Self-replication is any behavior of a dynamical system that yields construction of an identical or similar copy of itself. Biological cells, given suitable environments, reproduce by cell division. During cell division, DNA is replicated and can be transmitted to offspring during reproduction. Biological viruses can replicate, but only by commandeering the reproductive machinery of cells through a process of infection. Harmful prion proteins can replicate by converting normal proteins into rogue forms. Computer viruses reproduce using the hardware and software already present on computers. Self-replication in robotics has been an area of research and a subject of interest in science fiction. Any self-replicating mechanism which does not make a perfect copy (mutation) will experience genetic variation and will create variants of itself. These variants will be subject to natural selection, since some will be better at surviving in their current environment than others and will out-breed them.
Overview
Theory
Early research by John von Neumann established that replicators have several parts:
A coded representation of the replicator A mechanism to copy the coded representation A mechanism for effecting construction within the host environment of the replicator Exceptions to this pattern may be possible, although almost all known examples adhere to it. Scientists have come close to constructing RNA that can be copied in an "environment" that is a solution of RNA monomers and transcriptase, but such systems are more accurately characterized as "assisted replication" than "self-replication". In 2021 researchers succeeded in constructing a system with sixteen specially designed DNA sequences. Four of these can be linked together (through base pairing) in a certain order following a template of four already-linked sequences, by changing the temperature up and down. The number of template copies is thus increased in each cycle. No external agent such as an enzyme is needed, but the system must be supplied with a reservoir of the sixteen DNA sequences. The simplest possible case is that only a genome exists. Without some specification of the self-reproducing steps, a genome-only system is probably better characterized as something like a crystal.
Origin of life Self-replication is a fundamental feature of life. It was proposed that self-replication emerged in the evolution of life when a molecule similar to a double-stranded polynucleotide (possibly like RNA) dissociated into single-stranded polynucleotides and each of these acted as a template for synthesis of a complementary strand producing two double stranded copies. In a system such as this, individual duplex replicators with different nucleotide sequences could compete with each other for available mononucleotide resources, thus initiating natural selection for the most "fit" sequences. Replication of these early forms of life was likely highly inaccurate producing mutations that influenced the folding state of the polynucleotides, thus affecting the propensities for strand association (promoting stability) and disassociation (allowing genome replication). The evolution of order in living systems has been proposed to be an example of a fundamental order generating principle that also applies to physical systems.
Classes of self-replication Recent research has begun to categorize replicators, often based on the amount of support they require.
Natural replicators have all or most of their design from nonhuman sources. Such systems include natural life forms. Autotrophic replicators can reproduce themselves "in the wild". They mine their own materials. It is conjectured that non-biological autotrophic replicators could be designed by humans, and could easily accept specifications for human products. Self-reproductive systems are conjectured systems which would produce copies of themselves from industrial feedstocks such as metal bar and wire. Self-assembling systems assemble copies of themselves from finished, delivered parts. Simple examples of such systems have been demonstrated at the macro scale. The design space for machine replicators is very broad. A comprehensive study to date by Robert Freitas and Ralph Merkle has identified 137 design dimensions grouped into a dozen separate categories, including: (1) Replication Control, (2) Replication Information, (3) Replication Substrate, (4) Replicator Structure, (5) Passive Parts, (6) Active Subunits, (7) Replicator Energetics, (8) Replicator Kinematics, (9) Replication Process, (10) Replicator Performance, (11) Product Structure, and (12) Evolvability.
A self-replicating computer program
In computer science a quine is a self-reproducing computer program that, when executed, outputs its own code. For example, a quine in the Python programming language is:
a='a=%r;print(a%%a)';print(a%a) A more trivial approach is to write a program that will make a copy of any stream of data that it is directed to, and then direct it at itself. In this case the program is treated as both executable code, and as data to be manipulated. This approach is common in most self-replicating systems, including biological life, and is simpler as it does not require the program to contain a complete description of itself. In many programming languages an empty program is legal, and executes without producing errors or other output. The output is thus the same as the source code, so the program is trivially self-reproducing.
Self-replicating tiling
In geometry a self-replicating tiling is a tiling pattern in which several congruent tiles may be joined to form a larger tile that is similar to the original. This is an aspect of the field of study known as tessellation. The "sphinx" hexiamond is the only known self-replicating pentagon. For example, four such concave pentagons can be joined to make one with twice the dimensions. Solomon W. Golomb coined the term rep-tiles for self-replicating tilings. In 2012, Lee Sallows identified rep-tiles as a special instance of a self-tiling tile set or setiset. A setiset of order n is a set of n shapes that can be assembled in n different ways so as to form larger replicas of themselves. Setisets in which every shape is distinct are called 'perfect'. A rep-n rep-tile is just a setiset composed of n identical pieces.
… excerpt ends here. Continue reading the full article.
