Most genes occur as alleles, which give rise to differences in phenotype. Some phenotypes are better than others at helping the individual compete, survive, and reproduce. These alleles, therefore, increase in the population while others decrease. A population is a group of individuals of the same species that occupy a given area. In theory, the entire population shares a gene pool, which is the genetic resources that is passed to the next generation.
Alleles are two or more slightly different molecular forms of the same gene.
Movement of alleles between populations is called gene flow. Individuals, within the population, inherit different combinations of alleles, that results in variation in phenotype. Population genetics is the study of the properties of genes within a population. The progressive change within allele frequencies is called microevolution. The theoretical state when gene frequencies do not change from one generation to the next is called Hardy-Weinberg Equilibrium.