When is it co-evolution? A reply to Steen and co-authors

David Steen and co-authors in this journal offer a philosophical argument to support an Evolutionary Community Concept to identify what they call evolutionary communities. They describe these as unique collections of species that interact and have co-evolved in a given geographic area and that include co-evolved dependencies between different parts of a community. Steen et al. refer to the coevolution of assemblages, collections, communities, dependencies, interspecific and abiotic interactions, and traits, but they do not define co-evolution or provide an example in which co-evolution (as distinct from interaction) has been demonstrated. There may be as many ways to explain interactions among species as these interactions themselves; they may result, for example, from ecological fitting and phenotypic plasticity. In this paper, I argue that standard Darwinian Theory explains intraspecific microevolution or descent with modification within an interbreeding and potentially continuous population. The power, mechanism, or force that causes interspecific co-evolution, especially at the community scale, requires further explanation.