So much for single-step selection of random variation. What about cumulative selection; how much more effective should this be? Very very much more effective, perhaps more so than we at first realize, although it is almost obvious when we reflect further. 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:
WDLMNLT DTJBKWIRZREZLMQCO P
It now 'breeds from' this random phrase. 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. In this instance the winning phrase of the next 'generation' happened to be:
WDLTMNLT DTJBSWIRZREZLMQCO P
Not an obvious improvement! But the procedure is repeated, again mutant 'progeny' are 'bred from' the phrase, and a new 'winner' is chosen. This goes on, generation after generation. After 10 generations, the phrase chosen for 'breeding' was:
MDLDMNLS ITJISWHRZREZ MECS P
After 20 generations it was:
MELDINLS IT ISWPRKE Z WECSEL
By now, the eye of faith fancies that it can see a resemblance to the target phrase. By 30 generations there can be no doubt:
METHINGS IT ISWLIKE B WECSEL
Generation 40 takes us to within one letter of the target:
METHINKS IT IS LIKE I WEASEL
And the target was finally reached in generation 43. A second run of the computer began with the phrase:
Y YVMQKZPFJXWVHGLAWFVCHQXYOPY,
passed through (again reporting only every tenth generation):
Y YVMQKSPFTXWSHLIKEFV HQYSPY
YETHINKSPITXISHLIKEFA WQYSEY
METHINKS IT ISSLIKE A WEFSEY
METHINKS IT ISBLIKE A WEASES
METHINKS IT ISJLIKE A WEASEO
METHINKS IT IS LIKE A WEASEP
and reached the target phrase in generation 64. In a third run the computer started with:
GEWRGZRPBCTPGQMCKHFDBGW ZCCF
and reached METHINKS IT IS LIKE A WEASEL in 41 generations of selective 'breeding'.
