I've often used the variety found within dogs and the role of selective breeding in evolution discussions. Frankly, given the advancements in medicine I see the human race over the next three generations getting equal diversity (if it hasn't already!). Folk who wouldn't have survived or been able to pass on their genes 20 years ago, now can. That is going to massively widen the gene pool of the human race.
And in my opinion that's a good thing....The fewer variations found within a species, the closer that species is to extinction.
All according to my untrained pop science of course....
If so, then the gene pool will sharply narrow again once parents can select the traits of their children, since the range of desirable traits is relatively narrow, and narrower than the current range of traits.
Socialism has an even bigger impact on human diversity. It broadens the gene-pool, mostly in undesirable directions, while jail serves to fine tune it. War's role in human evolution is as a big reset button, bringing the gene pool back into a very narrow range. It's why humans are remarkably the same the world over, and why the Neanderthals no longer exist.
This has been happening since the dawn of time but the pace has picked up over the past century. Every time we overcome a genetic trait that impedes survival or reproductive success the result is that the trait becomes more prevalent. Just two examples: The invention of eyeglasses led to an increase in nearsightedness as the almost-blind prospered, and the discovery of insulin caused diabetes to spike as diabetics could survive long enough to have children of their own. I think we'll see a similar rise in cystic fibrosis; when I was a kid the life expectancy of a CF patient was about 15 years but now it's not uncommon for them to make it past 25.
Frankly, I see it going the other way. Geographic isolation and social taboos led to populations that are diverse, but fairly homogenous within each; more mobility and more acceptance of intermarriage have begun to blur the lines. Tiger Woods -- 1/4 white, 1/4 black, 1/2 Thai -- for example.
You can look at England, where Celts, Saxons and Normans have essentially ceased to be discernibly separate ethnic groups. Now, you're right that this mobility and mixing of populations will lead to greater diversity in terms of combinations of genetic traits that didn't combine before, but it won't be anywhere near as obvious as it is with dogs, and it won't happen in three generations.
The range of traits among domesticated dogs is the result of hundreds of different eugenics programs designed to select different traits depending on human needs; for the same phenomenon to take place with humans, you'd have to have someone selectively breeding dwarves with dwarves, 7' men with 7' women, athletes with athletes, physicists with physicists, etc.