The weasel program or Dawkins' weasel is a

thought experiment A thought experiment is a hypothetical situation in which a hypothesis, theory, or principle is laid out for the purpose of thinking through its consequences. History The ancient Greek ''deiknymi'' (), or thought experiment, "was the most anci ...

and a variety of

computer simulation Computer simulation is the process of mathematical modelling, performed on a computer, which is designed to predict the behaviour of, or the outcome of, a real-world or physical system. The reliability of some mathematical models can be dete ...

s illustrating it. Their aim is to demonstrate that the process that drives

evolution Evolution is change in the heritable characteristics of biological populations over successive generations. These characteristics are the expressions of genes, which are passed on from parent to offspring during reproduction. Variation ...

ary systems—random variation combined with non-random cumulative

selection Selection may refer to: Science * Selection (biology), also called natural selection, selection in evolution ** Sex selection, in genetics ** Mate selection, in mating ** Sexual selection in humans, in human sexuality ** Human mating strategie ...

—is different from pure

chance Chance may refer to: Mathematics and Science * In mathematics, likelihood of something (by way of the Likelihood function and/or Probability density function). * ''Chance'' (statistics magazine) Places * Chance, Kentucky, US * Chance, Mary ...

. The thought experiment was formulated by

Richard Dawkins Richard Dawkins (born 26 March 1941) is a British evolutionary biologist and author. He is an emeritus fellow of New College, Oxford and was Professor for Public Understanding of Science in the University of Oxford from 1995 to 2008. An ath ...

, and the first simulation written by him; various other implementations of the program have been written by others.

Overview

In chapter 3 of his book ''

The Blind Watchmaker ''The Blind Watchmaker: Why the Evidence of Evolution Reveals a Universe without Design'' is a 1986 book by Richard Dawkins, in which the author presents an explanation of, and argument for, the theory of evolution by means of natural selecti ...

'', Dawkins gave the following introduction to the program, referencing the well-known

infinite monkey theorem The infinite monkey theorem states that a monkey hitting keys at random on a typewriter keyboard for an infinite amount of time will almost surely type any given text, such as the complete works of William Shakespeare. In fact, the monkey would ...

: The scenario is staged to produce a string of

gibberish Gibberish, also called jibber-jabber or gobbledygook, is speech that is (or appears to be) nonsense. It may include speech sounds that are not actual words, pseudowords, or language games and specialized jargon that seems nonsensical to outsider ...

letters, assuming that the selection of each letter in a sequence of 28 characters will be random. The number of possible combinations in this random sequence is 27²⁸, or about 10⁴⁰, so the

probability Probability is the branch of mathematics concerning numerical descriptions of how likely an Event (probability theory), event is to occur, or how likely it is that a proposition is true. The probability of an event is a number between 0 and ...

that the monkey will produce a given sequence is extremely low. Any particular sequence of 28 characters could be selected as a "target" phrase, all equally as improbable as Dawkins's chosen target, "METHINKS IT IS LIKE A WEASEL". A

computer program A computer program is a sequence or set of instructions in a programming language for a computer to execute. Computer programs are one component of software, which also includes documentation and other intangible components. A computer program ...

could be written to carry out the actions of Dawkins's

hypothetical A hypothesis (plural hypotheses) is a proposed explanation for a phenomenon. For a hypothesis to be a scientific hypothesis, the scientific method requires that one can test it. Scientists generally base scientific hypotheses on previous obser ...

monkey, continuously generating combinations of 26 letters and spaces at high speed. Even at the rate of millions of combinations per second, it is unlikely, even given the entire lifetime of the universe to run, that the program would ever produce the phrase "METHINKS IT IS LIKE A WEASEL". Dawkins intends this example to illustrate a common misunderstanding of

ary change, i.e. that DNA sequences or

organic compound In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen or carbon-carbon bonds. Due to carbon's ability to catenate (form chains with other carbon atoms), millions of organic compounds are known. The ...

s such as

proteins Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, respo ...

are the result of atoms randomly combining to form more complex structures. In these types of computations, any sequence of

amino acids Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although hundreds of amino acids exist in nature, by far the most important are the alpha-amino acids, which comprise proteins. Only 22 alpha am ...

in a protein will be extraordinarily improbable (this is known as

Hoyle's fallacy The junkyard tornado, also known as Hoyle's fallacy, is an argument used to deride the probability of abiogenesis as comparable to "the chance that a tornado sweeping through a junkyard might assemble a Boeing 747." It was used originally by En ...

). Rather, evolution proceeds by

hill climbing numerical analysis, hill climbing is a mathematical optimization technique which belongs to the family of local search. It is an iterative algorithm that starts with an arbitrary solution to a problem, then attempts to find a better solution ...

