In the science of biology, a mechanism is a system of causally interacting parts and processes that produce one or more effects. Scientists explain phenomena by describing mechanisms that could produce the phenomena. For example, natural selection is a mechanism of biological evolution; other mechanisms of evolution include genetic drift, mutation, and gene flow. In ecology, mechanisms such as predation and host-parasite interactions produce change in ecological systems. In practice, no description of a mechanism is ever complete because not all details of the parts and processes of a mechanism are fully known. For example, natural selection is a mechanism of evolution that includes countless, inter-individual interactions with other individuals, components, and processes of the environment in which natural selection operates. Characterizations/ definitions Many characterizations/definitions of mechanisms in the philosophy of science/biology have been provided in the past decades. For example, one influential characterization of neuro- and molecular biological mechanisms by Peter K. Machamer, Lindley Darden and Carl Craver is as follows: mechanisms are entities and activities organized such that they are productive of regular changes from start to termination conditions. Other characterizations have been proposed by Stuart Glennan (1996, 2002), who articulates an interactionist account of mechanisms, and William Bechtel (1993, 2006), who emphasizes parts and operations. The characterization by Machemer et al. is as follows: mechanisms are entities and activities organized such that they are productive of changes from start conditions to termination conditions. There are three distinguishable aspects of this characterization: Ontic aspect The ontic constituency of biological mechanisms includes entities and activities. Thus, this conception postulates a dualistic ontology of mechanisms, where entities are substantial components, and activities are reified components of mechanisms. This augmented ontology increases the explanatory power of this conception. Descriptive aspect Most descriptions of mechanisms (as found in the scientific literature) include specifications of the entities and activities involved, as well as the start and termination conditions. This aspect is mostly limited to linear mechanisms, which have relatively unambiguous beginning and end points between which they produce their phenomenon, although it may be possible to arbitrarily select such points in cyclical mechanisms (e.g., the Krebs cycle). Epistemic aspect Mechanisms are dynamic producers of phenomena. This conception emphasizes activities, which are causes that are reified. It is because of activities that this conception of mechanisms is able to capture the dynamicity of mechanisms as they bring about a phenomenon. Analysis Mechanisms in science/biology