Population fragmentation is a form of population segregation. It is often caused by habitat fragmentation

Habitat fragmentation

Habitat fragmentation as the name implies, describes the emergence of discontinuities in an organism's preferred environment , causing population fragmentation...

. Population fragmentation causes inbreeding depression

Inbreeding depression

Inbreeding depression is the reduced fitness in a given population as a result of breeding of related individuals. It is often the result of a population bottleneck...

, which leads to a decrease in genetic variability in the species involved. This decreases the fitness of the population for several reasons. First, inbreeding forces competition with relatives, which decreases the evolutionary fitness of the species. Secondly, the decrease in genetic variability causes an increased possibility a lethal homozygous recessive trait may be expressed; this decreases the average litter size reproduced, indirectly decreasing the population.