, as in

adaptive landscape Adaptation, in biology, is the process or trait by which organisms or population better match their environment Adaptation may also refer to: Arts * Adaptation (arts), a transfer of a work of art from one medium to another ** Film adaptation, a ...

s. Dawkins then goes on to show that a process of ''cumulative'' selection can take far fewer steps to reach any given target. In Dawkins's words: By repeating the procedure, a randomly generated sequence of 28 letters and spaces will be gradually changed each

generation A generation refers to all of the people born and living at about the same time, regarded collectively. It can also be described as, "the average period, generally considered to be about 20–⁠30 years, during which children are born and gr ...

. The sequences progress through each generation: :Generation 01: Note: the 4th character of line 1 is missing in Dawkins' text; however line 2 suggests it was probably a T :Generation 02: :Generation 10: :Generation 20: :Generation 30: :Generation 40: :Generation 43: Dawkins continues:

Implications for biology

The program aims to demonstrate that the preservation of small changes in an evolving string of characters (or

gene In biology, the word gene (from , ; "...Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredity..." meaning ''generation'' or ''birth'' or ''gender'') can have several different meanings. The Mendelian gene is a ba ...

s) can produce meaningful combinations in a relatively short time as long as there is some mechanism to select cumulative changes, whether it is a person identifying which traits are desirable (in the case of artificial selection) or a criterion of survival ("fitness") imposed by the environment (in the case of natural selection). Reproducing systems tend to preserve traits across generations, because the offspring inherit a copy of the parent's traits. It is the differences between offspring, the variations in copying, which become the basis for selection, allowing phrases closer to the target to survive, and the remaining variants to "die." Dawkins discusses the issue of the mechanism of selection with respect to his "biomorphs" program: Regarding the example's applicability to biological evolution, he is careful to point out that it has its limitations: Dawkins-Weasel

More complex models

In ''The Blind Watchmaker,'' Dawkins goes on to provide a graphical model of gene selection involving entities he calls biomorphs. These are

two-dimensional In mathematics, a plane is a Euclidean (flat), two-dimensional surface that extends indefinitely. A plane is the two-dimensional analogue of a point (zero dimensions), a line (one dimension) and three-dimensional space. Planes can arise as s ...

sets of

line segments In geometry, a line segment is a part of a straight line that is bounded by two distinct end points, and contains every point on the line that is between its endpoints. The length of a line segment is given by the Euclidean distance between i ...

which bear relationships to each other, drawn under the control of "genes" that determine the appearance of the biomorph. By selecting entities from sequential generations of biomorphs, an experimenter can guide the evolution of the figures toward given shapes, such as "airplane" or "octopus" biomorphs. As a simulation, the biomorphs are not much closer to the actual genetic behavior of biological organisms. Like the Weasel program, their development is shaped by an external factor, in this case the decisions of the experimenter who chooses which of many possible shapes will go forward into the following generation. They do however serve to illustrate the concept of "genetic space," where each possible gene is treated as a

dimension In physics and mathematics, the dimension of a Space (mathematics), mathematical space (or object) is informally defined as the minimum number of coordinates needed to specify any Point (geometry), point within it. Thus, a Line (geometry), lin ...

, and the actual genomes of living organisms make up a tiny fraction of all possible gene combinations, most of which will not produce a viable organism. As Dawkins puts it, "however many ways there may be of being alive, it is certain that there are vastly more ways of being dead". In ''Climbing Mount Improbable'', Dawkins responded to the limitations of the Weasel program by describing programs, written by other parties, that modeled the evolution of the

spider web A spider web, spiderweb, spider's web, or cobweb (from the archaic word '' coppe'', meaning "spider") is a structure created by a spider out of proteinaceous spider silk extruded from its spinnerets, generally meant to catch its prey. Spid ...

. He suggested that these programs were more realistic models of the evolutionary process, since they had no predetermined goal other than coming up with a web that caught more flies through a "trial and error" process. Spiderwebs were seen as good topics for evolutionary modeling because they were simple examples of biosystems that were easily visualized; the modeling programs successfully generated a range of spider webs similar to those found in nature.

Example algorithm

Although Dawkins did not provide the source code for his program, a "Weasel" style algorithm could run as follows. # Start with a random string of 28 characters. # Make 100 copies of the string (''reproduce''). # For each character in each of the 100 copies, with a probability of 5%, replace (''mutate'') the character with a new random character. # Compare each new string with the target string "METHINKS IT IS LIKE A WEASEL", and give each a score (the number of letters in the string that are correct and in the correct position). # If any of the new strings has a perfect score (28), halt. Otherwise, take the highest scoring string, and go to step 2. For these purposes, a "character" is any uppercase letter, or a space. The number of copies per generation, and the chance of mutation per letter are not specified in Dawkins's book; 100 copies and a 5% mutation rate are examples. Correct letters are not "locked"; each correct letter may become incorrect in subsequent generations. The terms of the program and the existence of the target phrase do however mean that such 'negative mutations' will quickly be 'corrected'.

References

*Dawkins, R. (1986) ''

'' Oxford University Press.

External links