Abstract
Genes are thought to have evolved from long-lived and multiply-interactive molecules in the early stages of the origins of life. However, at that stage there were no replicators, and the distinction between interactors and replicators did not yet apply. Nevertheless, the process of evolution that proceeded from initial autocatalytic hypercycles to full organisms was a Darwinian process of selection of favourable variants. We distinguish therefore between Neo-Darwinian evolution and the related Weismannian and Central Dogma divisions, on the one hand, and the more generic category of Darwinian evolution on the other. We argue that Hull’s and Dawkins’ replicator/interactor distinction of entities is a sufficient, but not necessary, condition for Darwinian evolution to take place. We conceive the origin of genes as a separation between different types of molecules in a thermodynamic state space, and employ a notion of reproducers.
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Notes
For example, in some organisms the UAG codon codes for the amino acid glutamine, rather than “stop”, as in the mRNA SGC (Schultz and Yarus 1996).
Peter Godfrey-Smith’s significant book (Godfrey-Smith 2009: 63f) has a critique of replicators that is very complementary to the one offered here. Godfrey-Smith conceives of selection processes in a conceptual manifold formed by the variables “fidelity of heredity”, (H) “abundance of variation”, (V) “continuity, or smoothness of the fitness landscape”, (C) and “dependence of reproductive differences on intrinsic character” (S). Since this is a continuous space, he, too, conceives of replication as occurring at one corner of the space, rather than there being some qualitative difference between replicators and other objects in evolution.
Arguments to this effect have been made by several authors e.g., Weber (2005).
Not, as the figure might suggest (though not the caption) developmental states of a typical lifecycle.
Hull’s notion of an interactor, discussed below, is an ecological entity with economic properties.
The chief difference between our account and Dawkins (1976), chapter 2 is that for Dawkins, any molecule that could replicate itself is a replicator, but he seemed to suggest that other molecules were not “part” of the replicator. In the account given in this paper, molecules evolve stability through their involvement in interactions of differential rates and stability, and the entire process is protometabolic cf. Deamer and Weber (2010).
What counts as reproduction is open for debate. We think that it is broadly when the degree of structural similarity of progeny with parents exceeds similarity by chance, but this is not sufficient. A reproducer is any system that produces systems that are better than chance in their identity conditions (sequence identity, topological relations, etc.) but similarity is a vague notion in this context, and so the critical issue is what kind of identities matter. Reproducers are a class of entities, formed by populations of objects, whose interaction networks result in the creation and growth in number of the objects and thus the size and possibly number of the entities. Those entities with extremely high levels of fidelity can be described as interactor/replicator systems; those with merely enough reproductive fidelity to hold themselves together (through homo- and hetero-dimerisation with like molecules) are not interactor/replicator systems. Note that whether the entities are bounded (e.g., by a cell membrane) will determine whether they increase in number (cells) or just get bigger (hypercycles in the chemoton [see below]).
Kitcher outlines the explanatory schema for neo-darwinian selection in terms of genetic trajectories, initial distributions and frequencies of alleles, and fitness of alleles in a given environment and simple individual selection. The small capitals denote a core doctrine of the various Darwinian views of evolution. The overall argument is a straight nomological-deductive explanation schema in his view, as it is in Mary Williams’ and Hull’s generalised axiomatic views of evolution by selection. Kitcher also considers the views of Darwin himself as setting up a minimal Darwinism, which do not adhere to the centrality of selective mechanisms.
Unfortunately, the term “interactor” has a meaning also in chemistry, where it refers to any kind of stoichiometric or catalytic reaction between molecules. To avoid confusion, we will qualify the term where necessary.
Despite Szathmáry’s objections (Szathmáry 1988), the term “hypercycle” is here applied to any chemical cycle that is self-reproducing, and not those that are merely autocatalytic.
We are not committed to the detailed proposals of Eigen and Schuster here. Any autocatalytic process with several catalysts and reactants will exhibit the formal properties required by this argument cf. Wicken (1987).
The assumption here is that the reactor is a Malthusian reactor, i.e., one in which the rate of reaction is higher than the rate of the provision of new oligomers that act as raw material in the interactions of the hypercycle. Reactors in which the rate of reaction is less than or equal to the rate of new material are termed hyperbolic, after the growth curve. Malthusian reactors exhibit a growth curve similar to hyperbolic reactors but form a logistic curve as the raw material is exhausted, and competitive exclusion commences (i.e., as the carrying capacity is approached).
Although the efficiencies of various components will also be subjected to selection.
See Schrum et al. (2010) for a review.
David Hull, in conversation with JSW, posed this objection.
That is, if the conditions in R are neither severely Malthusian, nor entirely open to hyperbolic growth and thus free the selective coefficients.
In the context of speciation, Eldredge (1989) suggested “moremaker”, but this is a little awkward. We may quibble about some aspects of Griesemer’s definition, as Godfrey-Smith (2009) does, that the condition of “material overlap” between parent and progeny entities is too strong, and require only a causal interaction between these two entities, but that does not materially affect this paper’s argument.
Griesemer (1999) made a proposal to reduce genetics to developmental biology, which shows that reduction need not follow physical scale or containment. Reduction of theories itself is at least in part a matter of theoretical and explanatory convenience.
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Acknowledgments
We are grateful to the late and much-missed David Hull, Paul Griffiths, and Jim Greisemer for correspondence and conversation with JSW on this subject. Thanks also to the reviewer and to the editor, Kim Sterelny, for helpful comments.
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Wilkins, J.S., Stanyon, C. & Musgrave, I. Selection without replicators: the origin of genes, and the replicator/interactor distinction in etiobiology. Biol Philos 27, 215–239 (2012). https://doi.org/10.1007/s10539-011-9298-7
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DOI: https://doi.org/10.1007/s10539-011-9298-7