We report a measurement of the top-quark mass Mt in the dilepton decay channel tt→bl'+ν′lbl−νl. Events are selected with a neural network which has been directly optimized for statistical precision in top-quark mass using neuroevolution, a technique modeled on biological evolution. The top-quark mass is extracted from per-event probability densities that are formed by the convolution of leading order matrix elements and detector resolution functions. The joint probability is the product of the probability densities from 344 candidate events in 2.0  fb−1 of pp collisions collected with the CDF II detector, yielding a measurement of Mt=171.2±2.7(stat)±2.9(syst)  GeV/c2.