In this talk, we focus on the speed selection problem for monotone dynamical systems of monostable type, including reaction-diffusion equations, nonlocal diffusion equations, and the Lotka-Volterra competition system. For the first time, we propose a sufficient and necessary condition for this long-standing problem from a new perspective. Moreover, our results reveal the essence of the linearly selected problem from the observation of the decay rate of the minimal traveling wave solution.