Which evolutionary effect results in reduced genetic variation in a community?

The founder effect.

Explanation:

The founder effect does result in a reduction in the genetic variation in a community. When only a handful of organisms reach a new area with suitable resources, such as an island, many of the gene pool in the original community is lost and this increases the chances for evolution of something new as time and adaptation to new resources and conditions exert their effect.

d. the weight of the book