Morphological evolution in the growth of thin films of Si1-xGex on Si(100) by gas-source molecular beam epitaxy has been investigated by in situ spectroscopic ellipsometry. When the ellipsometric trajectories are monitored at 3.4 eV and depicted in the plane of (psi,Delta) angles, two-dimensional (2D) growth is seen to produce a short one-turn spiral. The onset of three-dimensional nucleation can be detected by the smooth connection of one initial spiral to another small branch before convergence at the destination point of 2D growth or by deviation toward the region of lower Delta. These features correspond to the development of surface undulations that form a sinusoidal-like ripple structure. The second-stage transition from the ripple to coherent pyramidal islands occurs with the appearance of the cusp-shaped inflection point between the first and second branch, which is due to the engraving of troughs between islands. With yet higher content of Ge in the film and at high growth temperatures, the trajectory curve is shifted rapidly toward the lower Delta region after moving along the 2D growth spiral for a while, and this observation reflects the nucleation of isolated dome-shaped islands.