Darwinian Thinking: 150 Years after The Origin

Abstract

hypothesis that adaptive divergence leads to speciation and to the evolution of traits and behaviors that influence the prospects for species coexistence. A key issue here is the role of character displacement in the origin and mainte- nance of diversity. Pfennig and Pfennig find substantial evidence that character displacement plays an important role in the evolution of species interactions and identify a number of additional areas that Darwin had not con- sidered, including sexual selection and phenotypic plas- ticity. Their paper clearly illustrates how Darwin's original observations continue to shape our thinking on the in- terface between ecology, behavior, and evolution. Of the many insightful observations presented in The Origin, perhaps none is more remarkable than Darwin's discussion of the evolution of hybrid sterility. He under- stood that sterility could not have evolved as an adaptation of individual species and that interactions in the hybrids stemming from the contributions of each parent caused incompatibilities, but without today's specific knowledge of genetics, he was unable to reach a satisfactory expla- nation. Presgraves (2010) reviews this topic in detail, in- cluding an historical perspective on the contributions by Darwin, Muller, and Dobzhansky to the problem. He also provides an overview of the recent advances in our un- derstanding of the genetic mechanisms that contribute to the evolution of hybrid incompatibilities. This includes evidence that genetic conflict, and not adaptive divergence, may be a primary cause of hybrid sterility and inviability. Darwin considered multiple sources of evidence to gain support for his theory of natural selection, including the evolution of both living and extinct organisms. Hunt (2010) describes Darwin's attempts to use the fossil record to test the hypothesis that evolution by natural selection is a gradual process. Darwin argued at the time that the record was too incomplete to provide an adequate test. Hunt reviews the current paleontological record and con- cludes that over short periods, rates of morphological evo- lution are sometimes considerably faster than Darwin had proposed. Sticklebacks provide a particularly illustrative case, where a nearly continuous record extending over the last 10 million years shows that most phenotypic evolution occurred over a brief period of approximately 1,000 gen- erations. Moreover, for a wide variety of organisms and