Evolution question for its adherents?

To the evolutionarily unbaptized, it seems that evolution might occur slowly, by the gradual accretion of random point-mutations over millions of years, but certainly could occur rapidly by the spread of genes already available in the population. For example, genes presumably exist among us for the eyes of Ted Williams, the endurance of marathon runners, the general physical plant of Mohammed Ali, the intelligence of Gauss, and so on. (This of course umes genetic determinism, which not all geneticists buy.) Are, or were, these becoming general? Perhaps. Show me. If not, one must conclude either that these qualities do not confer fitness, or that fitness does not become general. It seems odd to believe that mive structural changes can occur slowly through the aculation of accidental changes, but much more rapid increases in fitness do not occur through existent genes. Can we get answers, please? Concrete, non-metaphysical, demonstrable answers?