Answer:
An increase in pollution.
Step-by-step explanation:
Say we have 100 species of tuna in a lake. If all 100 species are given access to a food surplus, none are going to have the need to compete for survival, which encourages diversity in a population. Similarly, A decrease in predators and a decrease in fishing (which could be considered a predator) will not provide challenges for natural selection to occur in this population. However, if an increase of pollution occurs, whichever species of tuna is most adept at surviving and/or thriving in this environment will survive, and whichever species cannot survive in this environment will either change location or simply die out.