Energy and spatial order in niche and community

Abstract

The concept of a varying evolutionary tempo that is regulated by energy was first suggested in the 1950s. It was based on the observation that energy-rich habitats appear to be the centres of evolutionary change, producing novel characters more frequently and having greater speciation rates. Subsequently, a number of studies have found positive relationships between evolutionary rate and energy. Gradients of energy occur across a range of scales and these have been invoked to explain, for example, higher tropical species richness. Precipitation has also been found to influence evolutionary rate, suggesting that biologically available energy and productivity are the important variables in the relationship rather than solar energy alone. Here, we take the theoretical step of investigating at smaller scales the implications of faster evolutionary tempo where productivity is greater: first at the level of the species population and, subsequently, for the same-guild co-habitants in a community. To facilitate, this we begin by applying the concept of gradients of available energy, from more productive to less productive sectors within the species niche, in the context of source-sink theory. We then propose that a species population will have its highest rate of evolution, for changes that confer a positive selection coefficient throughout the niche, in that sector where it has best adaptive fit and greatest per capita energy flux. We also posit that, where it conferred a shift in fitness, an evolutionary change at niche optimum could subsequently affect conspecific populations occupying lower energy niche sectors via the selection-mediated dispersal of the apomorphy into the more marginal components of the niche. We then infer that this type of change in a species with a generally higher per capita energy flux might negatively affect adjacent more slowly evolving species (living in less productive peripheral niches) in situations where the increased fitness conferred by a particular apomorphy was relevant to conditions occurring beyond the limits of the progenitor niche hypervolume. We therefore suggest a directional component to evolution at the compressed scales of niche and community level microevolution whereby populations and species occupying more productive conditions have greater tempos of change with attendant enhancements of both their competitive influence and their evolutionary potential. In this manner, previously recorded macro-evolutionary patterns indicating faster evolution with increased energy at larger scales are interpreted in the context of proposed micro-evolutionary relationships at intraspecific and locally interspecific scales.