One of the most exciting recent developments in astrophysics has been the observation by large international consortia of hundreds of thousands of high-resolution spectra of galaxies in the local and distant Universe. The interpretation of these spectra in terms of stellar ages and metallicities is the key to reconstructing the star formation and chemical enrichment histories of the Universe. So far, however, such interpretations have been hampered by the fact that current models rely on spectral libraries of observed stars in the Milky Way and Magellanic Clouds, which have solar metal-abundance ratios at high metallicities. In contrast, the spectra of external galaxies appear to often be dominated by stars with non-solar metal abundance ratios (e.g., super solar alpha/Fe for massive galaxies). Until now, therefore, no spectral evolution model could help us fully exploit the wealth of information on chemical enrichment that is encoded in galaxy spectra.Only a few attempts were made to take into account the dependence of some spectral features on metal-abundance ratios. During my PhD thesis, I computed a comprehensive grid of high-resolution stellar spectra for non-solar mixtures of light elements at different metallicities. In collaboration with stellar evolution experts, we have assembled a grid of stellar evolutionary tracks for the same set of element mixtures. I plan to combine my spectral library with these evolutionary tracks to obtain the first fully consistent population synthesis models allowing the spectral interpretation of star clusters and galaxies in a wide range of chemical compositions.I will exploit this extremely powerful tool to quantify the effects of changes in abundance ratios on standard diagnostics of age and metallicity in galaxy spectra. By appealing to efficient techniques to interpret the large number of spectra gathered by modern galaxy surveys, I will be able to derive unprecedented constraints on the chemical evolution of galaxies.