Final answer:
According to the neutral theory of molecular evolution, the primary driving force when sequences evolve by drift and negative selection is genetic drift. Negative selection, or purifying selection, acts to remove harmful mutations from a population. This helps to maintain the stability and functionality of sequences.
Step-by-step explanation:
According to the neutral theory of molecular evolution, when sequences evolve by drift and negative (a.k.a. purifying) selection, the primary driving force is genetic drift. Genetic drift refers to random changes in allele frequencies in a population over time. In the case of neutral evolution, these changes are not influenced by natural selection, but rather by chance. Negative selection, on the other hand, refers to the removal of deleterious or harmful mutations from a population through natural selection. In this context, negative selection can act to maintain the stability and functionality of certain sequences by removing mutations that negatively affect fitness.