Final answer:
Genetic diversity in a population can be decreased due to inbreeding and chance events like natural disasters or human-caused events like the bottleneck effect.
Step-by-step explanation:
The diversity of alleles and genotypes within a population is called genetic variance. When scientists are involved in the breeding of a species, such as with animals in zoos and nature preserves, they try to increase a population's genetic variance to preserve as much of the phenotypic diversity as they can. This also helps reduce the risks associated with inbreeding, the mating of closely related individuals, which can have the undesirable effect of bringing together deleterious recessive mutations that can cause abnormalities and susceptibility to disease.
A decline in genetic diversity can also occur due to chance events such as natural disasters or human-caused events like the bottleneck effect, where a large portion of the population is randomly wiped out, leading to a drastic change in the genetic structure of the survivors.