We derive observed Hα and R-band luminosity densities of an H I-selected sample of nearby galaxies using the SINGG sample to be limg1.gif = (9.4 ± 1.8) × 1038 h70 ergs s-1 Mpc-3 for Hα and limg2.gif = (4.4 ± 0.7) × 1037 h70 ergs s-1 Å-1 Mpc-3 in the R band. This R-band luminosity density is approximately 70% of that found by the Sloan Digital Sky Survey. This leads to a local star formation rate density of log(img3.gifSFR [Script Msun yr-1 Mpc-3]) = -1.80img4.gif(random) ± 0.03(systematic) + log(h70) after applying a mean internal extinction correction of 0.82 mag. The gas cycling time of this sample is found to be tgas = 7.5img5.gif Gyr, and the volume-averaged equivalent width of the SINGG galaxies is EW(Hα) = 28.8img6.gif Å (21.2img7.gif Å without internal dust correction). As with similar surveys, these results imply that img3.gifSFR(z) decreases drastically from z ~ 1.5 to the present. A comparison of the dynamical masses of the SINGG galaxies evaluated at their optical limits with their stellar and H I masses shows significant evidence of downsizing: the most massive galaxies have a larger fraction of their mass locked up in stars compared with H I, while the opposite is true for less massive galaxies. We show that the application of the Kennicutt star formation law to a galaxy having the median orbital time at the optical limit of this sample results in a star formation rate decay with cosmic time similar to that given by the img3.gifSFR(z) evolution. This implies that the img3.gifSFR(z) evolution is primarily due to the secular evolution of galaxies, rather than interactions or mergers. This is consistent with the morphologies predominantly seen in the SINGG sample.