Replicators

Replicators

The concept of replication is central to biology. For instance, Richard Dawkins makes heavy and illuminating use of it in The Selfish Gene and elsewhere. Here I will offer some analytical remarks about the concept, with special reference to the question of what counts as replication: what is replication and how far does it extend? Let’s start with word “replica”: the OED gives us this, “an exact copy or model especially on a smaller scale than the original”. One thinks of model planes or effigies of famous people. Two elements stand out in this definition: the idea of an “exact copy” and the idea of an “original”. There must be (a) a close resemblance and (b) the replica must be derived from the original. Close resemblance alone is not sufficient to be a replica, or else random similarity would suffice; there has to be some sort of relation of dependence whereby the replica is derived from the original. Thus atoms are not replicas of each other, despite close resemblance, because there is no process of derivation leading from one to the other. The entry for “replicate” captures this nicely: “make an exact copy of; reproduce”. Replication is a process in which a copy is reproduced from an original: the original is part of the causal story about how the replica came into existence. Replication is a generative process that produces exact similarity. Finally the OED turns explicitly biological for “replicate itself”: “(of genetic material or a living organism) reproduce or give rise to a copy of itself”. Again, two elements stand out: (a) both organisms and genes are said to replicate themselves, and (b) the original “gives rise” to the copy—causes it, generates it, brings it into being. So self-replication is not the same as replication in general: some replication is driven by the original itself, while some derives from an external source, possibly a conscious agent. There is internally driven replication and externally driven replication: the former is performed by genes and organisms, while the latter requires an external agency, such as a photocopier or an artist or a modeler.[1]

One might be tempted to suppose that replication constitutes the essence of the biological: it is necessary and sufficient to qualify something as biological. The world falls into two big categories, the biological and the non-biological, and replication is the feature that makes the difference. However, there appear to be counterexamples to such a criterion of the biological. First, it is not clear that replication can’t occur outside the biological realm, as with the formation of crystals. Here we have derivative duplication but the process is entirely chemical and inanimate. Perhaps it could be argued that this is not replication proper because of some further ingredient in the notion (it is not purposive, for example), but on the face of it this looks like non-biological reproduction—exact copies produced from an original. Second, it is not clear that everything biological is replicative: isn’t sex biological, but is sex a replicator? What about homeostasis or digestion or respiration? In what sense could these be described as replicators? They are not like organisms or genes, discrete entities that generate copies of themselves; they are more like processes in which such entities participate. They aren’t spatially bounded things, chunks of matter, lumps of stuff. So it looks like replication is not the very essence of the biological, just one of its chief features. There are replicating non-biological entities and there are biological non-replicating entities.

But let’s examine this more closely. Putting aside the crystals type of case, is it really true that only organisms and genes replicate, as the dictionary suggests? What about cells? Surely they replicate, using a mixture of internal and external machinery: they literally divide into exact copies. They self-replicate according to endogenous and exogenous factors. And don’t larger bits of tissue also sometimes replicate—such as the re-grown tails of lizards? In fact not much of this sort of macro-replication goes on, but it is logically possible that it should, and the result would be bona fide replication. Body parts could be replicas of earlier body parts generated by some sort of constructive process. What about organs of the body—can we say that they too are replicators? Well, they don’t produce copies of themselves by division, like cells, but isn’t there an indirect way that they work to produce copies of themselves? That is, an organ contributes to the success of an organism in such a way that an organ just like it crops up in the next generation. It does this because the body in which it lives contains genes that construct organs like it. An efficient heart in one animal leads to a similar heart in its offspring because that heart helps the organism reproduce and pass on its genes. We can thus say that the original heart plays a causal role in the process whereby a copy of it comes to exist in a later generation. It isn’t just a coincidence that the heart of an offspring animal resembles the heart of its parents; the latter heart “gives rise” to the former heart (with a little help from the genes). The whole organism replicates itself by exploiting the genes, and the organs that make it up do much the same. So the organs are replicators too—they engage in the process of producing copies of themselves. The same holds for height, weight, musculature, eye color, etc.—all these are things that get replicated.

