How evolution favors cooperation is a fundamental issue in social and economic systems. By incorporating multiple learning mechanisms in project selection into the threshold public goods game, we have investigated the coupling effect of mutation and imitation on the evolution of cooperation. Compared with the situation where there is no project selection mechanism, the existence of project selection may suppress or promote cooperation depending upon different learning mechanisms. There exists a critical ratio between the imitators and the mutants in the population, below which cooperation is suppressed while above which cooperation is promoted. With the coevolutionary mechanism of individual strategies and individual preferences, a higher level of cooperation corresponds to a larger scale of projects. A theoretical analysis indicates that, as most of the individuals are mutants, the coevolutionary mechanism is governed by the mutation process, which leads to a lower level of cooperation. As most of the individuals are imitators,