Is it easy to move and easy to evolve? Evolutionary accessibility and adaptation.

Recent phylogenetic studies have documented high levels of conservatism in ecological traits that seem at odds with the traditional view that organisms can readily adapt to different environments. We highlight the need for a new level of rigour in interpreting such patterns from both organismal and biogeographical perspectives. A handful of closely studied systems are revealing a greater number of ecological transitions than anticipated, but these are typically phylogenetically clustered, suggesting that the relative ease or difficulty of such adaptations is strongly context-dependent. We believe that this differential evolutionary 'accessibility' to certain adaptations is pervasive across the tree of life and we illustrate this with reference to several important ecological syndromes in plants. Differential accessibility derives in large part from the attributes of the organisms themselves - certain traits may act as 'enablers' that increase the likelihood of particular innovations. So far, we have made minimal progress in identifying precursor traits that underlie the evolution of ecological syndromes, but we are hopeful that improved phylogenetic resolution will allow for a surge of new insight. However, the accessibility of particular adaptations also derives from external factors, such as the relative location and extent of certain habitats and the competitive ability of the lineages that already occupy them. Better understanding of where particular lineages have existed in the past, and of the adjacency or connectivity of different environments through time, will also be necessary to explain how both dispersal and ecological diversification have jointly contributed to the assembly of the world's ecosystems.