Allele frequencies can change in response to natural selection or in response to a reduction in selection pressures. As some of the selective forces that have acted on humans in the past are eliminated — humans no longer need to run away from big, fierce, hungry animals, for example — other alleles can increase in frequency as a result of genetic drift (random events; refer to Chapter 6).
In an environment in which running fast doesn't confer any particular benefit, not being able to run fast doesn't mean that humans are in any way less fit. As long as no tigers are around, our slow alleles don't have a lower probability of being passed on to the next generation. So we may get slower, but we don't get less fit. (Note that there isn't selection for slower running; there just isn't selection either way.)
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