Observation of the interplay between interacting energy levels of two spin species is limited by the difficulties in continuously tracking energy levels and thus leaves spin transport in quantum wires still not well understood. We present a dc conductance feature in the nonequilibrium transport regime, a direct indication that the first one-dimensional subband is filled mostly by one spin species only. How this anomalous spin population changes with magnetic field and source-drain bias is directly measured. We show the source-drain bias changes spin polarization in semiconductor nanowires, providing a fully electrical method for the creation and manipulation of spin polarization as well as spin-polarized currents.