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In the Light of Evolution Volume III: Two Centuries of Darwin
lation genetics into the emerging evolutionary synthesis (Dobzhansky, 1937). Today, in the genomics era, scientists routinely extend studies of natural selection and trait evolution to the level of DNA itself, as several chapters in this book will attest. Genomic dissections are also providing fresh insights into the ancient mystery alluded to in the title of Darwin’s seminal work: how species originate. Ironically, The Origin says relatively little about the evolution of reproductive isolating barriers, which under the modern biological species concept are key to understanding cladogenetic (speciational) processes.
In the opening chapter of this book, Sara Via takes a fresh perspective on the origin of species by characterizing genomic regions that appear to be diverging early in a speciation process. She calls this the “magnifying glass” approach for speciation in action, and contrasts it with the more traditional “spyglass” approach in which each completed speciation is characterized retrospectively by scrutinizing genetic differences between established sister taxa. Via develops and presents genetic evidence for a model in which incipient species become, in effect, genealogical mosaics in which ecologically important genomic regions (i.e., those under divergent ecological selection, sometimes even in sympatry) become resistant to genetic exchange, while gene flow remains possible over most of the genome. The key genomic regions under divergent selection become focal points for “divergence hitchhiking” by linked loci, as they reduce the porosity of the emerging species boundary to gene exchange. Under this scenario, Via views divergent selection as the motivator of genealogical differences (in these particular genomic regions) that later will crystallize into the branching pattern in the species phylogeny. Eventually, in responses to selection, genetic drift, and mutation, gene genealogies in the remainder of the genome will come into topological concordance with the species phylogeny, but these additional genetic differences will have been the effect of speciation rather than its cause.
Some of the richest biological quarries for extracting information about natural selection and speciation involve clades (monophyletic groups) that have arisen via rapid adaptive radiations. Darwin presaged such evolutionary analyses in his considerations of different forms of mockingbirds in the Galápagos Islands, and in the various finch species he collected there that now bear Darwin’s name (but whose evolutionary appraisal mostly awaited later researchers). In Chapter 2, Scott Hodges and Nathan Derieg take a modern approach to speciation analysis by integrating observations from field studies with molecular and phylogenetic dissections of genes for traits (especially flower color) that probably played key cladogenetic roles in a spectacular evolutionary radiation of Aquilegia (columbine) plants. The authors describe how molecular investigations of genomes can