I.9 Only sufficiently complex systems containing competing elements capable of reproduction, variability and inheritance can become the subject of biological evolution
When a biologist attempts to name the characteristic properties of living organisms, i.e. properties that differentiate them from nonliving systems, he usually first names apparent properties, such as irritability andmetabolism.If we ignore the fact that organisms such as viruses do not exhibit either metabolism or irritability in the classical sense, the fact remains that irritability, i.e. the ability to receive signals from the external environment, is exhibited by a great many nonliving systems, such as the security devices in cars or a central heating regulator. Again, the ability of metabolism, material conversion, is exhibited by a great many chemical dissipation systems.
Apparently the only actually unique property of living systems, i.e. organisms (including the above-mentioned viruses), remains the capability of biological evolution.At the same time, it is quite probable that this is simultaneously a necessary and sufficient condition. It can be assumed that any system capable of undergoing biological evolution, whatever its physical nature, will sooner or later develop into a living system, i.e. also acquire the other types of traits encountered in contemporary organisms.
The capability of “biological evolution” must be defined in a manner so that we will avoid unacceptable circular definitions. We will attempt to demonstrate that the ability to undergo biological evolution overlaps to a certain degree with the ability to undergo natural selection and that this is a property consisting of a number of individual components. Only sufficiently complex systems, capable of undergoing natural selection, i.e. containing mutually competing elements capable of reproduction, variability and inheritance, can (and probably will) become a subject of biological evolution.