Final answer:
Population size changes are governed by four factors: birth rates, death rates, immigration, and emigration, along with the life expectancy which affects the time individuals are part of a population.
Step-by-step explanation:
The four factors that govern changes in population size are birth rates, death rates, immigration, and emigration. Birth rates contribute to the increase in population when they exceed death rates. Conversely, if death rates are higher than birth rates, the population is likely to decrease. Immigration (the movement of individuals into a population) and emigration (the movement of individuals out of a population) also affect the overall population dynamics. For example, a high rate of immigration can lead to population growth, while high emigration can contribute to population decline. These factors, combined with life expectancy, which determines the length of time individuals contribute to the population's genetic pool, are critical for understanding how populations change over time.