In the book The Blind Watchmaker, Richard Dawkins explains how evolution happens through accumulated small random variations (or mutations) filtered by the natural selection, as opposed to the mainstream misinterpretation that evolution happens in big leaps. He proposes a computer program, called weasel, to demonstrate how the accumulated small improvements (mutations that bring a benefit to the individual so that it is “chosen” by the natural selection) produce faster results.
I don’t know who it was first pointed out that, given enough time, a monkey bashing away at random on a typewriter could produce all the works of Shakespeare. The operative phrase is, of course, given enough time. Let us limit the task facing our monkey somewhat. Suppose that he has to produce, not the complete works of Shakespeare but just the short sentence ‘Methinks it is like a weasel’, and we shall make it relatively easy by giving him a typewriter with a restricted keyboard, one with just the 26 (capital) letters, and a space bar. How long will he take to write this one little sentence?
[...]
We again use our computer monkey, but with a crucial difference in its program. It again begins by choosing a random sequence of 28 letters, just as before … it duplicates it repeatedly, but with a certain chance of random error – ‘mutation’ – in the copying. The computer examines the mutant nonsense phrases, the ‘progeny’ of the original phrase, and chooses the one which, however slightly, most resembles the target phrase, METHINKS IT IS LIKE A WEASEL.
The phrase “METHINKS IT IS LIKE A WEASEL” is from Hamlet:
Hamlet: Do you see yonder cloud that’s almost in shape of a camel?
Polonius: By the mass, and ’tis like a camel, indeed.
Hamlet: Methinks it is like a weasel.
The algorithm as described on Wikipedia, is as follows:
- Start with a random string of 28 characters.
- Make 100 copies of this string, with a 5% chance per character of that character being replaced with a random character.
- Compare each new string with the target “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.
A character is an upper case letter or space, the number of copies per iteration could be 100 and the mutation rate 5%.
The following C# code is an implementation of the algorithm.
class Weasel1
{
private const string ALLOWED_CHARS = "ABCDEFGHIJKLMNOPQRSTUVXYWZ ";
private const string TARGET = "METHINKS IT IS LIKE A WEASEL";
Random rand = new Random();
private int Fitness(string candidate)
{
int score = 0;
for (int i = 0; i < candidate.Length; ++i)
{
if (candidate[i] == TARGET[i])
score++;
}
return score;
}
private string Mutate(string parent)
{
StringBuilder builder = new StringBuilder();
for (int i = 0; i < parent.Length; ++i)
{
bool mutate = rand.Next(20) == 10;
builder.Append(
mutate ?
ALLOWED_CHARS[rand.Next(ALLOWED_CHARS.Length)] : parent
[i]);
}
return builder.ToString();
}
public void Run(int copies)
{
StringBuilder builder = new StringBuilder();
for (int i = 0; i < TARGET.Length; ++i)
{
builder.Append(ALLOWED_CHARS[rand.Next(ALLOWED_CHARS.Length)]);
}
string parent = builder.ToString();
int step = 1;
Console.WriteLine("{0:D3} {1}", step, parent);
do
{
step++;
string[] children = new string[copies];
for (int i = 0; i < copies; ++i)
{
children[i] = Mutate(parent);
}
int indexMax = 0;
int scoreMax = 0;
for (int i = 0; i < copies; ++i)
{
int score = Fitness(children[i]);
if (score > scoreMax)
{
scoreMax = score;
indexMax = i;
}
}
parent = children[indexMax];
Console.WriteLine("{0:D3} {1}", step, parent);
} while (parent != TARGET);
}
}
class Program
{
static void Main(string[] args)
{
Weasel1 weasel = new Weasel1();
weasel.Run(100);
}
}
Function Fitness() computes how similar an offspring is to the parent, function Mutate() makes a copy of the parent with a 5% chance for each character to mutate. Function Run() runs the algorithm until the target string is reached. A possible output of the program is shown below. Different runs require different number of iterations (even more than 100).
