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Rapid Evolutionary Change in Homogocene

The pace of evolution is accelerating as a result of the widespread introduction of species to new geographic regions. This is the central theme of Alien Species and Evolution, a comprehensive text that unites concepts from evolutionary biology and ecology to explain the processes and effects of a largely unrecognized form of global change. Driven by human activities, the global mixing of biota is occurring at unprecedented rates (prompting some ecologists to refer to the current period of Earth’s history as “the Homogocene”). Consequently, species are rapidly being exposed to novel biotic and abiotic selection pressures. In a well-written synthesis, George Cox examines the evolutionary responses of alien species to their new environments and the responses of native species to alien interlopers. There are 20 chapters grouped within four major sections. The first section, “Basic Concepts of Alien Invasion and Evolution,” reviews the fundamental aspects of evolutionary biology applied to alien species, including adaptations for dispersal and establishment, traits conferring invasiveness, founder effects, and the importance of genetic variability. It also briefly summarizes genetic methods for identifying donor regions, invasion routes and invasion rates, all of which yield valuable information for management and policy. The second section examines processes of evolutionary change and adaptation of alien species in their new environment. Its major focus is on genetic changes in native and alien species through hybridization. Cox relates the story of an alien plant that appeared in salt marshes on the coast of England in the late nineteenth century. The plant spread slowly at first, suddenly became invasive, and eventually colonized estuaries throughout the British Isles and into mainland Europe. It was revealed to be the hybrid of a native cordgrass (Spartina maritima) and a closely related North American cordgrass (Spartina alterniflora). The hybrid was sterile but became invasive after it underwent a genetic mutation that doubled its chromosome number, which restored its fertility and created a new, highly competitive species, Spartina anglica. Because novel combinations of genes can play novel (alien) ecological roles, Cox also devotes several pages to genetically engineered organisms and their potential for genetic exchange with other species. The remaining chapters in this section deal with abiotic and biotic factors (community resistance and enemy release) that stimulate adaptations in alien species. The third section, “Evolutionary Interaction of Aliens and Natives,” provides examples of rapid adaptation of invaders to new plant and animal hosts, prey, and enemies. An example is the Eurasian codling moth (Cydia pomonella), a worldwide pest of apples, pears, and plums. Within a few decades of invading California, it became a pest of English walnuts. The ubiquity of host shifts by introduced predators and herbivores serves as a warning against predicting an invader’s impact based on its behavior in its natural range. Another topic concerns evolutionary shifts by native herbivores to alien plants. In some cases, the alien host is more favorable to the herbivore’s development than its natural host. Cox raises the intriguing possibility that the widespread introduction of potential host species may be paving the way for the rapid evolution of new pest insects. He cites the example of the European budworm Choristoneura murinana, which has become exposed to a variety of introduced exotic conifers and thus been given the opportunity to broaden its host repertoire, becoming preadapted to exploiting resources in other regions where it may be introduced. The last section, “Global Evolutionary Consequences of Alien Invasions,” explores trajectories of global changes wrought by alien species. The context-dependent nature of invasions and their effects is emphasized throughout the book, but nowhere more dramatically than in this section. For example, Cox describes the near extinction of the American chestnut tree by the most destructive forest disease on record, chestnut blight fungus. The pathogen was introduced with Asian chestnut trees planted as ornamentals in New York State during the early twentieth century. Unlike its Asian congener, the American chestnut lacks the immunity gained by evolutionary experience with the fungus. Cox provides several other examples of the