OK, I've just given myself a 2 hour genetics lesson from:
http://www.dla.utexas.edu/depts/anthro/cbramblett/ant301/four.html and
http://www.zoology.ubc.ca/~otto/Bio336/Lectures.html (lectures 6 and 7)


Imagine a population of 100,000 in which a mutation occurs.
Assume the mutation has a 50% chance of being passed on to offspring.
Also assume the mutation gives the bearer a 2% advantage over a non-bearer.
This advantage could be a greater fertility rate, a resistance to a disease,
a greater ability to raise young etc.

That mutation will have 'infected' 10% of the population after 471 generations.
Would this be where middle-eastern humans are now - 10% grain tolerant?

Here's a table for mutation 'infection' rate based on different percentage
of advantage rendered by the mutation.

Percentage      Generations required to achieve that percentage
'infected'      1% adv  2% adv  3% adv  4% adv  5% adv
0.001           1       1       1       1       1
0.01            231     117     78      59      48
0.1             463     233     156     118     95
1               696     350     235     177     142
10              937     471     316     238     191
25              1047    526     353     266     214
50              1158    582     390     294     236
75              1268    637     427     322     259
90              1378    693     464     350     282
99              1619    814     545     411     331
99.9            1851    931     623     470     378

 ...Richard.