Sources of variation

Founder effect
It is the effect on the resulting gene pool that occurs when a new isolated population is founded by a small number of individuals possessing limited genetic variation relative to the larger population from which they have migrated.
Sewall wright effect
The sudden and random change in the allele frequency occurring in small or isolated populations by chance alone is called the Sewall wright effect.
Bottle neck effect 
The phenomenon causing the reduction in the allele frequency and loss of some alleles from the gene pool in a random fashion is called bottle neck effect.