Variation in gene frequencies within populations can occur by chance rather than by natural selection. This is referred to as

Change in allelic frequency occurring by chance is called genetic drift. Bottleneck effect and founder effect are examples of genetic drift. 

Gene flow is the introduction of genetic material (by interbreeding) from one population of a species to another, thereby changing the composition of the gene pool of the receiving population.

Genetic load is the existence of a harmful allele in a population. E.g., sickle cell trait. Random mating is a totally haphazard mating, with no regard to the genetic makeup (genotype) of the mate.