Some have even argued that it makes no sense to define life in terms of evolution, as evolution is purely a mechanism for change. Francisco Varela points out that ``reproduction requires a unity to be reproduced; this is why reproduction is operationally secondary to the establishment of the unity, and it cannot enter as a defining feature of the organisation of living systems'' [Varela 79] (p.33). Similarly, in discussing the distinction between genealogical (evolutionary) and ecological hierarchies of nature, Stanley Salthe questions whether organisms (or similar entities) have a place in the genealogical hierarchy. ``After all, much of population genetics deals directly with gene frequencies in demes and does not bother with genotypes per se'' [Salthe 85] (p.235). An organism ``has no significant extension in time; it cannot evolve. The organism as we usually think of it (ourselves) is clearly an ecological entity'' (ibid.).
In looking at an individual organism, it is remarkable that there is very little that stays constant over its lifetime. Energy is continuously expended as it goes about its activities, and this lost energy is replaced by the regular ingestion of food or by the direct harnessing of sunlight. Even the very molecules from which an organism is made are generally in a state of flux.2.12 In contrast to most other objects in the world, organisms also have a remarkable capacity for self-maintenance and repair in the face of environmental perturbations. As Waddington puts it, the characteristic form of living organisms is ``not only, in many cases, a complex one, but the entity by which it is expressed is more nearly comparable to a river than to a mass of solid rock'' [Waddington 57] (p.2).
The image of organisms as self-maintaining structures that are open to both energy and matter has led many to consider them as dissipative systems (e.g. [Haskell 40], [O'Neill et al. 86]). The vortex produced when the plug is pulled from a basin, a flame, and, to take a larger example, the Earth's atmosphere, are all examples of physical dissipative structures. They all require a continuous input of energy to maintain their structure, and dissipate this energy as a result. However, organisms differ from physical dissipative structures in their ability to exert control on the flux of energy and matter through them. We are drawn to the idea of organisms as self-producing and self-maintaining wholes.
This view of life dates back to Immanuel Kant in the late eighteenth century, if not earlier. Kant's view of an organism was one in which ``each part existed both for and by means of the whole, while the whole existed for and by means of the parts'' (from [Kauffman 95] (p.274)). The Chilean biologists Humberto Maturana and Francisco Varela have attempted to formalise this notion of self-production and self-maintenance in their definition of `autopoietic systems'2.13 (e.g. [Maturana & Varela 80], [Varela 79]). The formal definition of autopoiesis is:2.14
``An autopoietic system is organized (defined as a unity) as a network of processes of production (transformation and destruction) of components that produces the components that: (1) through their interactions and transformations continuously regenerate and realise the network of processes (relations) that produced them; and (2) constitute it (the machine) as a concrete unity in the space in which they exist by specifying the topological domain of its realisation as such a network.'' [Varela 79] (p.13).In other words, an autopoietic organisation is one which actively produces and maintains its own structure. Maturana and Varela also suggest an informal definition:
``an autopoietic machine is a homeostatic (or rather a relations-static) system that has its own organization (defining network of relations) as the fundamental invariant.'' (ibid.)
Kauffman claims that the intellectual lineage starting from August Weismann's view of the ``germ line'' has led to the loss of the earlier image of cells and organisms as self-creating wholes [Kauffman 95]. Varela agrees, saying that ``the great developments of molecular biology have led to an overemphasis on isolated components, and to a disregard of questions pertaining to what makes the living system a whole, autonomous unity that is alive regardless of whether it reproduces or not'' [Varela 79] (p.5).
Some computer models of autopoiesis are mentioned in Section 3.2.3, and I will discuss the relationship of concepts such as autopoiesis and self-maintenance to evolutionary processes in Sections 7.1.4, 7.3.2 and 7.3.3.