001 E PVBQVRYRNQSRBCDLOCFOGEIB J 002 E PVBQVRYRNQSRBCDLOCFOWEIB J 003 E PVIQVRYRNQSRBCDLOCFOWEIB J 004 EEPLIQVUYRNQSRBCDLOCFOWEIB J 005 EEPLIQVU RNQSRBCDLOCFOWEIB J 006 EEPHIQLU RNQSRBCDLOCFKWEIB J 007 EEPHIQLU RNQSRHCDKOCFKWEIB J 008 EEPHIQLU RNQSSHCDKOCFKWEIB J 009 EEPHIQLU RNQSS CDKOCFKWEIB J 010 EEPHIQLU RNQSS CDKOCWKWEKB L 011 EEPHIQLU RNQSS CDKOCOKWEKB L 012 EEPHIQLU RN SS CDKOGOKWEKB L 013 EEPHIQLU RN SS LDKOGOKWEKB L 014 EEPHIQLU RT SS LDKOGOKWEKB L 015 EEPHIQLU RT SS LDKOGOKWEKBEL 016 EEPHIQLU RT SS LPKOGOKWEKBEL 017 EEPHIQLU RT SS LPKOGOKWEKSEL 018 EEPHIQLU IT SS LPKOGOKWEKSEL 019 EEPHIQLU IT SS LIKOGOKWEKSEL 020 EEPHIQLU IT SS LIKOGOKWEKSEL 021 EEKHIQLS IT SS LIKOGOKWEKSEL 022 EEKHIQLS IT SS LIKOGOKWEKSEL 023 EEKHIQLS IT SS LIKO OKWEKSEL 024 EEKHIQLS IT SS LIKO OFWEKSEL 025 EEKHIQLS IT ES LIKO OFWENSEL 026 EEKHINLS IT ES LIKO OFWENSEL 027 EEKHINLS IT ES LIKO O WENSEL 028 EEKHINLS IT ES LIKO O WECSEL 029 EEKHINLS IT BS LIKO O WEHSEL 030 EEKHINLS IT QS LIKO O WEASEL 031 EETHINSS IT QS LIKO O WEASEL 032 EETHINSS IT QS LIKO O WEASEL 033 EETHINSS IT IS LIKO O WEASEL 034 EETHINSS IT IS LIKO O WEASEL 035 EETHINBS IT IS LIKO O WEASEL 036 XETHINBS IT IS LIKO O WEASEL 037 XETHINBS IT IS LIKO O WEASEL 038 XETHINBS IT IS LIKO O WEASEL 039 XETHINBS IT IS LIKO O WEASEL 040 XETHINBS IT IS LIKK O WEASEL 041 XETHINBS IT IS LIKK O WEASEL 042 XETHINBS IT IS LIKK O WEASEL 043 XETHINKS IT IS LIKK O WEASEL 044 XETHINKS IT IS LIKK WEASEL 045 XETHINKS IT IS LIKK WEASEL 046 XETHINKS IT IS LIKK WEASEL 047 XETHINKS IT IS LIKE WEASEL 048 XETHINKS IT IS LIKE WEASEL 049 METHINKS IT IS LIKE WEASEL 050 METHINKS IT IS LIKE WEASEL 051 METHINKS IT IS LIKE A WEASEL
We can change this algorithm to randomize the chance of mutation. The changes to the code are small (basically function Mutate changes to take not only the parent string but only the chance for the mutation rate), but this increases the number of iterations needed to reach the target string.
class Weasel2
{
const string TARGET = "METHINKS IT IS LIKE A WEASEL";
const string ALLOWED_CHARS = "ABCDEFGHIJKLMNOPQRSTUVWXYZ ";
Random rand = new Random();
private double Fitness(string candidate)
{
int score = 0;
for (int i = 0; i < candidate.Length; ++i)
{
if (candidate[i] == TARGET[i])
score++;
}
return score;
}
private string Mutate(string parent, double rate)
{
StringBuilder builder = new StringBuilder();
foreach(char c in parent)
{
builder.Append(
rand.NextDouble() < rate ?
