Final answer:
The term 'inbreeding depression' describes the reduced reproductive success and viability in offspring due to mating of closely related individuals, which can lead to a loss of genetic diversity and increased homozygosity for harmful mutations.
Step-by-step explanation:
The reduced reproductive output of a population that is due to inbred individuals having low fertility is referred to as inbreeding depression. This occurs because inbreeding, the mating of closely related individuals, increases the likelihood that offspring will inherit homozygous recessive alleles for deleterious traits. As a result, these offsprings may experience various problems such as increased chance of offspring dying, decreased life spans, and decreased chances of mating. Additionally, inbreeding can lead to a loss of genetic diversity within a population, which in turn reduces the population's ability to adapt to environmental changes and can increase the frequency of harmful genetic mutations. This is particularly detrimental for small populations and for species with already low genetic variation.