The adaptive continuum
Why do species fail to adapt and what limits the expansion of their distributions? It has often been observed that single species are not found everywhere and so they must often fail to adapt. Implicit in this question is what happens within a single species that limits its adaptive potential and relegates it to a smaller niche than could otherwise be gained through increased genetic variation. One problem with this question is it views species in isolation, as separate units, instead of as branches of a great phylogenetic tree. Answering this question may require looking beyond a single biological species and asking where the species came from and what other species it may have given rise to. There are innumerable forms of life on Earth and we now know that speciation through natural selection is the primary "origin of species." We also know that because selection is such a strong driver of diversification that the process of speciation is often adaptive, a concept described by terms such as "ecological speciation." To understand adaptation, and its limits, requires viewing life from the widest and smallest perspectives.
Adaptation is essentially a niche expanding process, and niche expansion occurs at micro-and macroevolutionary scales. The tree of life is itself an adaptive tree, and it has its own cumulative niche formed from the many reproductively isolated units or branches we call species. As these individual branches adapt and expand their niche they produce new branches, not unlike the life cycle of an individual organism. In the life cycle analogy, a species' inception represents 'birth.'
Niche and geographic expansion, including dispersal to new environments and adaptive microevolution, represents stages of 'development.' Cladogenesis, or the inception of new species from an ancestor represents 'reproduction.' Finally, extinction represents the 'death' of a species. Like individuals, species age, and thus the character of species changes over time (i.e. anagenesis). Like a planarian or plant that has been cut in two, a species can be segregated and the separate parts can develop or age independently, often leading to eventual speciation through vicariance. During the development of a species, genetic variation (the basis of all adaptation) builds through mutation. When and if species enter a reproductive phase, new species can evolve, or "bud" off. These new branching events through speciation, although adaptive in the broad, tree-of-life sense, may actually limit the adaptive potential of the original or progenitor species. For the progenitor species of these buds (or the segregants resulting from vicariance events), this is an adaptation-limiting process. That is to say, progenitors lose significant and important genetic variation to their offspring species. Thus, a highly adaptive branch (or 'reproductive' species) will be one with frequent and great losses in genetic variation. Restated, the most adaptive species lose genetic variation to the birth of new species at the cost of their own niche expansion. These losses in genetic variation, importantly, serve to limit adaptation within a species. Nevertheless, these adaptive speciation events, as with all types of adaptation, represent expansion of the niche for the tree of life. Thus, we can acknowledge several limits to adaptation. First, there is time, which limits the building of genetic variation through mutation and the colonization of new habitats, thus providing opportunities for reproductive isolation. Next there is speciation, which results in losses in novel genetic variation.
Finally, there are external, stochastic factors that lead to extinction. Examples of these are catastrophes, new, negative species interactions, and population losses from rapid environmental changes that limit niche availability and kill individuals. Viewing species and the tree of life in this way (an adaptive continuum) allows us to 1) reconcile the conundrum between the diverse, adaptive tree of life and the oft-observed limits of species to adapt, 2) understand adaptation as a holistic process that includes speciation as well as microevolution, 3) frame research to better understand these adaptive processes and the tipping point between adaptive micro- and macroevolution, and 4) develop new approaches to biological conservation that better understand and incorporate adaptive limits within single species as well as conserving the potential for future adaptive speciation, which, looking over the tree of life, is by far the most adaptive aspect of biological diversity.