Does genetic drift increase or decrease genetic variation in populations and why?

It is important to understand that each individual has different genes. Genes can be lost if an individual dies without reproducing. To answer your question: There are two type of effects caused by Genetic Drift. The founder effect happens when a few species inhabit a new territory. If only those species reproduce then there are less genes in the gene pool and that leads to less variation. This can happen if storms sweep birds to a previously uninhabited island. The other effect is the bottleneck effect. This happens if a disease or poaching drastically reduces the number of individuals in a population. Since there are less individuals who can reproduce there are not as many genes that can be passed down.