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The distribution of gravitational torques and bar strengths in the localUniverse is derived from a detailed study of 163 galaxies observed in thenear-infrared. The results are compared with numerical models for spiral galaxyevolution. It is found that the observed distribution of torques can beaccounted for only with external accretion of gas onto spiral disks. Accretionis responsible for bar renewal - after the dissolution of primordial bars - aswell as the maintenance of spiral structures. Models of isolated, non-accretinggalaxies are ruled out. Moderate accretion rates do not explain theobservational results: it is shown that galactic disks should double their massin less than the Hubble time. The best fit is obtained if spiral galaxies areopen systems, still forming today by continuous gas accretion, doubling theirmass every 10 billion years.Comment: 4 pages, 2 figures, Astronomy and Astrophysics Letters (accepted

Gravitational torques in spiral galaxies: Gas accretion as a driving mechanism of galactic evolution