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.