c:
ALLOWED_CHARS[rand.Next(ALLOWED_CHARS.Length)]);
}
return builder.ToString();
}
public void Run(int copies)
{
StringBuilder builder = new StringBuilder();
for (int i = 0; i < TARGET.Length; ++i)
{
builder.Append(ALLOWED_CHARS[rand.Next(ALLOWED_CHARS.Length)]);
}
string parent = builder.ToString();
int step = 1;
Console.WriteLine("{0:D4} {1}", step, parent);
do
{
string[] children = new string[copies];
for(int i = 0; i < copies; ++i)
{
children[i] = Mutate(parent, rand.NextDouble());
}
int indexMax = 0;
double scoreMax = 0;
for (int i = 0; i < copies; ++i)
{
double score = Fitness(children[i]);
if (score > scoreMax)
{
scoreMax = score;
indexMax = i;
}
}
parent = children[indexMax];
Console.WriteLine("{0:D4} {1}", step, parent);
step++;
} while (parent != TARGET);
}
}
Here is the output, but because of the bigger number of iterations, only every 20th iteration is shown:
0001 BXZZBDJSZTVYFWRSGNKYEULISWRK 0020 MSTCFNBSTITVIS L KPLEDBRASEC 0040 METCINQSDITVIS LIKW A IRISEW 0060 METLINKSOITJIS LJKW A IGASEW 0080 METLINKSOITJIS LJKX A WEAUEL 0100 METLINKSTKTXIS LIKK A WEAOEL 0120 METHINKS KTXIS LIKH A WEAOEL 0140 METHINKSDKTXIS LIKA A WEASEL 0160 METHINKSEGTXIS LIKA A WEASEL 0180 METHINKSEGTEIS LIKE A WEASEL 0200 METHIMKSEGTMIS LIKE A WEASEL 0220 METHIMKSTITNIS LIKE A WEASEL 0240 METHIAKSTIT IS LIKE A WEASEL 0260 METHIPKSTIT IS LIKE A WEASEL 0280 METHJKKSTOT IS LRKE A WEASEL 0300 METHYQKSCOT IS LRKE A WEASEL 0320 METHFPKSCOT IS LRKE A WEASEL 0340 METHFPKSCGT IS LRKE A WEASES 0360 METHINKS XT IS LMOE A WEAIEL 0380 METHINKS YT IS LMZE A WEAIEL 0400 METHINKS IT IS LMHE A WEAIEL 0420 METHINKS IT IS LEDE A WEANEL 0440 METHINKS IT IS LIPE A WEANYL 0460 METHINKS IT IS LIPE A WEAOYL 0480 METHINKS IT IS LIWE A WEARWL 0500 METHINKS IT IS LIWEDA WEAFWT 0520 METVINKS IT IS LILE A WEABBG 0540 METVINKS IT IS LIJE A WEABTG 0560 METHINKS IT IS LIJE A WEAMRV 0580 METHINKS IT IS LIZE A WEAMRV 0600 METHINKS IT IS LIZE A WEA OV 0620 METHINKS IT IS LIZE A WEAHE 0640 METHINKS IT IS LIJE A WEAHEH 0660 METHINKS IT IS LIFE A WEAHEH 0680 M THINKS IT IS LINE I WXAHEL 0700 MQTHINKS IT IS LIAE I WXASEL 0720 MZTHIRKS IT IS LIKE T WXASEL 0740 METHINKR IT IS LIKE T WRASEL 0760 METOINKI IT IS LIKE T WRASEL 0780 METMINKM IT IS LIKE A WQASEL 0800 METMINKS IT IS LIKE A WQASEL 0820 METJIXKS IT IS LIKE A WQASEL 0840 METJILKS IT IS LIKE A WUASEL 0860 METJIPKS IP IS LIKE A WAASEL 0880 METJIPKS IW IS LIKE A WLASEL 0900 METIIBKS QT IS LIKE A SEASEL 0920 METIIPKS PT IS LIKE A SEASEL 0940 METKINKS PT IS LIKE A TEASEL 0960 METKINKS BT IS LIKE A QEASEL 0980 METWINKS IT IS LIKE A QEASEL 1000 METOINKS IT IS LIKE ANQEASEL 1020 METUINKS IT IS LIKE AHTEASEL 1040 METUINKS IT IS LIKE AMWEASEO 1060 METQINKS IT IS LIKE AMWEASEO 1080 MZTYINKS IT ISULIKE AKWEASEL 1100 MZTYINKS IT ISULIKE APWEADEL 1120 MJTHINKS IT ISULIKE AEWEAQEL 1140 MJTHINKS IT ISULIKE AFWEAUEL 1160 MJTHINKS IT ISCLIKP AVWEABEL 1180 MJTHINKS IT IS LIKP AVWEAGEG 1200 MNTHINKS IT IS LIKM AVWEACE 1220 MNTHINKS IT