There has been considerable ambiguity, if not downright confusion, in the literature of evolutionary biology regarding specific technical usage of the terms replication and replicator. As far as I am aware, the abstract, technical, idea of a replicator was first introduced by Dawkins (1976, 1978a). Hull subsequently elaborated the idea (Hull 1980, 1981), and Dawkins has since extended his own analysis somewhat further (Dawkins 1982a).
I shall argue that "replicator" has sometimes been used to refer to actors - individuals which are capable of replication - and sometimes to refer to lineages formed as a result of such replication (Fig.).
I suggest that the distinction between these two kinds of entities is actually very significant - and that it is only the latter (lineage) interpretation which can do the job Dawkins wants done.
Where necessary in the following, I shall explicitly distinguish references to actors with an A- prefix, and references to lineages with an L- prefix.
Figure: A-replicator vs. L-replicator
The ambiguity of usage can be illustrated by considering the concept of replicator longevity. In The Selfish Gene, Dawkins first defines longevity as relating to the lifetime of an individual replicator, i.e. an actor or A-replicator:
Certain molecules [supposed primordial replicators], once formed, would be less likely than others to break up again. These types would become relatively numerous in the soup, not only as a direct logical consequence of their `longevity', but also because they would have a long time available for making copies of themselves. Replicators of high longevity would therefore tend to become more numerous and, other things being equal, there would have been an `evolutionary trend' towards greater longevity in the population of molecules.
(Dawkins 1976, p. 18)
He goes on to argue that there would be an overall trend toward the evolution of "varieties" (?) of replicator with high "longevity/fecundity/copying-fidelity" (Dawkins 1976, p. 19). Thus, by "longevity" he must evidently mean the lifetime of individual actors, or A-longevity.
Somewhat later in the same source, Dawkins specifies that "Copying fidelity is another way of saying longevity-in-the-form-of-copies and I shall abbreviate this simply to longevity" (Dawkins 1976, p. 30, emphasis added). Now this version of longevity evidently refers to a replicator viewed as a lineage ( "in-the-form-of-copies" ); so this is L-longevity.
So far, any confusion is latent: as long as we remember that Dawkins is using "longevity" in two quite different ways, and judge his meaning from the context, it should not cause too much trouble. In particular, we might reasonably suppose that the slogan "longevity/fecundity/fidelity" will always refer to actors, not to lineages - i.e. the "longevity" in question will be A-longevity rather than L-longevity. I say this for two distinct reasons. Firstly, A-longevity is the sense of longevity with which Dawkins first introduced the slogan. But secondly, and more significantly, Dawkins claims that L-longevity is effectively equivalent to copying fidelity (a dubious equation in any case, but let it stand). It follows that, if the longevity in the slogan were interpreted as L-longevity, the slogan would become synonymous with "fidelity/fecundity/fidelity" - which is at least redundant and confusing, if not actually incoherent.
Unfortunately, however, Dawkins did indeed subsequently use the slogan in precisely this confusing way:
The qualities of a good replicator may be summed up in a slogan reminiscent of the French Revolution: Longevity, Fecundity, Fidelity [Dawkins 1976, 1978b]. Genes are capable of prodigious feats of fecundity and fidelity. In the form of copies of itself, a single gene may persist for a hundred million individual lifetimes.
(Dawkins 1978a, p. 68, emphasis added)
So we have the slogan, which I have just argued must imply A-replicator, followed immediately by an elaboration that obviously implies L-replicator. Given Dawkins' own confusion here, it is hardly surprising that Hull then compounded the error further:
According to Dawkins [1978a p. 68], the qualities of a good replicator may be summed up in a slogan reminiscent of the French Revolution: Longevity, Fecundity, Fidelity. As striking as this slogan is, it can easily be misunderstood. The fidelity which Dawkins is talking about is copying-fidelity, and the relevant longevity is longevity-in-the-form-of-copies [Dawkins 1976 p. 19, p. 30].
(Hull 1981, p. 31)
Hull's last citation here refers to the two locations in The Selfish Gene (1976 edition), which I have already identified above, where "longevity" was defined - but he omits to mention that these are two different, and incompatible, definitions!
Hull is certainly correct that Dawkins' slogan may be easily "misunderstood" . On my view, both he (and Dawkins himself) have suffered from just such misunderstanding. The interpretation Hull gives here is not the "correct" one - i.e. that which accompanied the original formulation of the slogan in (Dawkins 1976, p. 19), and which referred to A-replicators - but the confusing and redundant one which refers to L-replicators (Dawkins 1976 p. 30; 1978a p. 68).
Hull also uses Dawkins' slogan "longevity, fecundity and fidelity" in another paper (Hull 1980, p. 317), but, on this occasion, citing only (Dawkins 1978a) as the source. Again, Hull goes on to specify that the "relevant longevity concerns the retention of structure through descent" (i.e. L-longevity). Again, he does not comment on the fact that (according to Dawkins) this version of longevity is synonymous with (Dawkins' version of) fidelity, and is therefore redundant.
The problem is further compounded by Dawkins (1982a, p. 84) where he quotes, at length, from (Hull 1980), and specifically endorses Hull's interpretation of longevity in this context - thus reinforcing the confusion he himself originated in (Dawkins 1978a).
To summarise, while it seems that Dawkins typically uses "replicator" in the sense of a lineage or L-replicator rather than an actor or A-replicator, he also alternates between the two usages - sometimes with quite bewildering speed. Thus, we have the following two comments (quoted from consecutive paragraphs):
A germ line replicator (which may be active or passive) is a replicator that is potentially the ancestor of an indefinitely long line of descendant replicators...
[Evidently this refers to actors.]
... But whether it succeeds in practice or not, any germ line replicator is potentially immortal. It `aspires' to immortality but in practice is in danger of failing.
[Yet now we must be talking about lineages.]
(Dawkins 1982a, p. 83)
This confusion between A-replicator and L-replicator is counterpointed (presumably with unconscious irony) by Dawkins' approving remark that Hull (1980, 1981) is "particularly clear about the logical status of the lineage, and about its distinction from the replicator and the interactor" (Dawkins 1982a, p. 100).
In conclusion: the point of this rather laborious discussion has been to establish that, though Dawkins and Hull make considerable use of the term "replicator" , their usage is quite generally ambiguous as between A-replicator and L-replicator, and calls for very careful interpretation. The recognition of this fact is an essential prerequisite for the analysis of the "unit of selection" controversy which follows.