The paper first presents a comprehensive description of the selection pressure degradation problem when the traditional multi-objective evolutionary algorithms are adopted to solve many-objective optimization problems. According to the selection strategies, the Pareto-based, decomposition-based and indicator-based many-objective evolutionary algorithms(MaOEAs) are proposed to address the selection pressure degradation problem. Then, the definition of R2 indicator proposed as a new performance indicator is provided, and a series of R2-MaOEAs are infroduced. After analyzing the nature of R2-MaOEAs, this paper gives a general review of these algorithms in consideration of their four main components. Finally, the inherent basis and the future research arevintroduced.