Starting with:

 The target string: "METHINKS IT IS LIKE A WEASEL".
 An array of random characters chosen from the set of upper-case letters together with the space, and of the same length as the target string. (Call it the parent).
 A fitness function that computes the ‘closeness’ of its argument to the target string.
 A mutate function that given a string and a mutation rate returns a copy of the string, with some characters probably mutated.
 While the parent is not yet the target:
 copy the parent C times, each time allowing some random probability that another character might be substituted using mutate.
 Assess the fitness of the parent and all the copies to the target and make the most fit string the new parent, discarding the others.
 repeat until the parent converges, (hopefully), to the target.


See also

   Wikipedia entry:   Weasel algorithm.
   Wikipedia entry:   Evolutionary algorithm.

Note: to aid comparison, try and ensure the variables and functions mentioned in the task description appear in solutions



A cursory examination of a few of the solutions reveals that the instructions have not been followed rigorously in some solutions. Specifically,

 While the parent is not yet the target:
 copy the parent C times, each time allowing some random probability that another character might be substituted using mutate.
Note that some of the the solutions given retain characters in the mutated string that are correct in the target string. However, the instruction above does not state to retain any of the characters while performing the mutation. Although some may believe to do so is implied from the use of "converges"

(:* repeat until the parent converges, (hopefully), to the target.

Strictly speaking, the new parent should be selected from the new pool of mutations, and then the new parent used to generate the next set of mutations with parent characters getting retained only by not being mutated. It then becomes possible that the new set of mutations has no member that is fitter than the parent!

As illustration of this error, the code for 8th has the following remark.

Create a new string based on the TOS, changing randomly any characters which
don't already match the target:

NOTE: this has been changed, the 8th version is completely random now

Clearly, this algo will be applying the mutation function only to the parent characters that don't match to the target characters!

To ensure that the new parent is never less fit than the prior parent, both the parent and all of the latest mutations are subjected to the fitness test to select the next parent.




 
/* Weasel.rex - Me thinks thou art a weasel. - G,M.D. - 2/25/2011 */
arg C M
/* C is the number of children parent produces each generation. */
/* M is the mutation rate of each gene (character) */
 
call initialize
generation = 0
do until parent = target
   most_fitness = fitness(parent)
   most_fit     = parent
   do C
      child = mutate(parent, M)
      child_fitness = fitness(child)
      if child_fitness > most_fitness then
      do
         most_fitness = child_fitness
         most_fit = child
         say "Generation" generation": most fit='"most_fit"', fitness="left(most_fitness,4)
      end
   end
   parent = most_fit
   generation = generation + 1
end
exit
 
initialize:
   target   = "METHINKS IT IS LIKE A WEASEL"
   alphabet = "ABCDEFGHIJKLMNOPQRSTUVWXYZ "
   c_length_target = length(target)
   parent  = mutate(copies(" ", c_length_target), 1.0)
   do i = 1 to c_length_target
      target_ch.i = substr(target,i,1)
   end
return
 
fitness: procedure expose target_ch. c_length_target
   arg parm_string
   fitness = 0
   do i_target = 1 to c_length_target
      if substr(parm_string,i_target,1) = target_ch.i_target then
         fitness = fitness + 1
   end
return fitness
 
mutate:procedure expose alphabet
arg string, parm_mutation_rate
   result = ""
   do istr = 1 to length(string)
      if random(1,1000)/1000 <= parm_mutation_rate then
         result = result || substr(alphabet,random(1,length(alphabet)),1)
      else
         result = result || substr(string,istr,1)
   end
return result
 