Definition of Terms

Before continuing it is worth clarifying some of the terminology relating to evolutionary systems that will be used in subsequent chapters. It has been noted by various people that some confusion exists in the use of terms such as genes, replicators and lineages (e.g. [Hull 80], [Salthe 85], [McMullin 95]). The confusion arises because terms such as gene and replicator are sometimes used to refer to an individual instance of a specific form, e.g. a specific portion of DNA within one particular organism, and sometimes to refer to a class of forms, members of which can be considered identical from the point of view of their acting in an evolutionary process (e.g. it is in this sense that we refer to a gene being passed on (replicated) from one generation to the next).^2.26

In an attempt to remove this ambiguity between referring to tokens (individual instances) and classes, McMullin introduces two prefixes, `A-' (standing for Actor) and `L-' (standing for Lineage) [McMullin 95]. The former signifies an individual token, and the latter signifies a class (read the `L-' prefix as `a lineage of ...'). For example, an A-replicator is an individual (molecule, strand of DNA, etc.), and an L-replicator is a lineage of A-replicators. McMullin goes on to single out lineages of replicators in which all members are identical to the founder of the lineage with respect to a particular selectively relevant character. These lineages, which he calls similarity lineages, or S-lineages, are, he argues, the fundamental units of selection in a Darwinian evolutionary process.^2.27 Phrases such as ``genes are potentially immortal'' refer to L-genes, and, in particular, to S-lineages. Although McMullin regrets the introduction of yet more terminology (as do I), he sees no other way to disambiguate confusing terms such as `replicator'. I shall therefore use these prefixes throughout this thesis in situations where potential ambiguities may otherwise arise.

We are now in a position to be somewhat more precise about the meaning of a number of concepts that were used in Chapter 1.