The Theory of evolution is most predictive when the time it takes to observe a generation of organisms is less than the average (career) lifetimes of the individuals making the observation. For instance, a scientist can make predictive assessments on the mutation of bacteria to confer immunity to an antibiotic or to metabolize a novel organic compound, mainly due to the fact that bacteria can reproduce on the order of hours. Likewise, scientists can observe mosquito speciation because their generations are about a dozen per year- London subway mosquitoes are a new species, that, unlike their bird-feeding parent species, now feed on humans. Similarly for guppies and other fast-breeding fish (Most of my examples are from Finding Darwin's God by Kenneth Miller, who is also a Catholic scientist, just so you know).
Predictive power for the ToE for organisms with long generational times is minimal. This is mainly because we don't know how the environment is going to change on the order of 20-30 years, especially with how we are changing it. Additionally, technological innovation is currently more rapid than biologic change, so it's anyone's guess as to what evolutionary changes we'll eventually see.
However, to answer your question, I think that humans will probably become shorter and may eventually be able to metabolize a twinky
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