  Using our updated stellar evolutionary code, we quantitatively evaluate the
effects of the uncertainties in the main physical inputs on the evolutionary
characteristics of low mass stars from the main sequence to the zero age
horizontal branch (ZAHB). We calculated more than 3000 stellar tracks and
isochrones, with updated solar mixture, by changing the following physical
inputs within their current range of uncertainty: 1H(p,nu e+)2H,
14N(p,gamma)15O, and triple-alpha reaction rates, radiative and conductive
opacities, neutrino energy losses, and microscopic diffusion velocities. We
performed a systematic variation on a fixed grid, in a way to obtain a full
crossing of the perturbed input values. The effect of the variations of the
chosen physical inputs on relevant stellar evolutionary features, such as the
turn-off luminosity, the central hydrogen exhaustion time, the red-giant branch
(RGB) tip luminosity, the helium core mass, and the ZAHB luminosity in the RR
Lyrae region are statistically analyzed. For a 0.9 Msun model, the cumulative
uncertainty on the turn-off, the RGB tip, and the ZAHB luminosities accounts
for $\pm$ 0.02 dex, $\pm$ 0.03 dex, and $\pm$ 0.045 dex respectively, while the
central hydrogen exhaustion time varies of about $\pm$ 0.7 Gyr. The most
relevant effect is due to the radiative opacities uncertainty; for the later
evolutionary stages the second most important effect is due to the triple-alpha
reaction rate uncertainty. For an isochrone of 12 Gyr, we find that the
isochrone turn-off log luminosity varies of $\pm$ 0.013 dex, the mass at the
isochrone turn-off varies of $\pm$ 0.015 Msun, and the difference between ZAHB
and turn-off log-luminosity varies of $\pm$ 0.05 dex. The effect of the
physical uncertainty affecting the age inferred from turn-off luminosity and
from the vertical method are of $\pm$ 0.375 Gyr and $\pm$ 1.25 Gyr
respectively.
