First, however, we turn in rather a different direction--to consider systems where the agent level constraints assumed above are relaxed; that is, where the agents are open. By this we mean that the potential variability in agent behaviours is very large (if not infinite); but this variability is not merely parametric modulation of still fixed and stereotyped behaviours, but rather is of a scope which exceeds the capability of a designer to pre-conceive (never mind design) it.
At heart this means that among the methods (perhaps the only method) of an agent there is a general purpose (Turing complete) computer; and among the state variables (perhaps the only variable), there is a programme. On one level, the agents are still as rigid as ever--the available agent methods cannot be varied. Yet, on another level, there is literally unlimited scope for variation in agent behaviours. Indeed, this behavioural variation will even be formally unpredictable (in the sense of Turing halting).
Perhaps the best known example of this genre is Tierra (Ray, 1996); beside this original there are a number of related ALife systems with a similar basic idea such as for example the Avida systems (Adami, 1998). We will henceforth refer to those systems collectively as ``Tierra-like systems''.4
Whenever, as is the case in Tierra-like systems, the ``code'' which essentially defines the behaviour of the agents is created at run-time and not at compile time, then it seems clear that many of the restrictions which limited novelty creation in ABMs with closed agents should disappear. In particular, if the conditions exist for darwinian selection (the agents can reproduce, but with population growth limited by competition), and the embedded code in the agent state variables is subject to mutation, then it seems that there should be scope for continuing, open-ended (``perpetual'') evolutionary creation of novelty. Tierra-like systems offer exactly this. The agents and their behaviours are not bound by the pre-conceiving imagination of the modeller/programmer, but can freely emerge from the evolutionary process.
There is no doubt that Tierra and similar systems do in fact show interesting evolutionary phenomena; but as already noted in the section On Novelty, it is also a well-known fact that the creativity of the evolution in those systems seems to be limited and cannot be sustained for a long period of time. Attempts to build Tierra-like systems with a more long-term evolution have largely failed.
The nature of these limitations is still very much an open question; however we can consider at least a few specific conjectures as to the explanation.
A first possible factor may be the rather small size of the artificial worlds which were initially studied. These allowed only a comparatively small number of creatures to exist simultaneously. This hypotheses is easily testable. The rapid increase in available computing power means that, even after a relatively short waiting time, the implementation of much larger experimental worlds has already become possible. However, at least for the case of Tierra, these larger worlds do not seem to have led to a significantly more interesting phenomenology.
A second possible explanation for the rather limited phenomenology of Tierra is that it is due to a lack of structure and complexity of the environment of the digital organisms. The environment as experienced by the agents may simply lack features to be adapted to. An interesting study of the dependence of adaptability on environmental complexity in ALife systems is provided by Fletcher et al. (1997).
In the case of Tierra-like systems this points at a dilemma. The more complex their artificial worlds are designed, the more potential niches they offer to the agents. This should prolong the evolutionary process and increase the potential for the evolutionary development of interesting agent behaviours. On the other hand, the appeal and elegance of Tierra-like systems is exactly the simplicity of their initial world. To much designed and programmed complexity would to some degree defeat the purpose of those systems.
In the original Tierra the environment in which the agents live is quite deliberately minimalist. It essentially consists only of a ``memory'' space which the digital creatures inhabit; the ``slicer'' which allocates CPU time; and the ``reaper'' which kills off agents to ensure a continuing supply of free memory for further reproduction (and thus--hopefully--evolution).
It seems that we can therefore safely discount the emergence of, for example, predation in Tierra--that is, agents which kill other agents to exploit their resources. As Tierra is implemented, there is simply no mechanism for one organism to attack or kill another; and, in any case, the resources of a killed organism go back to a common pool and are immediately available for the next organism which attempts to reproduce--they would not be preferentially available to a (nascent) lineage of predators.
In order for predation to emerge, there would have to be generic mechanisms--presumably implemented by some ``world'' agent(s)--for redistribution of resources between agents. But the Tierran world agents (memory, slicer, reaper) are closed; they are not capable of evolving, but simply offer a fixed, stereotyped repertoire of interactions for the organism agents to engage with.
Of course, in a suitably enhanced and modified version of Tierra we may certainly observe more phenomena than in the original system. Specifically, one might add precisely the sorts of attack and resource redistribution capabilities that could underpin predation phenomena. Presumably one might then observe classical evolutionary arms races between predators and prey, with elaboration and refinement of more or less sophisticated strategies for both.
But of course, this all misses the point. Again, this world will ultimately be limited by the closed set of interaction possibilities. So, to pursue the example even further, we could certainly still not expect to see organisms burying themselves in ``sand'' in order to avoid detection by predators--simply because there is nothing of the nature of ``sand'' in this world, nor any mechanism for it to spontaneously appear!
It should be clear that there are two, at least, conceptually distinct kinds of limitation being identified here:
In summary then, the properties of the world in which the artificial organisms live is crucial to the evolutionary potential of the model-system; its ``physics'' and ``chemistry'', the ways in which organisms may be born, die and receive resources all constrain its evolutionary potential. Yet engineering a complex--but still fixed--world is no solution: it merely delays the still inevitable plateau of evolutionary exhaustion.
We conclude that, for our purposes at least, current ALife models with open agents (in the sense of evolutionarily programmable) turn out to be, at best, inadequate--and, at worst, a dead end. The challenge, of course, is to formulate a system where the world itself, including the very mechanisms or interfaces for agent interaction, is open and indefinitely mutable and creative. We see this as a major challenge ahead.
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