Home > Work > The Extended Phenotype: The Long Reach of the Gene
21 " I define a replicator as anything in the universe of which copies are made. "
― Richard Dawkins , The Extended Phenotype: The Long Reach of the Gene
22 " Replicators may be classified in two ways. They may be ‘active’ or ‘passive’, and, cutting across this classification, they may be ‘germ-line’ or ‘dead-end’ replicators. "
23 " An active replicator is any replicator whose nature has some influence over its probability of being copied. For example a DNA molecule, via protein synthesis, exerts phenotypic effects which influence whether it is copied: this is what natural selection is all about. "
24 " A passive replicator is a replicator whose nature has no influence over its probability of being copied. "
25 " A further important point is that Maynard Smith was seeking the ‘best’ strategy in only a special sense. In fact he was seeking an ‘evolutionarily stable strategy’ or ESS. "
26 " 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. A gene in a gamete is a germ-line replicator. So is a gene in one of the germ-line cells of a body, a direct mitotic ancestor of a gamete. "
27 " The ESS has been rigorously defined (Maynard Smith 1974), but it can be crudely encapsulated as a strategy that is successful when competing with copies of itself. "
28 " If a program or strategy is successful, this means that copies of it will tend to become more numerous in the population of programs and will ultimately become almost universal. It will therefore come to be surrounded by copies of itself. If it is to remain universal, therefore, it must be successful when competing against copies of itself, successful compared with rare different strategies that might arise by mutation or invasion. "
29 " A dead-end replicator (which also may be active or passive) is a replicator which may be copied a finite number of times, giving rise to a short chain of descendants, but which is definitely not the potential ancestor of an indefinitely long line of descendants. "
30 " To recapitulate, the significance of the difference between growth and reproduction is that reproduction permits a new beginning, a new developmental cycle, and a new organism which may be an improvement, in terms of the fundamental organization of complex structure, over its predecessor. "
31 " The whole purpose of our search for a ‘unit of selection’ is to discover a suitable actor to play the leading role in our metaphors of purpose. "
32 " Most of the DNA molecules in our bodies are dead-end replicators. They may be the ancestors of a few dozen generations of mitotic replication, but they will definitely not be long-term ancestors. "
33 " When we talk of a program as ‘doing better’ or as being ‘successful’ we are notionally measuring success as capacity to propagate copies of the same program in the next generation: in reality this is likely to mean that a successful program is one which promotes the survival and reproduction of the animal adopting it. "
34 " Adoption and contraception, like reading, mathematics, and stress-induced illness, are products of an animal that is living in an environment radically different from the one in which its genes were naturally selected. "
35 " The point about recurrent reproduction life cycles, and hence, by implication, the point about organisms, is that they allow repeated returns to the drawing board during evolutionary time. "
36 " A DNA molecule in the germ-line of an individual who happens to die young, or who otherwise fails to reproduce, should not be called a dead-end replicator. Such germ-lines are, as it turns out, terminal. They fail in what may metaphorically be called their aspiration to immortality. Differential failure of this kind is what we mean by natural selection. "
37 " 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. "
38 " As I said, the active/passive distinction cuts across the germ-line/dead-end distinction. All four combinations are conceivable. "
39 " Particular interest attaches to one of the four, the active germ-line replicator, for it is, I suggest, the ‘optimon’, the unit for whose benefit adaptations exist. "
40 " The integrated multicellular organism is a phenomenon which has emerged as a result of natural selection on primitively independent selfish replicators. It has paid replicators to behave gregariously. The phenotypic power by which they ensure their survival is in principle extended and unbounded. In practice the organism has arisen as a partially bounded local concentration, a shared knot of replicator power. "