Answer:
True.
Explanation:
Natural selection may be defined as the process by which the organism are selected by the nature and adapt in the environment. Darwin explains the theory of natural selection.
In the given situation, the adult fish is kept but over time, the population of the adult fish declines over time. To maintains the population of adult fish, the mutation might occur in nature that reduces the size of the fish and naturally select the fish, fishes increase their number and changes their characteristics according to the environment. The average length of the adult population decline by the process of natural selection.
Thus, the correct answer is true.
