Answer:
Inbreeding depression
Explanation:
When inbreeding or breeding between the related individuals occurs within a population, a reduction is seen in the biological fitness of the individuals of that population. This reduction in the biological fitness of the individuals of the population is called inbreeding depression.
The same case happened with the population of fruit flies that were grown in the laboratory. Due to the inbreeding for several generations, their biological fitness got decreased and thus many of them are not viable in that environment.