**Figure 6.** **Time-evolution of mortality dynamics in the mathematical model of a heterogeneous population.** The model of a heterogeneous population composed of four subpopulations is modified to contain time-dependent parameters and is used to fit period Swedish death rates for ages 0 to 100 and for the entire 20^{th} century period (1900-2000). The resulting fitted surface of the modified model to the age- and time-related Swedish data is shown in panel (**D**). The initial mortalities and the mortality coefficients of subpopulations are assumed to change linearly over time (fits are shown in panels (**A**) and (**B**) respectively) while their initial fractions change exponentially (shown in panel (**C**)). Note that the plot in panel A is shown on a semi-logarithmic scale.