Kinematic Self-Replicating Machines
© 2004 Robert A. Freitas Jr. and Ralph C. Merkle. All Rights Reserved.
Robert A. Freitas Jr., Ralph C. Merkle, Kinematic Self-Replicating Machines, Landes Bioscience, Georgetown, TX, 2004.
5.1.1 Dawkins Classification of Replicators (1976)
According to a brief summary by Brandon , biologist Richard Dawkins in his classic 1976 work The Selfish Gene  defines a replicator as “anything in the universe of which copies are made”. Dawkins observes that three characteristics affect the quality of replicators: fecundity, fidelity, and longevity: “What is of evolutionary importance is that it produces copies of itself so that it is potentially immortal in the form of copies. Of course, everything else being equal, the more copies [that] are replicated (fecundity) and the more accurately a replicator produces them (fidelity), the greater its longevity and evolutionary success.” In addition to these three characteristics, Dawkins offers two additional useful design dimensions in his classification of replicators.
First, replicators may be active or passive. An active replicator is an entity that influences its probability of being replicated – for example, any DNA molecule which, either through protein synthesis or the regulation of protein synthesis, has some phenotypic effect. On the other hand, a passive replicator is one which has no influence on its probability of being copied, such as a section of DNA that is not transcribed.
Second, replicators may be germ-line or dead-end, depending on whether they are potential ancestors of an indefinitely long line of descendant replicators. For instance, the gene in a gamete or germ-line cell in a body is a germ-line replicator, whereas most of the genes in our bodies are not germ line and can replicate only a finite number of times through mitosis, thus are dead-end replicators. Brandon notes that “it is the potential, not the fact, of being in an indefinitely long ancestry that matters for this classificatory distinction. So a gene in a spermatozoon that fails to fertilize is still a germ line replicator.”
For living systems, evolution by natural selection is a two-step process – (1) the direct replication of structure, and (2) an interaction with the environment so that replication is differential across different entities, traditionally called phenotypes. The notion of phenotypes seems adequate for discussions of organismic selection, but entities other than organisms such as chromosomes, groups of organisms, or even entire species can interact with their environment in a way that also makes replication differential. Thus Hull  suggests the more general term “interactor” as “an entity that directly interacts as a cohesive whole with its environment in such a way that replication is differential.” Dawkins regards organisms necessarily as vehicles that are secondary to their replicators (i.e., their “selfish genes”), with any change in replicator structure passed on during the process of replication. But Brandon  notes that “it is easy to imagine self repairing replicators where changes in structure would not necessarily be passed on in replication,” a potentially useful distinction in the case of artificial mechanical replicators as opposed to gene-driven biological replicators. Brandon continues: “What seems to be important is that replication be direct and accurate. Directness and accuracy are terms of degree, and if we allow some play in both, under certain circumstances an organism could be considered a replicator.” McMullin  has noted additional qualifications to Dawkins’ schema relating to the “units of selection” issue.
Last updated on 1 August 2005