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Replicators - An Exponentialist View
Replicator - "In discussions of evolution, a replicator is an entity (such as a gene, a meme, or the contents of a computer memory disk) which can get itself copied, including any changes it may have undergone. In a broader sense, a replicator is a system which can make a copy of itself, not necessarily copying any changes it may have undergone. A rabbit's genes are replicators in the first sense (a change in a gene can be inherited); the rabbit itself is a replicator only in the second sense (a notch made in its ear can't be inherited)." (Drexler, 1986)
The concept of the replicator was popularised by zoologist Richard Dawkins in The Selfish Gene (1976, 1989) in which Dawkins focuses on gene replicators and introduces the concept of a meme replicator. See Richard Dawkins - An Exponentialist View for more. Although Dawkins argued that organisms should not be regarded as replicators (but as "vehicles" for replicators), the most widely accepted definition of a replicator is the later and more general definition of Dawkins (Dawkins, 1982):
"I define a replicator as anything in the universe of which copies are made."
Today this broader definition has come to replace Dawkins' earlier, more restrictive use of the term (Drexler, 1986 & 1990; Witting, 1997; Freitas and Merkle, 2004). Note that the distinction Drexler (1986) makes between a primary case replicator (a rabbit's genes) and a secondary case replicator (the rabbit) is less relevant when considering populations rather than evolution.
In this short article, and throughout this Exponentialist web site, I argue that self-replication is the key to understanding life, evolution, and the ever present challenge of population growth within limits to growth and avoiding Malthusian catastrophe. Furthermore, in the realm of the replicator, the influence of Malthus runs far deeper than commonly understood as I will briefly demonstrate.
Examples Of Replicators
Large replicators (humans, animals and plants) are made by little replicators. Given that large replicators are multi-cellular organisms, it is fair to say that large replicators are built by Cellular Replicators and Bacterial Replicators. However even smaller replicators are involved, as genes drive cellular and bacterial replication. Other replicators hijack the ability of certain replicators to get built. Viral Replicators are a good example, as a virus hijacks a cell replicator to produce more viruses (and not more cells).
Some replicators exist in theory only. The von Neumann machine is an old hypothetical example of a "clanking" replicator - machines capable of building copies of themselves. Well, we do have robot factories that produce cars, so it wouldn't be too far-fetched to expect robots that could create more robots.
The most thorough analysis of artificial self-replication is without doubt Kinematic Self-Replicating Machines in which the authors claim that artificial self-replication is no longer theoretical and therefore no longer the exclusive preserve of living systems (Freitas, Merkle, 2004, p1).
One of the most interesting and revolutionary concepts for new replicators is molecular nanotechnology (MNT). Although still currently theoretical, the long-term goal for MNT is the assembler, capable of using artificial replicators to produce any object (given an adequate supply of the right raw materials). The object would be built molecule by molecule, mimicking the process of biological self-replication. See Grey Goo - An Exponentialist View for more on nanotechnology.
What Is Life?
Replication can take many forms, such as sexual reproduction, asexual reproduction, viral replication, DNA replication, memetic replication, clanking self replication, or MNT assembler self-replication. Each involves what I term a replication event. The Exponentialist view of life rejects cellular chauvinism and emphasises the fact that all living entities are the result of a replication event:
A living entity is the result of a replication event by one or more replicators, and is encoded with the instructions of its own assembly.
Given that not all living entities are able to individually or jointly replicate, the ability to metabolise is also central to the definition of a living entity:
A living entity can either metabolise or replicate, or it can do both.
These definitions are similar to the universal definitions attempted by reprogenetics advocate Lee M. Silver (Silver, 1998):
"First, as noted, an absolute requirement for life of any kind is the ability to use energy for the purpose of maintaining information and structure. Although this property alone does not define life, it is clear that life cannot exist without it."
"Second, living things generally have the ability to reproduce themselves. Life begets life. There are, of course, many exceptions to this rule, such as the sterile hybrid of a horse and a donkey called a mule. Some mutated forms of simple life can lose the ability to reproduce. But while many such individual living creatures have lost the ability to reproduce, each one is still the product of reproduction from some similar parent or parents."
Imperfect Replication Results In Evolution
However, as Silver argues, the key is that reproduction and replication are not 100% accurate. Hence, variations occur and evolution follows and so (Silver, 1998):
"...it does seem possible to provide a complete definition of life-in-general at the individual level as a product of reproduction and evolution that uses energy to maintain self-defining information and organisation. The inanimate becomes animate only upon achieving the ability to evolve."
