Is transportation making us unfit for our environments?

In chapters 2, 3, and 4, Dr. Moalem describes traits of populations originating from several different areas and how they were useful in the locations they arose in, but were detrimental when individuals moved to a different part of the globe (for example, diabetes aiding those in freezing climates with limited sugar intake but being deadly untreated today). And historically, whenever a group of people from one area moved to another (for example the Spanish conquistadors sailing to South and Central America), either the invaders or the invaded were often devastated by disease.

Now that people can move across the globe much more easily and often than in the past, is the human race becoming more or less fit for their surroundings? Does the genetic mixing of races provide extra benefits or liabilities for offspring? Or, does our intelligence and innovation (or the evolution of our intellect) mean that we no longer have to worry about being fit for our environment and instead can make our environment fit for us?