Evolutionary Rates: Stress and Species Boundaries

Abstract

From the molecular to the biogeographic levels of organization, stressful conditions underlie much evolutionary change. An understand­ ing of evolutionary rates therefore depends upon a fusion of ecology and genetics especially under stressful conditions (59). Following Hoffmann & Parsons (30), this article defines stress as an factor-usually a physical one-that causes a potentially injurious change in a system. Defined in this way, stress has a major impact on many evolutionary and ecological processes since it will cause irreversible change irrespective of the density of organisms. At the limits of resistance, stress can impose high selection intensities with resultant rapid population shifts (4). Even though fundamental changes in the natural order of systems are likely at these times, much of the literature of evolutionary biology focusses upon more benign situations (30). However, extreme stress periods are unpredictable in occurrence and tend to be of short duration so that observations covering many generations are needed for their doumentation. There are some situations where severe stress can be regarded as an environmental probe; analysis of such situations leads to generalizations difficult to perceive under more optimal conditions. For example, biological systems can be described in terms of energetic costs, from which suggestive associations between habitat, life history characteristics, and stress resistance