In Section 7.2.3 it was argued that the less restricted these interactions are, the more potential the system has for open-ended evolution. In biological evolution, for example, a fundamental aspect of the struggle for existence is that organisms act as potential resources of both matter and energy for other organisms (see Section 7.1.4).
Regarding the particular sorts of interactions required for open-ended evolution, it is possible that these might fall into two broad categories: synergisms and conflicts (e.g. [Buss 87], [Maynard Smith & Szathmáry 95], [Stewart 97]: see Section 2.3.4). In biological systems, some important general principles governing interactions at the molecular level include: specificity of reaction, control of reaction, and switching mechanisms (e.g. [Maynard Smith 86] p.70). Furthermore, Morán and colleagues have remarked that origin-of-life models typically rely upon four types of biochemical interaction: reaction (the construction of new compounds, or the decomposition of existing ones), diffusion, catalysis, and template replication [Morán et al. 97]. Morán et al. suggest that the latter two interactions can be considered as special cases of reaction, so reaction and diffusion may be considered as the fundamental interactions. Another basic interaction principle, concerning the use of information in biological system (e.g. in the genetic code, allosteric enzymes, hormones and nervous conduction), is the arbitrary nature of the messenger. Maynard Smith, following Monod, refers to this as the `gratuity' of the messenger [Maynard Smith 86]. It is an open question whether these principles are truly universal, and therefore also necessary for artificial systems.