that describes a simple model for the density N ( x , t ) of population. This equation has two steady states, N = 0 , and N = N 0 . This solution corresponds to extinction and saturation of population. Observe that this model is spatially-extended, because it includes a diffusion term given by the second derivative. The state N ≡ ≡ N 0 is stable as a simple linear analysis can show and the state N = 0 is unstable. There exist a family of front solutions connecting N = N 0 with N = 0 , and such solution are propagative. Particularly, there exist one solution of the form N ( t , x ) = N ( x − − v t ) , with v is a velocity that only depends on D and r