Well i think that it had little need to evolve over the years because it had no predators that would go after it.
closed as unclear what you're asking by Chenmunka, Paulie_D, Aegon, tobiasvl, Au101 Apr 6 '17 at 13:56
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Evolution is the process where life-forms change over successive generations, either as a response to natural selection, genetic drift, random mutation or gene migration. Changes due to natural selection (which includes predation) will usually take several generations.
Given that there have only been four Tyrannosaurs seen in the film series (1, 2, 3, 4) and given that all but one appear to be the result of genetic manipulation and artificial breeding rather than natural sexual gene transfer, there simply hasn't been scope for any selection-based changes to take place.