Biology 30 Diploma Practice Test

A closed population is defined by its isolation from external influences, meaning that it does not gain or lose individuals through migration. As a result, the size of a closed population changes only due to two primary factors: natality, which refers to the birth rate within the population, and mortality, which signifies the death rate.

In this context, natality contributes to the population's growth by increasing the number of individuals, while mortality can lead to a reduction in population size as individuals die. Both of these factors are intrinsic to the population and are the only mechanisms by which its size can fluctuate in a closed environment.

Therefore, the changes in a closed population are dictated directly by the interplay of births and deaths, making the understanding of these two factors crucial for studying population dynamics.