In this Rapid Communication we propose a most general equation to study pattern formation for one-species populations and their limit domains in systems of length . To accomplish this, we include nonlocality in the growth and competition terms, where the integral kernels now depend on characteristic length parameters and . Therefore, we derived a parameter space where it is possible to analyze a coexistence curve that delimits domains for the existence (or absence) of pattern formation in population dynamics systems. We show that this curve is analogous to the coexistence curve in classical thermodynamics and critical phenomena physics. We have successfully compared this model with experimental data for diffusion of Escherichia coli populations.