Over at Gene Expression there is more about the new research about the acceleration of human evolution due to the increase in human population.
So what changed? Note that because there are many more individuals there are many more replications, so more mutational events will occur. The expectation is now that there will be 100000 mutations out of a population of 100 million, more mutations means that there will be a greater absolute number of beneficial mutants assuming that the expected proportion remains the same. The probability of fixation of a mutant of additive power (that is, two copies is twice as beneficial as one copy) is 2s, where s is a selection coefficient. There is theoretically a 20% chance that any one of the 10 mutants will fix, but of course the model is more complex than the assumptions which underly this because there are so many copies in any given generation that the allele isn’t expanding into a space of wild type until fixation but rather competing with other alleles. In any case, the implication is the same, there are so many mutants that the question is not if it will fix and substitute at the locus but when. Most of the time even a beneficial mutant will go extinct, but if you have enough production of these mutants then the likelihood that they will all go extinct is rather low. That results in the inevitability of their increase in frequency and eventual fixation.
And there’s the key, if the population of a species (in this case, humans) is large enough, then the likelyhood of potential mutations becoming fixed is greater. Now that there are more than 6 billion of us, what does that say about the rate of mutation? AKA, we’re evolving right now at a faster rate than we ever have!