As a result of this reduction allele frequencies will have been altered at random. An example of this would be the cheetah. There was a huge reduction in the cheetah population. Parents mated with their own offspring when no other options were available. As a result of this, they have the same alleles and are almost like clones.
Founder effect is when a population is isolated.
They take with them the alleles that were in that group. This is based on chance, not natural section. An example of this would be the Amish.