The gene pool is the entire set of genes in a given population. It includes all the genes of all individuals. Individuals have the same genes but may have alternative forms of each gene, known as alleles. Imagine a population of pea plants whose gene pool contains the genes that determine the color of the flower.

The gene for flower color has two alleles, the allele that produces green flowers and the allele that produces yellow ones. When a given population is isolated, its gene pool cannot gain genetic diversity via exchanges with individuals from other populations. In this case, genetic drift can occur, especially if the population consists of few individuals.

Genetic drift is the change in the frequency of certain alleles with respect to the original population. Some alleles can be lost after several generations simply because no individuals are born with that specific version of the gene. Even if the original population had significant genetic variation, the composition of the gene pool could change after just a few generations.

The best ecosystems for studying genetic drift are on islands whose populations have migrated from a nearby continent. Species evolve much faster on islands than on continents because an island sustains fewer individuals, fewer species overall, and offers fewer resources.

Essentially, the island is a simpler ecosystem where it is easier to study how genetic drift can change the gene pool of a given population.