Limits to adaptation and patterns of biodiversity

Why do species have finite ranges in space and time? All species have limited ecological distributions, and all species eventually become extinct. At the heart of these distributional limits is the idea of trade-offs: a single population or species cannot maximize its fitness in all environments (Woodward and Kelly 2003). Each species therefore occupies a limited range of ecological conditions, or a particular period in history, and interacts in complex ways in ecosystems consisting of many co-existing species. These interactions may in turn generate more specialization (Nosil & Harmon, this volume; Schemske, this volume). However, from an evolutionary biology perspective this explanation is incomplete. Populations clearly adapt to novel environments in some circumstances, otherwise there would be no life on land, no mammals in the ocean, and only a few species on oceanic islands such as Hawaii (Wagner & Funk 1995). What processes, therefore, act to constrain adaptation to changing environments and continually prevent the expansion of species into new habitats at the edge of their range? Understanding the factors that limit the temporal or spatial persistence of species is of key practical importance, given ongoing changes in global climate (Root et al. 2003), coupled with rapid habitat loss and alteration by the introduction of exotic species of parasites, predators and competitors.