IS LIKM AQWEACEL 1240 MNTHINKS IT IS LIKMXAKWEAAEL 1260 MNTHINKS CT IS LIKMGAPWEASEL 1280 MPTHINKS IX IS EIK GA WEASEL 1300 METHINKS IX IS LIK RA WEASEL 1320 METHINKS IZ IS LIKEWA WEASEL 1340 METHINKS IZ ISILIKEWA WEASEL 1360 METHINKS IJ ISILIKE A WEASEL 1380 METHINKS IM ISELIKE A WEASEL 1400 METHINKS IM ISELIKE A WEASEL 1420 METHINKS IU ISELIKE A WEASEL 1440 METHINKS IY ISELIKE A WEASEL 1460 METHINKS IT ISVLIKE A WEASEL 1480 METHINKS IT ISCLIKE A WEASEL 1500 METHINKS IT ISCLIKF A WEASEL 1520 METHINKS IT ISTLIKF A WEASEL 1540 METXINKS IT ISOLIKF A WEASEL 1560 MZTXINKS IT ISOLIKC A WEASEL 1580 MATXINKS IT IS LIKC A WEASEL 1600 MATXINKS IT IS LIKE A WEDSEL 1620 METHINKSGITVIC LIKE A WEASEL 1640 METHINKSGITQIC LIKE A WEASEL 1660 METHINLSAITQIC LIKE A WEASEL 1680 METHINLSAITQIH LIKE A WEASEL 1700 METHINGS ITAIH LIKE A WEASEL 1720 METHINGS ITRIS LIKE W WEASEL 1740 METHINVS IT IS LIJE A WEASEL 1760 METHINVS IT IS LIGE A WEASEL 1780 METHINVS IT IS LIGE A WEASEL 1800 METHINVS IT IS LI E A WEASYL 1820 METHIN S IT IS LIKE A WTASYT 1840 METHINQS IT IS LIKE A STASYT 1860 METHINWA IT IS LIKE A IFASES 1880 METHINKA IT IS LIKE A IFASEJ 1900 METHINKA IT YS LIKE O IEAAEL 1920 METHINKU IT YS LIKE O WEAAEL 1940 METHOJKM IT JS LIKE O WEAPEL 1960 METHOJKF IT IS LIKE RWEATEL 1980 METHFGKF IT IS LIKE ARWBATEL 2000 METHYNKG IT IS LIKE ARWEALEL 2020 METHUNKD IT IS LIKE AAWEAWEL 2040 METHIWKN IT IS LIKE AYWEADEL 2060 METHIUKN IT IS LIKE AKWEAQEL 2080 METHIUKN IT IS LIKE A WEAQEL 2100 METHIAKN IT IS LIKE N WEAQEL 2120 METHINKK IT IS LIKE N WEEQEL 2140 METHDNKQ IT IS LIKE A WEBSEL 2160 METHDNKQ IT IS LIKE A WEBSEL 2180 METHDNKQ IT IS LIKE A WEBSEL 2200 METHMNKQ IT IS LIKE A WEBSEL 2220 MEKHBNKX IT IS LIKE A WEBSEL 2240 MEBHQNKS IT IS LIKE A WEBSEL 2260 METHQNKS IT IS LIKEZA WEFSEL 2280 METHGNKS IT IS LIKEZA WESSEL 2300 METHGNKS IT IS LIKEZA WEWSEL 2320 METHLNKS IT IP BIKE A WEOSEL 2340 METHYNKS IT IB SIKE A WEQSEL 2360 METHINKS ITQIJ LIKE A WEQSEL 2380 METHINKS ITIIJ LIKE A WEYSTL 2400 METQINKS ITIIJ LIKE A WEASGL 2420 METHINKS ITIIJJLIKE A WEASGL 2440 METHINKS ITQIWJLIKE A WEAS L 2460 METHINKS ITZIWJLIKE A WEASCL 2480 METHINKS IT IDLLIZE A UEASCL 2500 METHINKS IT ID LIZE A IEASZL 2520 METHINKS IT ID LIZE A IEASZL 2540 METHINKS IT IB LIZE A XEASOL 2560 METHINKS IT IB LIAE A EASOL 2580 METHINKS IT IBVLICE A EASEL 2600 METHINKS IT IBVLIKE A EASEL 2620 METHINKS IT IKDLIKE A EASEL 2640 METHINKS IT IKNLIKE A HEASEL 2660 METHINKS IT IKNLIKE A HIASEL 2680 METHINKS IT IZNLIKE A HIASEL 2700 METHINKS IT IZNLIKE A WOASEL 2720 METHINKS IT ISNLIKE A WOASEL 2740 METHINKS IT ISNLIKE M WSASEL 2760 METHINKS IT ISVLIKE M WXASEL 2780 METHINKS IT IS LIKE S WEASEL 2785 METHINKS IT IS LIKE A WEASEL
Notice that matching letters are not immutable. A letter, even if it matches one in the target string can change at any time.
You can find similar implementations in several programming languages on the Rosetta Code website.
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