It is no different for mind and behavior. Mental capacities and behavioral dispositions get passed on according to their contribution to an organism’s reproductive success: excellent memory, acute vision, sound reasoning, a tendency to strike while the iron is hot, etc. In so far as these are genetically encoded (i.e. very far), they get replicated in the usual way. This is not to say that genetic transmission is the only way to replicate: there is also behavioral contagion, learning, “osmosis”, meme transfer, etc. That is, mind and behavior can be replicated by several avenues and mechanisms not just by genetic transmission, so it is wrong to limit the concept of replication to the genes and whole organisms in the manner of the OED; we need to include much more under the concept.[2]The important point is that the concept is more elastic than the dictionary allows, at least so far as its entry for “self-replication” goes. In fact, replication is a common feature of a lot of biology: organisms, genes, cells, cell clusters, organs, mental traits, and behavioral dispositions. These items vary in mechanism and type of entity, but the concept of replication is common to all of them. The concept is abstract and inclusive (rather like the concept of natural selection).

This relates to something I have argued elsewhere, namely that genes are not the end of the line when it comes to selfishness.[3] Without repeating what I have already said, the point is that the biosphere, or parts of it, acts as the beneficiary of genetic and organismic action: the biosphere persists in virtue of the genes and their host organisms, because these entities enable biological processes and traits to continue. The point I am making now is that we can extend the notion of replication to include such processes and traits. Take sex: it continues in the world by a process of replication—past sex gives rise to future sex. The future sex is just like the past sex (a replica or duplicate or copy of it), and it is the past sex that “gives rise” to the future sex. Why? Because sexual behavior leads to the survival of organisms that engage in it, along with genes that transmit it. The sexual behavior of an elephant, say, is a copy of previous elephant sexual behavior, where the earlier behavior is part of the causal story of how later sexual behavior in elephants came to be. Sex is replicated across the generations just like anatomy and physiology. And it is subject to competition and natural selection in the same way too: the better you are at it, the better you do in the reproduction races. So these biological phenomena act as replicators (aided by the genetic and organismic machinery)—they are repeated, reproduced, duplicated. Thus they are not just beneficiaries of the activities of genes and organisms; they function in a replicative manner. Sex survives because it does well in a biosphere governed by natural selection, and it is passed on by a process of replication. Homeostasis is the same: it is duplicated across the animal kingdom because it works well as an aid to survival and reproduction. So traits like sex and homeostasis manifest the attributes common to biological phenomena: they can be beneficiaries of other entities (genes and organisms), and they are subject to replication. They become increasingly numerous by a process of replication. In addition there can be variations in the traits that give natural selection something to play with: a random mutation in sexual anatomy or behavior can lead to greater reproductive success than rivals, as is also the case for homeostasis. So, to use my preferred terminology, the biosphere (or sections of it) constitutes biological bedrock—the ultimate self-replicating, selfish, naturally selected stuff of biology. Not genes and not organisms: they exist one or two levels up. In particular, the gene functions as a survival machine for the biosphere, an aid to its propagation. This is not to say that the gene is not unique and central in its own way—as indeed the organism is unique and central in its own way—but it is not the be-all and end-all, biologically speaking. If I were to coin a term for these ultimate realities, I would call them “biemes”: biemes are such relatively abstract things as sex, respiration, digestion, homeostasis, growth, locomotion, sensation, thought, and so on. These things stay in existence because genes and organisms keep them in existence, and they do so because they are useful to the survival of both genes and organisms. There are thus three pillars of biological reality: organisms, genes, and biemes—with cells and organs sandwiched in between. The organisms serve the genes and the genes serve the biemes (the biosphere in effect). The most visible things are therefore the least basic: from the conspicuous organism to the microscopic gene to the abstract bieme. All are equally real, and equally vital, but the third has a claim to being the most fundamental.[4]

Colin McGinn

[1] Strictly speaking, replication is a three-place relation: “x is a replica of y in respect z”. Nothing is ever a replica in every respect. In the case of genes there are three respects to consider: chemical composition, functional properties, and information content. In principle replication might preserve one of these but not the others: for example, same information but not the same chemical composition or functional profile. In practice the three go together, but conceptually they are quite distinct. We can imagine possible worlds in which genes shed their chemical features and yet preserve their functional and informational features as they get passed on. We also have the notion of partial replication as well as complete replication (itself inevitably partial).

[2] I would also apply the concept of replication to historical phenomena: sometimes a population copies another population and hence produces a common historical change, e.g. revolutions or wars. This is large-scale replicative behavioral contagion. Given this, I would be happy to rate history as a branch off biology (it is really psychology writ large).

[3] See “The Selfless Machine” and “The Selfish Biosphere”.

[4] I see no competition between this perspective and standard selfish gene theory, merely a shift of emphasis. The gene can retain its starring role, though it works for another level of reality (like shady money men lurking in the shadows of movie production). The organism benefits the gene, but the gene benefits the biosphere (including the process of natural selection itself).