Final answer:
The proportional amount by which the fitness of a population is reduced due to deleterious alleles is known as inbreeding depression, which affects genetic diversity and can lead to abnormalities.
Step-by-step explanation:
The proportional amount by which the fitness of a population is reduced due to the number of deleterious alleles in that population is known as inbreeding depression.
This occurs primarily when related individuals breed, which increases the probability that offspring will inherit deleterious recessive alleles. Inbreeding depression can lead to lower reproductive success and survival rates, along with an increased susceptibility to disease and abnormalities.
It's a measurable consequence of a small initial population size that results in a decline of genetic diversity, thus making a population more vulnerable to environmental changes and deleterious mutations.