Intrinsic
evolution of
hardware means that the individuals of the evolving population receive fitness
scores according to their performance when instantiated as real physical
pieces of electronics. Evolution proceeds by taking note of the overall
behavioural effect of variations made to the real circuits; this is very
different from conventional design techniques, which proceed by manipulating
abstract models.
The use of abstract models simplifies design by allowing some aspects of reality to be ignored, but the properties of the real hardware that have been `abstracted away' must be suppressed: they must not be allowed to influence the final behaviour of the designed circuit. For example, a designer engaged in digital design does not need to think about the analogue behaviour of the transistors, but considers them as ON/OFF switches. To allow circuits designed at this level of abstraction to work in reality, the transistors must always be kept in either the ON state or the OFF state except during short transient periods while they are switching between them. Steps must be taken to ensure that these transients do not influence the overall behaviour of the system as predicted by the designer's digital model; this is manifest in the use of a global clock in synchronous design, and the more local co-ordination mechanisms of asynchronous design methodologies. Hence, the use of an abstract model at the design stage imposes constraints on what circuits can be produced, in order to ensure that the model is valid.
Intrinsic hardware evolution, by observing the consequences of variations made to the real hardware, avoids the need for design abstractions and the accompanying constraints. Our notion of the nature of electronic systems is heavily biased by our design methodologies and the constraints applied to facilitate their abstractions, so evolvable hardware demands a radical rethink of what electronic circuits can be. Both the spatial structure (modularity) and the temporal structure (synchronisation and the rôle of phase in general) need to be considered.