According to Dawkins, the three main attributes of a replicator are fecundity , fidelity, and longevity (Dawkins, 1976). In other words, the quantity of replication (how many copies), the quality of replication (how good are the copies), and the length of time the replicator exists. Fidelity in replication is never perfect. Thus, replicators do not necessarily produce replicas - they produce copies which may or may not be identical. For those biological replicators based on DNA, it is this key aspect of one of the attributes of a replicator - the lack of perfect fidelity - that leads to evolution (Dawkins, 1976):
"We do not know how accurately the original replicator molecules made their copies. Their modern descendants, the DNA molecules, are astonishingly faithful compared with the most high-fidelity human copying process, but even they occasionally make mistakes, and it is these mistakes that make evolution possible."
Or, to put it another way, self-replication is the origin of all evolved species (Witting, 1997, p. 4):
"...self-replication is the origin from which all living organism have evolved."
See Witting - An Exponentialist View for more.
Even when considering the futuristic replicators of molecular nanotechnology, there is acknowledgement that this lack of fidelity in replication determines the process of evolution, which thrives on variation and selection (Drexler, 1990):
"Evolution proceeds by the variation and selection of replicators."
See Drexler - An Exponentialist View for more.
As Dawkins explains, replicator populations grow via exponential growth until checked by limits to growth (Dawkins, 1996, my bolding):
"It is the nature of a replicator that it generates a population of copies of itself, and that means a population of entities that also undergo duplication. Hence the population will tend to grow exponentially until checked by competition for resources or raw materials. I'll develop the idea of exponential growth in a moment. Briefly, the population doubles at regular intervals, rather than adding a constant number at regular intervals. This means that there will soon be a very large population of replicators and hence competition between them."
Let's break down some of the (bolded) concepts in this Dawkins quote to establish the connection alluded to in my introduction between Malthus and replicators.
I explain the concept very Malthusian concept of population doubling in my article The Mechanism Of Population Doubling, and I re-examine the concept of exponential growth in my article What Is Exponential? Exponential growth is also known sometimes known as Malthusian Increase (modelled via the Malthusian Growth Model), and the concept of limits to growth is also sometimes known as the Malthusian Principle or the Malthusian Wall. This in turn leads to the concept of the Malthusian catastrophe, or population collapse due to a replicator population having hit those limits and exceed its resource base.
The competition between replicator populations is sometimes known as the struggle for existence which, as Darwin acknowledged, is a Malthusian concept (Darwin, 1859, p.8):
"In the next chapter the Struggle for Existence amongst all organic beings throughout the world, which inevitably follows from their high geometrical powers of increase, will be treated of. This is the doctrine of Malthus, applied to the whole animal and vegetable kingdoms."
In addition, the growth rate of a population is sometimes referred to as the Malthusian Parameter, and measures the fitness of a population in relation to its environment (Fisher, 1930). Hence the evolutionary concept of the survival of fittest, similar to Malthusian doctrine of the struggle for existence, relates to the differential replication of discrete replicators populations whose fitness is determined by their Malthusian Parameter.
One fairly recent and exciting addition to the Malthusian lexicon in relation to replicators and evolutionary processes is the concept of Malthusian Relativity - the new and unconventional scientific view that evolution is deterministic, and that complexity inevitably follows whenever self-replication is present (Witting, 1997):
""...self-replicating molecules automatically evolve toward the complex organisms on Earth."
See Witting - An Exponentialist View for more on this new idea.
Although I have noted the enormous influence of fundamental Malthusian concepts in connection to the study of replicators and evolutionary processes, I should note that Malthus never internationally wrote about evolution - see my article Malthus and Evolution for more. Nor did he use the term replicator, or discuss the concept of self-replication directly.
Finally, please note that Malthusian views are not necessarily the same as my Exponentialist views - refer to the summary on the Exponentialist Homepage for more.
Darwin, Charles. Origin Of Species The Illustrated Edition*. Sterling Publishing Co. 1859, 2008*.
Dawkins, Richard, The Selfish Gene. Oxford University Press.1976, 1989.
Dawkins, Richard, The Extended Phenotype. Oxford University Press. 1982
Dawkins, Richard, Climbing Mount Improbable. Penguin. 1996.
Drexler, K. Eric. Engines Of Creation - The Coming Era of Nanotechnology. Oxford University Press. 1990.
Drexler, K. Eric. Engines Of Creation - The Coming Era of Nanotechnology. Online Glossary (website accessed 23rd September 2009). 1986.
Fisher R. A.. The Genetical Theory of Natural Selection - A Complete Variorum Edition* - Oxford University Press. 1930, 1958, 1999, 2003*.
Freitas, Robert, A., Merkle, Ralph, C., Kinematic Self-Replicating Machines. Landes Bioscience. 2004.
Malthus, Thomas Robert, An Essay on the Principle of Population. J. Johnson. Library of Economics and Liberty. 1798. (1st edition)
Silver, Lee M. Remaking Eden. Cloning and Beyond in a Brave New World. Weidenfeld & Nicolson. 1998.
Witting, Lars, A General Theory of Evolution By Means of Selection by Density Dependant Competitive Interactions. Peregrine. 1997
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