles Darwin, the driving force behind all evolution was natural selection. More recently, evolutionary biologists have considered that many mutations are essentially neutral with respect to natural selection. Many questions remain. Are molecular differences between species adaptive? Are differences within species adaptive? Modern biotechnology has enabled us to identify precisely the actual DNA structure from many individuals within population, and thus to see how these DNA sequences have changed over time and to answer some of these questions. At the same time, this knowledge poses new challenges to our ability to understand the observed patterns. This volume outlines the biological problems, provides new perspectives on theoretical treatments of the consequences of natural selection, examines the consequences of molecular data, and relates molecular events to speciation. This book should be of interest to upper level (3rd/4th year) students, and graduate students or researchers in population biology, population genetics, evolutionary biology and theoretical biology.