In this paper, we describe a new mechanism for population size adaptation in a differential evolution algorithm. The proposed mechanism is based on a linear decrease in the population diversity rate and allows both a decrease in population size and an increase in population size. We experimentally compared the effectiveness of several variants of the differential evolution algorithm with and without the adaptive mechanism on a set of test functions for CEC 2014. In addition, we compared the mechanism with linear population size reduction. The comparison results show that the use of the proposed mechanism is beneficial in terms of the efficiency of the optimum search in more than half of the tested problems, while the results are rarely degraded by implementing the mechanism.