Gene pool is the total variety of genes and alleles present in a sexually reproducing population, the composition of gene pool may change constantly from generation to generation in any given population. New combination of genes produce unique genotypes which, when in physical terms as phenotypes, undergo environment selection pressures which continually select and determine which genes pass on to the next generation.
A population whose gene pool shows consistent change from generation to generation is said to be undergoing evolution change. A static gene pool represents a situation where genetic variation between members of the species is inadequate to bring evolutionary change.