  Taylor's fluctuation scaling (FS) has been observed in many natural and
man-made systems revealing an amazing universality of the law. Here we give
strong theoretical foundations for the origins and abundance of Taylor's FS in
different complex systems. The universality of our approach is validated
against real world data ranging from bird and insect populations through human
chromosomes and traffic intensity in transportation networks to stock market
dynamics. Using fundamental principles of statistical physics (both equilibrium
and non-equilibrium) we prove that Taylor's law results from the well-defined
density of states (DOS) function of a system that gives the number of states
characterized by the same value of a macroscopic parameter (i.e., the number of
birds observed in a given area, traffic intensity measured as a number of cars
passing trough a given observation point or daily activity in the stock market
measured in millions of dollars).
