I wanted to post a paragraph from an article on Evolution at Wikipedia.org.
From Wikipedia
Many aspects of genetic drift depend on the size of the population (generally abbreviated as N). This is especially important in small mating
populations, where chance fluctuations from generation to generation can be large. The relative importance of natural selection and genetic drift in
determining the fate of new mutations also depends on the population size and the strength of selection: when N times s (population size times
strength of selection) is small, genetic drift predominates. When N times s is large, selection predominates. Thus, natural selection is 'more
efficient' in large populations, or equivalently, genetic drift is stronger in small populations. Finally, the time for an allele to become fixed in
the population by genetic drift (that is, for all individuals in the population to carry that allele) depends on population size, with smaller
populations requiring a shorter time to fixation.

Here's the full article:
en.wikipedia.org...
I hope this helps a little to explain the point that I'm trying to make, since I talked to a few people about this and they seemed a little confused.
I recommend reading these sections, if not the entire page: Gene Flow, Population Structure, Drift, and Natural Selection.
Now, if a large population is to evolve
as a whole, then there need to be factors of natural selection. In the case of humans, we have an
extremely large population for our species, and thus an almost inconceivably large amount of genetic variation (far more than necessary). So
to help in our evolution, we would need to have
extremely large factors of natural selection. That's not to say that we are completely void
of these factors, just that they are too insignificant to affect our population as a whole. Nothing short of large scale natural disasters all over
the world would be that significant.
That's where I think the aliens come in. Assume for a moment that we are a part of some sort of breeding program... for whatever purpose. Now
apply to humans what we
already apply to the animals and plants that we breed for ourselves. Doesn't it make sense for the breeder to get rid
of undesirable traits, and keep the desirable ones?