Abstract: Mutations drive adaptive evolution due to their heritable effects on fitness. Empirical measures of the distribution of fitness effects of new mutations (the DFE) have been increasingly successful, and have recently highlighted the fact that the DFE changes during adaptation. Here, we analyze these dynamic changes to the DFE during a simplified adaptive process: an adaptive walk across an additive fitness landscape. First, we derive analytical approximations for the fitness distributions of both available and previously fixed alleles, and use these to derive expressions for the DFE at each step of the adaptive walk. We then confirm these predictions with independent simulations that relax several simplifying assumptions made in the analysis. Along with these quantitative predictions, we find that as de novo mutations accumulate, the DFE is reshaped in two important qualitative ways: the fraction of deleterious mutations increases (a shift to the left), and the variance of the distribution decreases. Finally, our analysis makes the surprising prediction that, at least in additive fitness landscapes, adaptation may be more limited by the availability of low-fitness alleles to be replaced, rather than by the availability of beneficial mutations.