Define Historical Example Population genetic theory?

Population genetic theory, first developed by Sir Ronald Fisher, JBS Haldane and Sewall Wright, demonstrated how several evolutionary and demographic forces will influence levels of genetic variation in a population. Whereas mutation increases genetic variation, random genetic drift removes it. The strength of drift is proportion to the opposite of the effective population size (Wright 1931). So, population genetic theory predicts that genetic variation within a population should increase along with population size.

If population sizes fluctuate over time, the long-term average effective population size is more sensitive to the lower population sizes as compared to the higher ones. For instance, consider an insect population that over the course of each year undergoes three generations with population sizes of 10, 100 and 1000. Were it to frequently fluctuate among these three population sizes, its effective population size would be approximately 30-- far lower than the arithmetic mean of 370. So, periodic bottlenecks in population size will sharply decrease the effective population size and therefore genetic variance.